A narrow (∼7 km wide) fold and thrust belt in west Texas that represents the northernmost extent of a Grenville-age collisional belt along the southern margin of Laurentia (Grenville Front), records a complex history of deformation and associated fluid flow. The Streeruwitz thrust that emplaced ca. 1.35 Ga high-grade metamorphic rocks over ca. 1.25 Ga foreland sedimentary and volcanic rocks postdates polyphase deformation in the footwall and is complexly folded into domes and basins. Four phases of tectonism, recording a changing kinematic setting, affected the area and formed: (1) pre-Streeruwitz ductile polyphase folds (F1–F3) and associated foliations (S1–S2) consistent with northward tectonic transport; (2) dextral oblique-slip, high-angle, west-northwest–trending faults associated with upright vertical sheath folds (F4); (3) Streeruwitz and related subsidiary imbricate thrusts that truncate F1–F4 folds at a high angle and cause localized folding (F5) consistent with north-northeast to northeastward tectonic transport; and (4) complex southeast- and northwest-trending domes and basins (F6) of the thrusts, resulting from continual Grenville-age transpression.
Fluids with an evolving chemistry over time were channelized along the thrusts, metasomatically altering the adjacent rocks. Early siliceous fluids caused replacement of mafic dikes and dolostones that preserve F1 and S1 and formation of extensive talc bodies with talc aligned axial planar to F2, forming the dominant S2 fabric. Initial thrusting at depth produced mylonites in both footwall (syn-S2) and hanging-wall rocks that were later brecciated in the final stage of thrusting along the Streeruwitz thrust. Further evolution of fluids along the thrusts is recorded in altered rocks adjacent to thrusts, breccias, and veins, starting with silica- and alkali-rich fluids. Lastly, carbonate-rich fluids replaced footwall rocks and cemented breccias in both the hanging wall and footwall.
This study documents a previously unrecognized complex structural, metamorphic, and metasomatic history, and fluid evolution in the foreland. This history, coupled with differences from that in the overriding older metamorphic rocks, requires a new kinematic model for the southern margin of Laurentia. In addition, the disparity in deformation timing and kinematic evolution between west Texas and the central Texas Llano uplift requires active subduction in west Texas after collision in central Texas. We propose that collision of a north-verging continental indenter with southern Laurentia initially occurred in the Llano area (ca. 1150–1120 Ma) and that continued subduction along strike caused clockwise rotation of the indenting continent and collision in west Texas (ca. 1060–980 Ma).
Mesoproterozoic Grenville orogenesis (ca. 1.3–0.9 Ga) resulted in the formation of the supercontinent Rodinia (e.g., Dalziel et al., 2000) with the Laurentian continental block in a central position. Grenville-age rocks crop out in a south-southwest trend from Canada’s Maritime Provinces through the Appalachian Mountains into the subsurface and are also exposed in central and west Texas. Numerous studies indicate that these Grenville-age rocks underwent polyphase deformation associated with arc-continent and continent-continent collision (e.g., see Tollo et al., 2004). One of the key debates for Grenville collisional orogenesis hinges on which continent or continents collided with Laurentia during the Proterozoic, resulting in the assembly of Rodinia (Hoffman, 1991; Dalziel, 1991; Karlstrom et al., 1999; Dalziel et al., 2000; Tohver et al., 2002; Torsvik, 2003; Meert and Torsvik, 2003; Whitmeyer and Karlstrom, 2007; Li et al., 2008; Ibanez-Mejia et al., 2011). Integral parts of testing Rodinia plate reconstructions are structural, kinematic, and geochronologic analyses from key areas.
Two tectonic models have been proposed for the Grenville orogeny along the southern margin of Laurentia. One of us (Mosher, 1998) proposed that a southern continent acted as an indenter that collided with Laurentia between the Llano uplift and the Van Horn region. This collisional model predicts northeast tectonic transport for the Llano uplift and northwest tectonic transport for the Van Horn region of west Texas. In this model, a different continental block collided with the eastern margin of Laurentia. Bickford et al. (2000) proposed dextral transcurrent motion along the southern margin of Laurentia resulting from continental collision of a single block along the eastern margin of Laurentia. This model also predicts northwest-directed tectonic transport for the west Texas exposures, although it does not explain the northeastward tectonic transport observed in the Llano uplift (e.g., Reese and Mosher, 2004; Mosher et al., 2004). Grimes and Copeland (2004) demonstrated that deformation in the west Texas exposures occurred ∼60 m.y. after orogenesis in the Llano uplift, requiring modification of both models (Mosher et al., 2008A).
Mesoproterozoic exposures near Van Horn, where high-grade metamorphic rocks were thrust over low-grade rocks in a 5–7-km-wide foreland fold and thrust belt (Fig. 1), provide a key area for testing tectonic models for Grenville orogenesis. Exposures in west Texas display a transect across a mid-crustal metamorphic core to upper crustal foreland rocks over a distance of ∼20 km. Furthermore, these rocks are north and west of the deformational fronts of Phanerozoic orogens and show little evidence of overprinting deformation or younger translation. Rocks of equivalent age to the north and west are undeformed, indicating that this area represents the Grenville Front in west Texas (Llano Front of Mosher, 1993).
In this paper we present results of a detailed structural analysis of new exposures in talc mines within the Grenville foreland that constrain its kinematic and geologic evolution and demonstrate that both the tectonic transport direction and the chemistry of fluids channelized along faults changed over time. Furthermore, we present a structural and kinematic model for the west Texas exposures and incorporate it into a tectonic model for Grenville orogenesis along the southern margin of Laurentia.
Precambrian exposures in Texas provide a unique transect through the Grenville orogenic belt, from a metamorphic core in the Llano uplift of central Texas to upper crustal and undeformed sections in west Texas. The Grenville Front trends southwestward in the subsurface from northeastern Texas to the Van Horn region in west Texas, where it is exposed in a Phanerozoic horst block (Ewing, 1990; Mosher, 1993). The Llano uplift of central Texas is 300 km inboard of the Grenville Front and exposes high-grade, polydeformed, metamorphic rocks and syntectonic to posttectonic granites (Mosher, 1998). Deformation across the Llano uplift is preserved in as many as six phases of folding (Nelis et al., 1989; Mosher, 1993, 1998; Reese and Mosher, 2004; Mosher et al., 2004; Levine and Mosher, 2010) and two metamorphic events (see Carlson, 1998, for review). Initial amphibolite transitional to granulite and eclogite facies metamorphism related to collisional tectonics occurred between 1150 and 1119 Ma, waning by 1115 Ma (Mosher, 1998; Carlson, 1998; Carlson et al., 2007; Mosher et al., 2008b); a second largely static, low-pressure, high-temperature metamorphic event associated with emplacement of syntectonic to posttectonic granitic plutons occurred between 1119 and 1070 Ma (Bebout and Carlson, 1986; Reed, 1999; Mosher, 1998). A model was proposed (Mosher, 1998) that included an arc-continent collision of an exotic terrane (Roback, 1996) with an unidentified southern continent between 1275 and 1256 Ma, and subsequent accretion of this arc-continent block to southern Laurentia between 1150 and 1120 Ma, with lower limit of deformation at 1098 Ma (Nelis el al., 1989; redated by Walker, 1992). Structural and kinematic analyses indicate that overall tectonic transport within the uplift was to the northeast (Reese and Mosher, 2004; Mosher et al., 2004). Evidence to support interpretations involving transcurrent motion in the uplift has not been observed (Mosher et al., 2004).
Mesoproterozoic exposures in west Texas include polydeformed metamorphic rocks of the 1.38–1.29 Ga Carrizo Mountain Group (CMG) thrust over ca. 1.26 Ga (Grimes, 1999; Bickford et al., 2000) foreland sedimentary rocks of the Allamoore and Tumbledown Formations, and younger Hazel Formation, of the Van Horn area (this study, Fig. 1A); undeformed sedimentary and igneous equivalents are found in the Franklin Mountains near El Paso (Roths, 1993; Pittenger et al., 1994), and in igneous rocks in the Hueco Mountains (Masson, 1956).
In the Van Horn region, the Streeruwitz thrust is the primary structural feature, which translated high-grade metamorphic rocks of the CMG ∼19 km northward over low-grade, highly deformed sedimentary and volcanic rocks of the Allamoore and Hazel Formations (King and Flawn, 1953; Wiley, 1970; Reynolds, 1985; Haenggi, 2001; Fig. 1B). The 40Ar/39Ar dating of mylonites near the Streeruwitz thrust indicates mylonitization ca. 1035 Ma (Bickford et al., 2000), and rapid exhumation and associated brittle faulting between ca. 1000 and 980 Ma (Grimes and Copeland, 2004). Deformation and metasomatism decrease markedly away from the thrust, and deformation is almost absent ∼7 km to the north.
Deformation in the CMG shows a complex and protracted history dominated by zones of dextral transpression and overall northwestward tectonic transport (Grimes, 1999; Grimes and Mosher, 2003). Metamorphic grade and deformational complexity decrease from the southeast to the northwest approaching the Streeruwitz thrust (King and Flawn, 1953; DuBois, 1998; Mosher, 1993, 1998; Grimes, 1999; Grimes and Mosher, 2003; Grimes and Copeland, 2004). The highest metamorphic grade CMG rocks (peak metamorphism: 640 ± 50 °C to 510–530 °C) record at least five phases of deformation (D1–D5) and a medium pressure-temperature metamorphic event dated as 1057 ± 6 Ma (Bristol and Mosher, 1989; Grimes and Mosher, 2003; Grimes and Copeland, 2004). D1 and D2 in the central and southeastern Carrizo Mountains were characterized by crustal thickening, formation of northwest-verging folds, and northwest-directed dextral transpression, whereas D3 and D4 refolded earlier folds under conditions of dextral shearing and retrograde metamorphism (Grimes and Mosher, 2003).
Precambrian sedimentary units within the foreland include the Allamoore, Tumbledown, and Hazel Formations. The ca. 1260 Ma Allamoore Formation is composed of shallow intrusive and extrusive mafic volcanic rocks interlayered with Mg-rich carbonates, thin layers of phyllite and talc, large massive talc bodies, minor felsic tuff, and cherty and stromatolitic limestone and dolostone (Gore, 1985). The sediments were deposited in an intertidal to supratidal setting within a restricted lagoonal environment with some component of ephemeral hypersaline lakes or sabkha (Bourbon, 1981; Nyberg and Schopf, 1981; Edwards, 1985); this accounts for the high magnesium content of Allamoore rocks, aiding the mineralization of talc. Massive talc deposits are restricted to the Allamoore Formation.
The ca. 1246 Ma Tumbledown Formation uncomformably overlies the Allamoore Formation and is a 168-m-thick succession of volcanic lithic sandstone, basaltic agglomerate, mafic volcanic flows, rhyolitic felsite, and large gravity-slide blocks of Allamoore carbonate exposed mostly in the Tumbledown Mountain area 12 km west-northwest of the study area (McLelland, 1996; Glahn, 1997; Bickford et al., 2000). Thrusting involving these two formations resulted in the nonconformably overlying Hazel Formation, an arid alluvial fan complex that accumulated in a tectonically active transpressional basin by erosion of advancing thrust sheets to the south (King and Flawn, 1953; Reynolds, 1985; Soegaard and Callahan, 1994). The basin is of unknown geometry and extent and was bounded on its southern margin by active uplift of the Allamoore and Tumbledown Formations, the source terrain for most of the Hazel clasts (Soegaard and Callahan, 1994). Granite and rhyolite clasts found within the Hazel conglomerate are similar to and coeval with the Red Bluff Granite Complex and rhyolite from the Thunderbird Group (see Thomann, 1980; Shannon et al., 1997), both exposed in the Franklin Mountains 160 km west (Roths,1993; Bickford et al., 2000). CMG clasts are notably absent in the Hazel Formation (King and Flawn, 1953; Soegaard and Callahan, 1994). Red granite clasts in Hazel conglomerate have been dated as 1123 ± 29 Ma and rhyolitic clasts as 1126 +100/–27 Ma (Roths, 1993), giving an older constraint on the age of deposition. Overlying the Mesoproterozoic units, the middle Cambrian Van Horn Sandstone (Spencer et al., 2014) is a postorogenic alluvial fan complex derived from a northern highland and deposited on a highly dissected Precambrian surface (McGowen and Groat, 1971). Only tilting and warping, with local offset on high-angle faults, affects this unit (King, 1965).
Grenville-age foreland deformation in west Texas had a long and protracted history of transpression. High-angle west-northwest–trending faults that cut the foreland show as much as 1400 m of strike-slip motion (Kwon, 1990). These faults show sinistral and dextral motion (Kwon, 1990; Glahn, 1997) that predates (Davis and Mosher, 2006; this study) and postdates major thrusting (King and Flawn, 1953). The protracted histories of strike-slip motion on these high-angle faults (i.e., Grapevine, Dallas, Yates Spring, and the Carrizo Springs fault zone) are partly responsible for the west-northwest foreland grain (King and Flawn, 1953). West-northwest–trending faults have been reactivated multiple times throughout the Phanerozoic in west Texas (King and Flawn, 1953; Muehlberger, 1980; Soegaard et al., 1993).
Although the CMG and the Allamoore Formation are moderately to well exposed, the Streeruwitz thrust contact between these two disparate groups is poorly exposed, and generally inferred (e.g., King and Flawn, 1953). This study presents an analysis of new exposures of this northwest- to north-northwest–striking thrust in three open-pit talc mines (Fig. 2). Footwall structures in the Allamoore Formation predate, postdate, and are coeval with the thrust. The Rosa Blanca mine is the farthest northwest with the Streeruwitz thrust exposed along the south wall. The Texola mine is <∼3 km southeast of the Rosa Blanca mine along the trace of the thrust, and the Streeruwitz thrust is exposed in three dimensions (Fig. 3). The thrust is folded into type 1 interference patterns, or dome and basin folds (Ramsey, 1967). The Glen Ray mine is located farthest inboard, between the two other mines (∼2 km southeast of the Rosa Blanca mine, ∼800 m northwest of the Texola mine; Fig. 2), and exposes an imbricate thrust within the Allamoore Formation that truncates early transpressional folds and faults.
The structural analysis presented here indicates that four distinct overprinting deformation phases affected the foreland (discussed in the following). Phase 1 is characterized by early folding (F1–F3), resulting in S1 and the primary S2 foliations, and regional metamorphism and metasomatism resulting in the large talc deposits, compatible with early thrusting under ductile conditions. Phase 2 is characterized by high-angle transpressional faults and related folds (F4) that refolded earlier structures. During phase 3, the Streeruwitz thrust formed, along with subsidiary imbricate thrusts and associated folds (F5). The thrust faults truncate earlier structures at high angles, indicating a change in the overall kinematics of deformation. Phase 4 resulted in dome and basin type 1 folds of the thrust sheets, and reoriented all previous structures (this causes the scatter in data observed on stereonets). This final phase is interpreted to have resulted from renewed transpression. All the structures discussed here are observed within the Allamoore Formation, unless otherwise noted.
Phase 1. Early Thrusting
Phase 1 is characterized by three generations of folds (F1–F3) distinguished on the basis of fold morphology and overprinting relationships between foliations and structures. F1 folds are rarely observed in the field, typically as rootless folds of silicified dolostone enveloped in talc (Fig. 4A). In thin section, rootless F1 fold hinges are observed in the talc. F1 are tight to isoclinal folds, have both arcuate- and chevron-shaped fold hinges, and are complexly refolded. F1 folds typically have a wavelength that ranges to 20–30 cm across; no larger scale F1 folds were identified.
In the Rosa Blanca mine, F1 axial planes are east striking and steep dipping, and fold axes are roughly on a steep east-trending girdle, which also parallels the later Streeruwitz thrust in this mine (Davis, 2007). Most of the fold axes were measured along a northeast-trending wall composed of intensely folded talc with numerous shear planes and shear duplexes. This later intense deformation may account for the spread in fold axis orientations, or the fold hinge lines may have originally been curved (i.e., sheath folds). F1 folds observed in the Glen Ray mine are part of a series of the tectonically dismembered segments of a silicified dolostone layer enveloped within the talc directly below the imbricate thrust on the west side of the mine.
F1 folds developed an axial planar foliation (Fig. 4A), which is rarely observed in the field. In one less altered dolostone, the S1 foliation in thin section is defined by very fine grained elongate quartz with a crystallographic preferred orientation. S1 is more commonly recognized in thin section in second-generation (F2) fold hinges where it has been refolded. S1 is defined by alignment of metamorphic or deformed minerals specific to the protolith, with both talc and tremolite forming S1. Because of intense metasomatism and later foliation development, the S1 foliation is difficult to distinguish, or nonexistent. S1 and S2 commonly form a composite foliation and are described in more detail in the following.
F2 folds can be found in all mines but are relatively uncommon. The best exposures are on the easternmost wall in the Rosa Blanca mine, where altered mafic dikes that preserve the S1 foliation are folded into upright, closed to isoclinal folds with the dominant S2 talc foliation parallel to the axial planes. These folds are rootless and have been detached in later shear zones (Figs. 5A, 5B). F2 folds are also within the main body of talc as centimeter-scale, open to tight, vertical folds with northeast-striking axial planes. F2 folds are seen, although not so spectacularly, in the other mines, usually as rootless folds of a more competent lithology incorporated in sheared talc.
Some F2 folds show a different morphology. On the easternmost wall of the Rosa Blanca mine, several meters north of the F2 mafic dike folds, are “flame-like” F2 folds. These are upright, isoclinal folds with millimeter-scale wavelengths that also fold altered mafic dikes and present a stark contrast in geometry and morphology to other F2 folds of the same lithology. The talc foliation is axial planar, indicating that these folds are coeval with the upright closed folds a few meters to the south. The contrast in fold morphology appears to result from a localized zone of intense shearing.
F2 folds show no consistent orientation between the mines (Figs. 6A–6C). Within the Rosa Blanca mine, F2 axial planes are east striking and steeply dipping, similar to the F1 fold axial planes. Poles to the axial planes roughly define a steep north-trending girdle, indicating that they may have been refolded by a shallow east-plunging fold. Fold axes plunge moderately east. In the Texola mine, F2 axial planes dip shallowly to moderately southeast to southwest. The poles to the axial planes fall along a steep northeast-trending girdle, indicating that they may have been refolded by a shallow southeast-plunging fold. Fold axes have a shallow to moderate south-southeast plunge. In the Glen Ray mine, F2 axial planes are east striking and shallowly to steeply south dipping. The poles to the axial planes are along a vertical north-trending girdle, indicating that they may be folded by a subhorizontal east-plunging fold. The F2 fold axes have a wide range of orientations; most plunge moderately southeast to southwest.
The S2 foliation is the dominant foliation in the mines and is a very closely spaced, penetrative foliation. It is typically defined by aligned metamorphic minerals such as talc or tremolite where it is best expressed in outcrop; in other lithologies, it is defined by elongate grains of calcite and/or quartz. The S2 foliation is most consistent in the Rosa Blanca mine, where it generally is west-northwest striking and steeply dipping (Fig. 6D). The S2 orientation in the Texola and Glen Ray mines shows a wide dispersion with an overall prevalence of northwest strikes and moderate southwest dips; however, the foliation is clearly folded. Three girdles are defined by the foliation poles: the dominant girdle has a π pole that plunges moderately (∼40°) southwest; the other two girdles have π poles that plunge ∼60° northwest and east (Figs. 6E, 6F). These indicate that the foliation has been refolded by multiple sets of post-F2 folds. The S2 is foliation is also truncated by later faulting.
In thin section, S1 and S2 form a composite foliation that varies by lithology. Within talc units, S2 is the dominant foliation and increases in structural complexity and purity with proximity to the Streeruwitz thrust. S2 is primarily defined by talc and other minerals such as tremolite, quartz, and carbonate; the percentage of other minerals defines the grade of talc (from lowest to highest: talc-rich phyllite, ceramic-grade talc, and paint-grade talc; see Kyle and Clark, 1990). In talc-rich phyllite units, located structurally furthest from the thrust front, S2 is a closely spaced, crenulation foliation (Fig. 7A). Here, S2 is dominantly defined by tremolite (to 80%), talc, and an increasing percentage of carbonate material away from the thrust. Most of the tremolite grains are aligned laths (1–3 mm long, 0.2 mm wide) forming a well-developed S2 foliation. The S2 tremolite and talc foliation contains microscale interfolial F2 folds of recrystallized S1 tremolite within F2 fold hinges (Fig. 7A). These recrystallized crenulation foliations of tremolite suggest two periods of tremolite mineralization; however, tremolite defining S2 is rare except in the talc-rich phyllites and may reflect either the rock’s bulk chemistry or later nearly complete replacement by talc.
In the most abundant gray to black, ceramic-grade talc unit, S2 is defined by acicular talc grains (to 60%; as much as 2 mm wide, 8 mm long), calcite and/or dolomite, tremolite, and minor quartz. Carbonate forms thin layers parallel to the foliation or millimeter-scale nodules; tremolite forms euhedral to subhedral equant or lath-like grains (to 5 mm long) with margins replaced by talc. Interfolial F2 folds of ultrafine-grained (<0.5 mm) talc that is dynamically recrystallized in the hinges indicates that talc also defined S1 (Fig. 7B). This relationship, coupled with tremolite forming both S1 and S2 in the talc phyllites, indicates that early fold generations (F1 and F2) occurred under conditions within the stability field of both talc and tremolite, with the second stage of talc and/or tremolite growth favoring talc. Talc shows undulose extinction and kink bands, particularly where folded by late (F5) folds.
The pure, white or pink, paint-grade talc is 90%–100% talc and found as complexly folded slivers beneath thrust sheets. S2 is a closely spaced crenulation foliation. Talc parallel to S2 forms centimeter-scale layers or single laths (to 1 cm long). At high magnification, the talc fibers have serrated boundaries, indicating grain boundary migration, undulose extinction, and incipient subgrains.
In silicified dolostones, the composite S1-S2 anastomosing foliation is defined by aligned tremolite, elongate quartz, talc, and elongate dolomite (Figs. 4B, 4C). Talc defines S2 in layers dominated by 0.1–0.5 mm, thin fibrous mats or as individual grains in quartz-rich layers. Larger tremolite grains are subhedral, 0.5–1 mm long, acicular to equant grains; some of the larger tremolite porphyroblasts are rotated and wrapped by smaller tremolite and/or talc that further defines the composite foliation, indicating two periods of tremolite growth, the earlier phase defining S1. Larger S1 tremolites have face-centered pressure shadows of quartz, talc, and/or tremolite that are parallel to the composite foliation. S1 tremolite grains show undulatory extinction; mimetic replacement by talc parallel to S2 is observed along the margins of S1 tremolite grains. Quartz-rich layers are either microcrystalline quartz or lenticular clusters (∼0.5 mm in length) of moderately large recrystallized quartz with some optical continuity, giving the rock a pseudomylonitic texture (Fig. 4C). Quartz recrystallization was dominated by rotational recrystallization with minor grain boundary migration evident by somewhat serrated grain boundaries on subgrain-sized quartz. Elongate recrystallized quartz defines the composite S1-S2 foliation observed in most of the dolostone units.
Carbonate units include carbonate phyllite, banded limestone, and non-banded limestone; only one foliation is observed, which on the basis of the interlayered relationships with talc and the presence of aligned talc with only minor tremolite, is interpreted to be S2. In the carbonate phyllites, S2 is defined by elongate calcite and/or dolomite, talc in thin seams between carbonate layers, and dynamically recrystallized quartz (Fig. 8C). Dolomite grains are typically larger, as much as 10 mm in diameter, and show undulose extinction and minor grain boundary migration. In banded limestones, S2 is defined by elongate calcite and/or dolomite, elongate microcrystalline quartz (in chert), quartz ribbons (1 mm thick, to 1.5 cm long) that were dynamically recrystallized, and small acicular talc grains. In some units, tremolite appears as millimeter-sized euhedral grains aligned parallel to the foliation. S2 locally is defined by millimeter-thick straight or semi-stylolitic seams wrapping dolomite grains, indicating pressure solution. The seams are folded by post-F2 isoclinal folds, and in the hinges another set of pressure solution seams forms an axial planar cleavage to the post-F2 folds, locally enhancing S2 foliation. S2 is poorly expressed in the non-banded limestone by slightly elongate calcite, discontinuous layers of elongate and recrystallized quartz, and some pods of aligned talc.
Altered Allamoore Formation igneous rocks observed in the mines include altered mafic dikes, chloritized olivine basalt, and chloritized basalt interlayered with carbonates. Outcrops of altered mafic dikes, best seen in the Rosa Blanca mine, are pale green or pinkish crystalline rocks with only one well-developed foliation (Figs. 5A, 5B). In outcrop, this foliation is parallel to the S2 talc foliation. In thin section, the S2 foliation is defined by elongate mineral grains in alternating quartz-dominated or chlorite- and/or talc-dominated layers. Quartz-rich layers have a smooth foliation defined by a groundmass of fine-grained (∼0.2 mm) elongated quartz and minute (<0.05 mm) laths of chlorite. Subparallel to the quartz foliation are 5-mm-thick layers of quartz ribbons. Deformational features in the quartz ribbons include continuous to discontinuous undulose extinction with boundaries that are subparallel to the foliation, numerous subgrains 1 mm in length, and some blurry or serrated grain boundaries. The dominant recrystallization mechanism in these layers is rotational recrystallization with a minor degree of grain boundary migration. In the quartz-poor layers, the S2 foliation is defined by aligned grains of talc, chlorite, and tremolite.
In the hinge of one of the F2 folds of these mafic dikes, the S1 foliation is observed and is defined primarily by aligned biotite with some albite grains oriented parallel to the foliation (Fig. 5C). The only deformational features observed in the albite are deformation twins and undulose extinction. Some grains have inclusions of biotite and overprint the foliation, whereas others are wrapped by the foliation, suggesting that original and metasomatic albite is present. An adjacent sample of the same rock shows that the S1 foliation has been completely destroyed by silica-rich metasomatism (Fig. 5D). Only relict thin seams of the S1 biotite foliation and some associated albite, which shows evidence of grain boundary migration, are preserved. The S2 foliation is defined by the elongate quartz and minute (∼0.5 mm) laths of chlorite. Quartz elongation was accomplished by lattice reorientation, rotational recrystallization, and grain boundary migration. Chlorite is roughly aligned with the elongate quartz, but in some parts of the sample it has a more random orientation.
The timing of foliation development in the chloritized basalts is not as evident. The foliation is similar to the S1 foliation described here for the less altered part of the mafic dike. The foliation is defined exclusively by aligned biotite and chlorite. It wraps relict olivine phenocrysts, which are now altered to epidote and albite. Some of the albite has altered to sericite. The foliation contains a mineralogy similar to the S1 foliation of the mafic dike, and therefore it is reasonable to assume that this foliation is probably the same foliation; however, it could also be a temporal equivalent to S2.
F3 folds are found in all the mines but are best exposed and exemplified in the limestone units in the Glen Ray mine (Fig. 8A). Third-generation folds are tight to isoclinal and refold previous folds and the S2 foliation (Fig. 8). Two morphologies for the isoclinal folds have been observed, parallel folds (class 1B or 1C; Ramsey, 1967) and similar folds (class 2). The orientations of the two sets of folds are similar; both fold the previous foliations and lack a penetrative axial planar cleavage, and both appear to have formed under less ductile conditions than previous fold generations and more ductile conditions than later fold generations. Therefore, they are treated together as a single fold generation. The morphological contrast is thought to be a reflection of rheological differences between heterogeneous limestone layers. F3 folds are observed from thin section (millimeter scale) to mesoscopic (wavelengths to 2 m) scale (Fig. 8); some larger folds display parasitic folds with wavelengths of ∼1 m.
Orientations for F3 folds are similar between the Glen Ray and Texola mines. Poles to F3 axial planes for both mines plot along a northeast-trending girdle where the associated π pole plunges moderately southeast (Fig. 9). Another girdle, from the poles to axial planes of F3 fold in the Glen Ray mine, can be defined that trends northwest with a steep southwest-plunging fold axis. Both girdle orientations can be explained by later refolding. F3 fold axes show a wide range of orientations but are mostly constrained to either the southeast quadrant or show a cluster plunging west. Despite the scattered stereonet pattern, fold axes for the Glen Ray mine in map view are within the overall plane of the north wall of the large west pit, which strikes west-northwest and dips steeply south (Fig. 10). Meter-scale upright isoclinal folds oriented parallel to the strike of the north wall of the Glen Ray mine were observed along the eastern portion of the wall. The orientation of this wall appears to be a result of these large meter-scale F3 isoclinal folds, which are oblique to the strike of the imbricate of the Streeruwitz thrust that cuts the wall in the northwest corner of the mine.
No penetrative and pervasive foliation is associated with F3 folds; the S3 surface is a weakly defined axial planar cleavage observed only in thin section and localized in F3 fold cores. The lack of an axial planar foliation and the folding of S1–S2 surfaces are characteristics of F3 and all the subsequent fold generations.
Phase 1. Interpretation
These earliest structures consist of fold generations F1–F3, and the associated S1 and S2 foliations. These folds are grouped together because they formed early under ductile conditions, have similar mineralogies defining S1 and S2, and could have progressively formed in a noncoaxial shear environment associated with thrust motion. Alternatively the fold generations could be genetically unrelated. Subsequent structures, however, clearly truncate these early structures and require a different kinematic setting. F1 and F2 folds are usually rootless folds and both have axial planar foliations, S1 and S2, respectively. F1–F3 folds are tighter and more ductile in nature than later fold generations. S1 is defined by biotite in mafic dikes, talc in ceramic-grade talc units, and tremolite in dolostone. The tremolite is replaced by S2 talc and is only found parallel to S2 in talc-rich phyllites, most likely indicating higher temperatures during S1, although changing fluid composition could also cause this change. F2 folds have the dominant talc foliation, axial planar, indicating that the main foliation in the mines is S2, and most talc formed during formation of the F2, S2 phase. The abundance of talc and a decrease in tremolite in talc-rich units approaching thrusts suggests later fluid influx altering the mineral assemblages. F3 are isoclinal folds that lack a pervasive axial planar foliation. Upright isoclinal F3 folds make up the north wall of the Glen Ray mine, which is oriented obliquely to the imbricate thrust in the mine, requiring a change in the kinematic setting (see Phase 3 discussion). The mineralogy (talc and/or tremolite) defining S1 and S2 and deformational and recrystallization features exhibited in quartz and calcite indicate that greenschist facies metamorphic conditions were achieved during F1 and F2. Thus, these folds are best explained by pre-Streeruwitz, noncoaxial shear deformation associated with thrusting at depth; tectonic transport was most likely northward.
Phase 2. F4 Folds and Transpressional Faulting
The second group of structures encompasses one recognized fold generation, F4, and associated transpressional motion on high-angle faults or shear zones. F4 folds are seen in each mine, but like F3 folds, they are best exemplified in the competent units of the Glen Ray mine. F4 folds are tight to isoclinal, fold previous folds and foliations, and did not develop an axial planar foliation. Some folds could not be uniquely assigned to either F3 or F4 fold generations, but most F4 folds have unique morphologies that aid in their recognition, described in the following, and many fold F3 folds (Fig. 8A).
F4 folds, termed rollover folds, are typically asymmetrical, moderately to steeply plunging folds with a consistent vergence. F4 folds have curved or anastomosing fold axes; some are incipient to moderately developed sheath folds (Figs. 11 and 12). These folds typically die out along their length and change morphology from tight, overturned folds in the middle of the fold trace to more open, upright folds where they die out. Unlike F3 folds, which are mainly constrained to limestone units, F4 folds are found in other lithologies. F4 folds in talc show the characteristic fold geometry of rollover folds; i.e., they have curved fold axes that appear to have been rolled differentially, and are usually steeply plunging (Fig. 11). The largest F4 folds have a wavelength of only ∼3 m, due to their isoclinal geometries.
The morphology of the F4 folds gives an indication of the type of kinematic setting in which they were formed. The steep north wall of the Glen Ray mine roughly parallels bedding (Fig. 10) and displays abundant F4 folds at multiple scales (Figs. 11A and 12). The best example is the largest F4 fold, which is steeply plunging, asymmetrical, and westward verging with a wavelength of ∼3 m. Its morphology shows that it underwent differential motion, as the top part of the fold is tighter and offset from the lower portion of the fold by a tear fault (Fig. 12). Numerous parallel en echelon tear faults that accommodated strain during F4 folding are present on the limbs of this and adjacent rollover folds, resulting in offset hinge lines. These characteristics indicate that the fold formed by oblique dextral motion under brittle and/or ductile conditions, which is consistent with the anastomosing, sheath-like geometries.
Timing for the F4 folding is well constrained. Field observations show F3 folds being refolded by F4 (Fig. 8A). Furthermore, many of the F4 folds are truncated by shear zones in the talc and the Streeruwitz and associated imbricate thrusts (Figs. 11 and 12). The orientations of these folds, the crosscutting relationships with faults, folding of the S2 surface, and fold superposition give a well-constrained timing for the formation of F4 folds.
The orientation of F4 folds, although different between mines (Fig. 13), is internally consistent, controlled by the orientation of the layers on which they formed. In the Texola mine, poles to F4 axial planes are along an east-trending, north-dipping girdle, suggesting that they have been folded by a moderately south-plunging fold (Fig. 13A). The majority of F4 fold axes plunge moderately south; some plunge steeply west. A few of the F4 folds were rotated as they were tectonically incorporated into a talc décollement; however, the majority appears to be unrotated. Only a few F4 folds in the Rosa Blanca mine were observed; these are nearly vertical, south-plunging folds with east-northeast–striking axial planes (Fig. 13B). This orientation is compatible with the F4 folds of the other mines, and some are truncated by the Streeruwitz thrust (Fig. 11B).
In the Glen Ray mine, poles to the axial planes of F4 folds are along a northeast-trending, northwest-dipping girdle, suggesting that they were folded by a shallow southeast-plunging fold (Fig. 13C). F4 fold axes generally plunge moderately southeast. Most were measured from the north wall of the main pit in the Glen Ray mine, along the east-striking, moderately to steeply southward dipping layers. Although the fold axes appear consistently oriented in the field, they show a spread when plotted on a stereonet. The spread results from anastomosing, curved hinges of sheath folds, reorientation by later broad folds (F6), and the folding by F4 of a previously folded (i.e., nonplanar) surface.
The abundant F4 folds in a plane showing the same sense of shear indicates that they are part of a dextral oblique-slip shear (or fault) zone that paralleled the length of the Glen Ray pit; the adjacent talc was removed by mining. Examination of the nearly inaccessible main pit east wall, where the fault zone is projected to intersect, shows layers of highly contorted paint-grade talc and a zone of intense deformation and localized metasomatism. Numerous contorted beds and duplication and folding of basaltic rocks suggest that the fault had a large component of strike-slip motion, and some possible later reactivation as a thrust. On the west end of the pit, the imbricate thrust would truncate the fault within the excavated pit; this map interpretation is supported by the truncation of associated F4 folds in the northwest corner by the lower segment of the imbricate thrust (Figs. 10 and 11A).
Phase 2. Interpretation
The steeply to moderately dipping layers that form the north wall of the west pit in the Glen Ray mine best expose the F4 folds and display the timing relationships with other structural generations. The orientation of the layers is a result of isoclinal F3 folding, and the F4 folds exposed along this wall formed on the limbs of these larger isoclinal F3 folds. The consistent vergence of the F4 folds coupled with their morphology (anastomosing, curved hinge lines that die out along their length with associated changes in tightness and degree of overturning and sheath fold geometry) indicate that they formed as a result of dextral oblique shear parallel to the layering. This shear is supported by the differential motion along associated tear faults exhibited by the largest F4 fold located in the center of this wall and adjacent smaller folds. The characteristics of these F4 folds suggest the presence of an east-striking, dextral oblique-slip fault parallel to the northern wall within the excavated pit (Figs. 10 and 12). The proposed fault is substantiated by the presence of sheared and deformed Allamoore Formation rocks, especially talc, along the eastern wall of the main pit in the Glen Ray mine, where the fault would intersect the exposed outcrop. Although the trace of this fault cannot be followed out of the pit, a regional view of the fault trend suggests that it is related to other high-angle west-northwest–trending faults mapped in the foreland.
King and Flawn (1953), Kwon (1990), and Glahn (1997) mapped a series of high-angle faults throughout the foreland (Fig. 1), all with at least some indication of strike-slip motion. Glahn’s (1997) map shows a segment of the Carrizo Springs fault trending toward the Glen Ray mine (Fig. 2). Although only mapped as far as the hills bordering the Texola mine to the east, extrapolating the fault along its trend projects it into the Glen Ray mine. Thus, it seems reasonable that one of the regionally mapped high-angle, oblique- and/or strike-slip faults was responsible for the formation of the F4 folds. The presence of oblique- and/or strike-slip faults, the nature of associated F4 folds, and relative timing indicate that this phase was dominated by transpressional deformation. Clear crosscutting relationships demonstrate that it postdates the earlier phases (F1–F3, S1–S2) and predates the Streeruwitz thrust and associated imbricate thrusts. This interpretation of transpression-dominated foreland deformation is also supported by the narrow width of foreland deformation.
Phase 3. Streeruwitz Thrust and Related Structures
The Streeruwitz thrust is the master structure emplacing high-grade metamorphic rocks of the CMG over low-grade metasedimentary rocks of the Allamoore Formation. The thrust is exposed in three dimensions in the Texola (Figs. 3 and 14) and Rosa Blanca mines, with an associated imbricate thrust exposed in the Glen Ray mine (Fig. 10) that places volcanic rocks over metasedimentary rocks.
The CMG in the hanging wall contains metarhyolite composed of fine-grained, well-foliated mylonite that is interleaved with amphibolite. A mylonitic foliation is defined by aligned elongate quartz, albite, and muscovite and by rotated porphyroclasts of albite and quartz. Matrix quartz was elongated by lattice reorientation, shows undulose extinction, and 120° grain boundaries. Muscovite shows undulose extinction and microscale kinks. Foliation wraps the porphyroclasts; albite porphyroclasts are rotated with pressure shadows of quartz parallel to foliation and shear sense is either sinistral or indeterminate. Quartz pressure shadows show continuous to discontinuous undulose extinction, elongation by lattice reorientation, and rotational recrystallization with minor grain boundary migration.
Unaltered CMG amphibolites have a coarse, anastomosing, spaced foliation defined by aligned elongate quartz and biotite altered to clinochlore, forming a groundmass that wraps albite and blue-green hornblende porphyroblasts. The porphyroblasts show a rough alignment parallel to the foliation. A crude metamorphic layering parallels the foliation and is defined by alternating felsic- and mafic-rich layers. Isoclinal millimeter-scale folds of the foliation were observed that augment the wavy cleavage. Opaque minerals, principally pyrite, are aligned with the foliation. Fe-oxide-rich pressure solution seams parallel the foliation and wrap porphyroblasts and postdate the foliation.
Open to isoclinal, centimeter-scale folds of the foliation were observed in both the metarhyolite and amphibolite. These folds have no axial planar cleavage and do not appear to be refolded. The only timing constraint is that they postdate the CMG mylonitic foliation. Large-scale folds of the CMG are observed and related to the folding of the Streeruwitz thrust. The timing of all CMG structures not related to or postdating the Streeruwitz thrust is unknown relative to those in the Allamoore Formation rocks in the footwall.
In the Texola mine, CMG foliation and compositional layering strike southeast and dip moderately southwest and are truncated by the thrust (Fig. 3). Localized zones of metasomatism, where CMG rocks were replaced by a mixture of albite, vein tourmaline, iron carbonate, and quartz, obscure the thrust and lithological contacts within the CMG (Fig. 15). Breccias composed of clasts of CMG mylonites and Allamoore limestones are observed along the thrust; they are cemented with ankerite, dolomite, albite, and quartz, with local veins of tourmaline. King and Flawn (1953) noted slickensides of tourmaline associated with the Streeruwitz thrust elsewhere in the foreland. Typically, the Streeruwitz thrust has CMG in thrust contact with talc, using talc as a décollement, although slivers of limestone and small imbricate thrusts are noted directly below the main thrust (Figs. 3 and 14).
In the Texola mine, the orientation of the thrust plane varies dramatically, indicating folding after thrusting. Poles to shear planes define two rough girdles indicative of folding on southwest- and northeast-plunging fold axes (Figs. 16A, 16B). Talc bodies directly below the thrust behave as a décollement, and some thrust-related strain is partitioned onto shear planes within the talc. The preexisting foliation, S2, is deflected into the shear planes and folded by shear-related F5 folds. The orientations of these shear planes vary because of the anastomosing morphology and later folding (Figs. 16D, 16E). In general, most dip moderately to steeply southwest or to a lesser extent northeast. Overall the apparent sense of shear appears to be to the northeast, on the basis of shear related folds, deflection of the foliation, and the regional orientations of the shear planes and the thrust. This direction of tectonic transport is essentially parallel to the strike of the truncated bedding and foliation.
Along the southeastern wall in the Texola mine, a meter-thick shear zone adjacent to the Streeruwitz thrust is composed of strongly foliated talc with inclusions of folded carbonate clasts (Fig. 14C). Along the upper portion of this zone, a thin (∼0.5 m) zone of mylonitized carbonate is in direct contact with altered CMG. The mylonitized carbonate is exclusively sheared iron carbonate concretions or elongate folded dolomite. This thrust zone cuts vertical beds of the S2 talc foliation and vertical F4 (rollover) folds (Figs. 11B and 14C). F3 folds were also truncated by the thrust in other areas of the mine.
In the Rosa Blanca mine the Streeruwitz thrust strikes east and dips ∼70° south along the southern portion of the mine. About ∼350 m north, in the Dees pit, the thrust strikes north and dips ∼60 ° west, indicating that the thrust sheet is complexly folded on a larger scale (Fig. 2). Instead of a discrete fault or zone, juxtaposed CMG and Allamoore Formation rocks are separated by a 3–5 m mixed zone of sheared, intensely metasomatized rock (Fig. 17) in which many various lithologic clasts were incorporated (southern boundary complex). CMG rhyolites and amphibolites are altered to a nearly indistinguishable nondescript pink rock (jasperoid; Fig. 15) with abundant albite forming the groundmass. Below the CMG rocks is an ∼3-m-thick zone of a hard, dark brown carbonate-rich rock. Also associated with this complex are off white to pale yellow sections of a quartz-ankerite(?) rock in which the protolith texture was destroyed by shearing and metasomatism. Within the plane of this complex shear zone is series of nested boudins; the largest boudin spans 2.5 m. Structurally below this zone and pinching out to the east is a sliver of chloritized olivine basalt. This entire Streeruwitz thrust package is riding on 25–30 m of a talc décollement.
Within the talc adjacent to the thrust are numerous shear planes, duplexes, and shear-related folds (Fig. 18). The shear planes generally strike northwest and are steeply dipping (Fig. 16E). Like those in the Texola mine, there is a range of orientations; however, in this mine the variation appears to be primarily related to the anastomosing nature of the shear planes rather than later folding. Folds within the talc with curved axes are common and record the north-northeast direction of thrusting.
In the Glen Ray mine, the westernmost wall is chloritized basalt and sheared talc of the Allamoore Formation thrust over previously deformed carbonate units of the Allamoore Formation. The thrust strikes southeast and dips moderately southwest, but is gently folded by a late southwest-plunging fold. The thrust is oriented at a high angle to the prominent east-striking, steeply dipping layers containing the F4 (rollover) folds on the northern wall of the mine, truncates these folds (Fig. 11A), and would also truncate the postulated dextral oblique-slip fault (Figs. 10, 11A, and 12). Thick chloritized basalt of the overriding plate is exposed on the far eastern edge of the mine as well, indicating that the imbricate thrust sheet extends across the mine and is folded like the Streeruwitz thrust. Associated with the thrust zone is intensely sheared talc with numerous anastomosing shear duplexes. In general, most shear duplexes dip moderately to the southwest or west; however, the poles to the planes define a rough girdle indicative of folding on a southwest-plunging fold axis (Fig. 16C). Folds within the talc have curved axes that record the northeastward direction of thrusting.
In all the mines, folds (F5 folds) are spatially related to thrusts or are in shear duplexes, and although they appear in other lithologies, they are best expressed within the talc décollement (Figs. 18A, 18B). F5 folds that fold the S2 foliation range from open to isoclinal with arcuate or chevron-shaped hinges (Fig. 8C). Some box-shaped folds are also observed away from thrust faults. Fold wavelengths range from millimeter-scale crenulations in talc to meter-scale folds (Fig. 18B). A poorly to moderately well defined crenulation cleavage (and kinking) in the talc appears to be related to these folds. In addition, the talc is extremely crenulated by multiple sets that are most abundant adjacent to shear duplexes within talc bodies; thus all crenulations of S2 are grouped with the F5 folds.
F5 fold axes directly below various thrust exposures are curvilinear and convex in the direction of thrusting, compatible with north-northeast–directed thrusting. F5 fold axes are almost exclusively in the southern half of all stereonets (Fig. 19), reflecting the generally southeast to southwest dip of the Streeruwitz and associated imbricate thrusts. Axial planes for all F5 folds in each mine strike roughly east and dip south (Fig. 19).
Phase 3. Interpretation
The Streeruwitz thrust is exposed in the Texola and Rosa Blanca mines, where it cuts previously formed structures at high angles in both the hanging walls (CMG) and footwalls (Allamoore Formation). Mylonite breccias observed along the fault indicate that the Streeruwitz exhumation history traversed from ductile to brittle conditions. The rocks adjacent to the Streeruwitz thrust were also intensely metasomatized by silica, alkali, and carbonate-rich fluids that acted as cementing agents for the breccias. The Streeruwitz thrust utilized talc as a décollement horizon, and some of the motion was partitioned into shear planes within the talc. Thrusting and development of shear duplexes in the talc and adjoining rocks formed curvilinear F5 folds.
Klippes of the Streeruwitz thrust north and northeast of the mines (Fig. 2) confirm that the Streeruwitz thrust was an out of sequence thrust that truncated all previous faults as it moved over the foreland. The imbricate thrust exposed in the Glen Ray mine, as well as those exposed in the Texola mine, also indicates that the foreland fold-thrust belt was the result of thin-skinned thrusting.
Kinematic indicators below the thrust sheets, such as deflected foliation and curvilinear fold axes, and the orientation of associated shears and the thrusts, indicate that motion on the Streeruwitz and related imbricate thrusts is toward the north-northeast to northeast. This orientation matches the structural grain of the foreland, and differs from the northeast structural grain of the Carrizo Mountains.
Earlier folds (F1–F4) are truncated by the Streeruwitz and associated imbricate thrusts, and are typically oriented at a high angle to these thrusts. In the Glen Ray and part of the Texola mines, the apparent thrust transport direction is nearly parallel (Texola) or at a low angle (Glen Ray) to the strike of the underlying beds and foliation. Thus, a change in the direction of shearing is required to explain these observations. If early F1–F3 folds resulted from noncoaxial shear, as proposed, then a change from apparent northward transport to north-northeast– to northeast-directed transport occurred between the formation of F1–F3 folds and the Streeruwitz thrusts. The proposed dextral oblique motion on approximately west-northwest–striking faults that produced the F4 folds occurred between these two phases.
Phase 4. Transpression: F6 folds
The final deformational phase to affect the foreland is evidenced by the refolding of all previous folds and folding of the thrust sheets, including the Streeruwitz thrust. At least two sets of nearly orthogonal folds are observed. These folds are not differentiated as separate phases because no conclusive timing relationships between them could be determined. They form type 1 fold interference patterns (Ramsey, 1967) or dome and basin structures. (Anomalous folds, termed FE folds, associated with eastward-directed thrust faulting, are described in the following.)
A three-dimensional view of the F6 folds of the Streeruwitz thrust can be seen in the Texola mine (Fig. 3). Two sets of folds oriented at high angles to each other are evident in the folded trace of the thrust throughout the mine. Figure 14A is a simplified map of the smaller east pit of the mine showing the complex folding of the thrust sheet. A syncline-anticline pair with a moderate plunge to the southwest forms an interference pattern with a south-southwest–plunging anticline (Figs. 14A–14C). The stereonets in Figure 16A show that one of the two sets of folds plunges either south-southeast or north-northwest, and the other set plunges east or southwest. Changes in plunge along the trend of the fold axes suggest curvilinear fold axes, consistent with these folds defining domes and basins.
The main northeast wall of the Texola mine does not expose the thrust, but the foliation and bedding surfaces form a well-expressed dome and basin morphology (Fig. 20A). This wall is directly below the Streeruwitz thrust and can be considered an example of the geometry of the thrust plane. Dome structures along this wall have a wavelength of ∼5 m. The axial planes and fold axes from these F6 folds have a “shotgun” pattern as would be expected from measurements of domed shaped structures (Fig. 20B). Many of the fold axes measurements are either part of one fold with a curvilinear fold axis curving over a domal structure, or were calculated from foliation measurements taken around domal structures. Calculated and measured fold axes from the Texola mine shows a preponderance of plunges to the northwest, southeast, and southwest. The large-scale dome and basin structures in the Texola mine created room problems where the already present talc décollement was pinched and squeezed into the core of one of the domes. F6 folds observed in the Texola can be traced northwest along strike into the Glen Ray mine, where they are also exposed.
F6 folds in the Glen Ray mine are broad to open, 5–10-m-wide folds, some of which have a dome-shaped morphology. They fold the foliation and bedding and have no axial planar cleavage. One of the best and largest examples of an F6 fold in the Glen Ray mine is along the north wall (Fig. 12); it plunges moderately to the southeast and F4 folds are found on both limbs. This dominant structure reorients all previous folds and foliations (i.e., all are along stereonet girdles that have π poles that plunge moderately to the southeast; Fig. 20C). Other, nearly perpendicular, large F6 folds are observed along and nearly parallel to the wall (inaccessible to measurement) and fold the generally northwest-trending imbricate thrust. The axial planes of F6 folds have a wide range of orientations but show a somewhat consistent northwest strike. Fold axes mostly plunge southeast or southwest (Fig. 20C).
The effects of these post-Streeruwitz folds can be seen in the Rosa Blanca and adjacent Dees mine. The southern wall of the Rosa Blanca mine is parallel to the roughly east-west trace of the Streeruwitz thrust. Mining company maps, based on drill hole data from the Rosa Blanca mine, show that the thrust trace bends to the north just west of the Rosa Blanca (Davis, 2007), indicating that the thrust sheet is folded. The Dees pit, which is ∼150 m north of the Rosa Blanca mine (Fig. 2), exposes a generally north-trending Streeruwitz thrust that has been folded by several northwest-plunging folds and some tight, nearly vertical folds with east-striking axial planes (Taylor and Mosher, 2011). Along the south wall of the Rosa Blanca mine, many boudinaged layers are observed with shallow east-southeast–plunging necks. These boudins could be related either to extension during thrusting or to extension localized on the limbs of the late folds.
An anomalous set of north-northeast–striking, steeply west dipping thrusts with associated brittle fault propagation folds, FE, are observed in the Glen Ray mine. Fold axes of these folds are along the adjacent fault planes, and the axial planes are similar to or somewhat steeper than related fault planes. The data are internally consistent, and these folds do not appear to be refolded. The relative timing of the FE folds has not been determined, other than to note that they are the result of a late event. These structures are classed with the domes and basins because of their more brittle nature and lack of refolding. As only one outcrop of the folds and faults was observed, they are interpreted as being related to a space problem associated with the F6 folding event.
Phase 4. Interpretation
F6 folds resulted in folding of the Streeruwitz-related thrusts and refolding of previous folds. The superposing of two sets of F6 folds resulted in type 1 fold interference patterns, dome and basin structures (Ramsey, 1967). The effect of the F6 dome and basin folding on the Streeruwitz thrust is the most obvious manifestation of this set of structures. However, the impact on previous fold generations is widespread, as shown by the wide variety of orientations of all previous fold axial planes and axes. Previous fold and foliation measurements and thrusts are along girdles with π poles that plunge either to the south or southeast and northwest or to the east and southwest, the orientations of F6 domal fold axes. F6 folds are interpreted as being the result of continual transcurrent motion on oblique- and/or strike-slip faults. A summary of events and their tectonic significance is presented in Table 1.
TIMING OF METASOMATISM AND METAMORPHISM
Several lines of evidence indicate that widespread metasomatism, with evolving fluid composition, affected foreland rocks during deformation (King and Flawn, 1953; Reid, 1974; this study), i.e., large monomineralic talc deposits, replacement of mafic dikes with silicates, partial to complete replacement of rocks adjacent to fluid pathways (faults), and the presence of veins and cemented breccias. Focused fluid flow along faults was also noted in the hinterland (Grimes, 1999, p. 208–210) and in the CMG in the hanging wall of the Streeruwitz thrust (this study). Fluid evolution starts with an early silica-rich phase, followed by an alkali-rich phase, and then a flux of Fe, Mg carbonate, and silica-rich fluids. Mineral assemblages associated with S1, however, suggest an earlier isochemical greenschist facies metamorphism.
Evidence for an earlier isochemical greenschist facies metamorphism derives from volcanic rocks and dolostones within the Allamoore Formation. The mafic volcanic suite typically has a mineral assemblage of biotite + chlorite + albite ± epidote ± tourmaline, indicating greenschist facies metamorphism. Edwards (1985) also noted the rare presence of epidote, clinozoisite, and zoisite in Allamoore volcanic rocks further from the thrust, indicating low greenschist facies metamorphism.
The dolostones have a mineral assemblage of talc + tremolite + quartz + dolomite ± calcite, indicating greenschist facies metamorphism, and tremolite is locally observed in limestones. In dolostones, the S1 foliation is defined primarily by tremolite and dynamically recrystallized quartz, with dolomite and calcite as accessory phases. Mimetic replacement by talc is observed along the margins of tremolite grains (Fig. 4C). The mineral assemblage defining S1 is a possible product of isochemical metamorphism, assuming that the siliceous carbonate protolith had the appropriate bulk chemistry. At pressures of 2 kbar, tremolite forms at 450 °C and is stable up to ∼650 °C and over a range of XCO2 values (Winkler, 1979). Talc replacing tremolite occurs as retrograde reactions as the temperature drops below 500 °C at values higher than ∼0.2 XCO2 (e.g., talc is stable at lower temperatures, see Winkler, 1979). Below that XCO2 level, fluid composition becomes the main variable between the two stability fields, with more water-rich fluid stabilizing tremolite and more CO2-rich fluid favoring talc.
Thus, the greenschist mineral assemblages displayed in dolostones and mafic volcanic rocks may be the result of isochemical metamorphism. Identification of tremolite in altered dolostone, limestone, and talc-bearing rocks indicates that metamorphic conditions in the Allamoore Formation were a higher grade than previously thought (e.g., Edwards, 1985).
Siliceous Fluid Phase
The most dramatic effect of silica-rich fluid influx was the formation of economic talc bodies. Throughout the region, talc deposits form lenticular bodies that pinch and swell from a few meters to as much as 60 m thick (Chidester et al., 1964). The main foliation of the talc throughout the study area is S2. Rarely preserved S1 foliation is defined by recrystallized tremolite and/or talc in F1 fold hinges. Talc defining S2 is observed replacing the S1 tremolite, although tremolite is also observed parallel to S2 (Fig. 7). Both tremolite and talc were stable during the formation of both foliations, but talc clearly postdates tremolite, and the main phase of talc mineralization occurred during the formation of S2. The prevalence of talc over tremolite could have been in response to more CO2-rich fluids and/or a decline in temperature. A major influx of silica-rich fluids occurred during the formation of S2, and most likely started during S1 formation.
Timing for silica-bearing fluid influx is supported by the metasomatism observed affecting an F2 folded mafic dike along the east wall of the Rosa Blanca mine (Fig. 5). S1 in an altered part of a dike is almost completely replaced by recrystallized microcrystalline quartz, indicating post-S1 metasomatism. S2 in the altered dike (axial planar to the F2 folding the dike) is defined by deformed quartz; thus the metasomatism of this dike was pre-S2 and post-S1.
Deformed limestone away from the Streeruwitz thrust contains as much as 60% secondary quartz. Thus, silica-bearing fluids were widespread along pre-Streeruwitz faults and affected rocks distal to the Streeruwitz thrust. Early quartz veins are folded, vein quartz has been dynamically recrystallized, and the early veins are cut by veins of albite and less deformed quartz, some of which are post F3 (Fig. 8B).
The purity of talc is observed to increase with proximity to the Streeruwitz thrust. The pre-Streeruwitz complex folding of talc and the truncation of these structures by the Streeruwitz thrust preclude this being an original compositional effect; therefore, it must be fluid controlled. The change from talc-rich phyllites to ceramic-grade talc involves a change from 80% tremolite to 60% talc, along with a decrease in carbonate. The paint-grade talc found along the thrust is 90%–100% talc. During fluid channelization along the fault, an increase in CO2 content in water-rich fluids could cause tremolite to retrograde to talc.
In summary, silica-rich fluid was dominant during the earliest ductile, locally high-temperature deformational phase, occurring syn-F1 to pre-F3, and predominantly syn-F2. These conditions indicate greenschist facies metamorphism during talc mineralization and silica-dominated metasomatism. The increase in purity (paint-grade talc) immediately adjacent to the Streeruwitz thrust suggests additional synthrusting to postthrusting fluid flow.
An alkali-rich fluid phase metasomatically altered rocks along the trace of the Streeruwitz and related thrusts in the foreland. Mineralization includes disseminated albite, veins of albite, tourmaline, and sodic-amphiboles richterite and magnesioriebeckite. Albitization adjacent to the thrust appears to have had a protracted history. F2 folds of veins with albite as a minor constituent indicates alkali fluids early in the deformational history; however, these are not as volumetrically significant as they are post-F2. Albite veins cut the S2 foliation, are observed offsetting an F3 fold (Fig. 8B), and crosscut early quartz veins, suggesting that the main alkali-fluid influx was at least post-F3.
The presence of albite growth or replacement adjacent to the Streeruwitz thrust and its general absence further from the thrust indicate that albite-rich fluids were focused along the Streeruwitz thrust. The presence of albite as an early cementing agent for associated breccias also implies that alkali-rich fluids were present during late thrusting. Tapering deformational twins, some bent, kink bands, and a minor degree of grain boundary migration (possibly enhanced by fluids) within the albite indicate subsequent low- to medium-grade deformation conditions (300–500 °C) for some of the albite. Thus, although some albitization occurred early, it is mainly a post-F3 to synthrusting (and F5) event.
Further evidence of albitization and silica-rich fluids adjacent to the Streeruwitz thrust is found in the highly metasomatized metarhyolite and amphibolite units of the CMG exposed in the Texola and Rosa Blanca mines. These units are the least deformed and lowest metamorphic grade of the CMG. In altered metarhyolites, albite that postdates the foliation is commonly confined to veins, but also may exist as disseminated grains distinct from the earlier porphyroblasts wrapped by the foliation. CMG amphibolite is locally metasomatized to a pink fine-grained rock that is difficult to distinguish from the metarhyolite (Fig. 15). The altered amphibolite is now composed of quartz, albite, iron oxides, and carbonate, and biotite is no longer present.
Intense albite metasomatism associated with Streeruwitz thrusting altered both CMG and Allamoore Formation rocks to a nondescript, pink fine-grained siliceous jasperoid, obscuring the normally sharp contact in the Rosa Blanca mine. Altered units have a groundmass of disseminated albite and quartz, and an absence of mafic minerals in the amphibolite units. Biotite inclusions in metasomatic albite grains define the relict foliation. Metasomatic alteration decreases markedly away from the thrust.
Tourmaline is in juxtaposed CMG and Allamoore Formation rocks, suggesting that the two formations were contiguous during the alkali metasomatic phase. In the footwall, tourmaline commonly parallels the foliation but is usually randomly oriented and occurs in veins that cut the foliation. Tourmaline is interspersed with disseminated albite, and albite veins cut the tourmaline-bearing veins, implying that tourmaline may have precipitated early in the alkali fluid phase. The best constraint on the tourmaline mineralization is post-S2 and F2 and coeval with the albitization phase.
King and Flawn (1953) proposed that Na-rich fluids introduced albite into rocks adjacent to the Streeruwitz thrust coeval with fluids responsible for the tourmaline found throughout the region in both the hanging walls and footwalls of the thrust. Tourmaline has also been reported to form slickensides along faults within the Hazel Formation (King and Flawn, 1953; Reid, 1974). Note, however, that the CMG jasperoid amphibolite (affected by albite- and silica-rich fluids) is now juxtaposed with chloritized Allamoore basalt lacking albitization and siliceous alteration. This comparative relationship shows a difference in the timing and spatial effects between these evolving fluids, indicating continued movement on the Streeruwitz thrust postdating local albite-rich fluid influx and juxtaposition with rocks not affected by those fluids.
Near end-member magnesioriebeckite (Davis, 2007) replaces albite and quartz in a siderite–iron oxide–dolomite carbonate. Magnesioriebeckite occurs as spherical clusters or as laths (Fig. 21) folded by F6 folds. Spherical clusters indicate that the minerals grew in an environment free from differential stresses, and the amphiboles overgrow albite. These relationships suggest that magnesioriebeckite in a host rock of dolomite and/or albite carbonate grew after the presence of albite-rich fluids, or late in the alkali phase.
In summary, an alkali-rich fluid phase that was focused along the Streeruwitz thrust produced a protracted history of metasomatism. Although present during F2, the alkali fluids were most abundant post-F3 and pre-F6. Tourmaline seems to have crystallized just before or synchronously with albite, and magnesioriebeckite crystallized afterward. Carbonate- and quartz-bearing veins cut albite veins, however, and cement breccias consisting of fragments with disseminated albite.
The final influx of fluids during deformation was dominated by iron-magnesium carbonate- and silica-rich fluids. These fluids mineralized the rocks in veins and cemented breccias that were localized along thrusts, mainly the Streeruwitz thrust. Carbonate and quartz veins crosscut all other veins and foliations, implying that these fluids postdate F2 and most of the albitization. Furthermore, because these fluids acted as cementing agents for breccias localized along thrust planes (Fig. 22), they are at least synchronous with if not younger than the Streeruwitz thrust. F6 folding of tectonic slivers of cemented Allamoore breccia along the thrust implies that they were at least partially cemented at that time. All the carbonate and quartz grains associated with this fluid flow phase are somewhat deformed, indicating that they predate the cessation of motion along the Streeruwitz thrust or were deformed during F6. The deformational features of calcite, quartz, and the iron-magnesium carbonates demonstrate that they were deformed under low-temperature conditions.
Several episodes of carbonate cementation of breccias were roughly coeval with each other. The earliest appears to have been dolomite cementation (Fig. 22B), which was closely followed by siderite and quartz cementation. Deformational features and mineralogical associations indicate that alkali-rich fluids were still present at the time of brecciation, but predate the cessation of movement on the Streeruwitz thrust. By the time the thrust sheet was folded, the voluminous fluids percolating through the foreland had ceased.
Deformational features of quartz that formed from the early silica-bearing fluids and the presence of brecciated mylonites along the thrust indicate that the rocks adjacent to the Streeruwitz thrust record a ductile to brittle history.
The Streeruwitz thrust is an out of sequence thrust that emplaces the CMG metamorphic rocks on greenschist facies polydeformed Allamoore Formation rocks, truncating all previous structures. The Streeruwitz thrust also truncates folded syndepositional thrusts that involve the unmetamorphosed Hazel Formation, therefore postdating thrusting at the surface. We interpret thrusting that resulted in Hazel Formation deposition as an early surface manifestation of shearing at depth farther south that resulted in the ductile deformation in the CMG and Allamoore Formation and ultimately led to emplacement of the CMG along the Streeruwitz thrust. CMG rocks underwent ductile deformation and mylonitization from 1057 to 1035 Ma and were exhumed along the Streeruwitz thrust between 1000 and 980 Ma (Grimes and Copeland, 2004). Mylonites near the thrust yielded the youngest deformational (1035 Ma; Bickford et al., 2000) and cooling ages (980 Ma; Grimes and Copeland, 2004). Note that Elston and Clough (1993), using paleomagnetic data from the Hazel Formation, suggested that Hazel Formation deposition was between ca. 1100 and 1080 Ma; however, this timing would predate the deformation in the CMG and be unrelated to emplacement of the Streeruwitz thrust and associated foreland thrust belt, and therefore is unlikely (see Mosher, 1998).
Structural and Fluid Flow Evolution
Structural analysis of new exposures in talc mines along the trace of the Streeruwitz thrust indicates that the Grenville-age foreland underwent polyphase deformation in four distinct phases. Deformation was accompanied by fluids with an evolving chemistry that were focused along foreland faults, including the major Streeruwitz thrust, and resulted in three phases of syntectonic metasomatism. Early polyphase folding (F1–F3, S1–S2) occurred at greenschist facies and was accompanied by early silica-rich fluids syn-F1 and pre-F3, resulting in formation of the economic talc bodies. Subsequently, oblique- and/or strike-slip faults and associated shear zones deformed the earlier structures producing vertical rollover folds. All these later structures were truncated at a high angle by the Streeruwitz and imbricate thrusts. Alkali-rich fluids were locally present pre-F2, but predominantly caused metasomatism early in the evolution of the Streeruwitz and imbricate thrusts. Later during thrusting, the fluids became iron magnesium carbonate and silica rich. Lastly, the region was folded into large-scale domes and basins. The model for structural and fluid flow evolution is discussed in detail in the following.
Initial deformation at shallow crustal levels in the foreland (Fig. 23A) began with thrusting in the Allamoore and Tumbledown Formations, causing surface exposure. Thrust tips were eroded into an alluvial fan complex that became the Hazel Formation (Reynolds, 1985, 1988; Soegaard and Callahan, 1994). Soegaard and Callahan (1994) interpreted the Hazel Formation as being deposited in a transpressional basin on the basis of temporal variation in clast composition and juxtaposition of source terrane and sedimentary basin coupled with aggradation of megasequences. This deformation could be related to initial deformation at depth (ductile shearing and polyphase folding, phase 1 of this study; mylonitization of CMG rocks and beginning of exhumation) with north to northwest transport, compatible with oblique convergence (Fig. 23A). Observed transport in the hinterland (CMG) was to the northwest within a dextral zone of transpression (Grimes, 1999; Grimes and Mosher, 2003). The early structures (F1–F3 of this study) are compatible with northward transport and dextral transpression. The Hazel Formation has no clasts of CMG; therefore, the CMG must not have been exposed or in the near vicinity at this time, but it probably was in the processes of being exhumed along oblique convergent shear zones.
Silica-bearing fluids of unknown origin metasomatically altered rocks adjacent to developing thrusts throughout the region. These fluids provided the necessary silica and mobilized other ions for the formation of talc in the Allamoore Formation. The juxtaposition of silicified (and albitized) mafic CMG rocks against Allamoore talc indicates that these fluids were widespread (i.e., affected rocks later juxtaposed). This first deformational phase was dominated by ductile deformation, peak metamorphism that reached greenschist facies, and an influx of silica-rich fluids.
As convergence and transpression continued, high-angle oblique- and/or strike-slip faults formed and were reactivated in the foreland (Figs. 23A, 23B). This second phase of deformation was dominated by steeply plunging folds along high-angle west-northwest–trending dextral oblique- and strike-slip faults under brittle and/or ductile conditions, and marks a shift from thrust-related noncoaxial shear in phase 1 to transcurrent motion in phase 2. This deformation is compatible with the dextral transpression observed in the CMG (e.g., Grimes and Mosher, 2003) and the proposed deposition of the Hazel Formation in transpressional basins (Soegaard and Callahan, 1994; Glahn, 1997).
As shortening progressed, thin-skinned imbricate thrusts associated with the thrust zone developed and cut upsection through the foreland (Fig. 23B). Allamoore and Hazel thrusts eventually ceased without the CMG being exposed, and the thrust package was folded into conical folds, inverting much of the foreland (Fig. 23C) (King and Flawn, 1953; Reynolds, 1985, 1988). Continued exhumation along the Streeruwitz thrust moved rocks through a ductile-brittle transition.
Fluids focused along the developing Streeruwitz thrust evolved to an alkali-rich fluid phase (Fig. 23B). Albite was emplaced as veins and disseminated grains in the groundmass of rocks adjacent to the Streeruwitz thrust. The CMG adjacent to the Streeruwitz thrust was extensively altered, as were rocks in the associated shear zone. Tourmaline was precipitated along shear zones and in faults further north of the Streeruwitz. King and Flawn (1953) noted tourmaline slickensides in faults found in the Hazel Formation and thought that tourmaline and albitization were coeval. Sodic amphiboles, including magnesioriebeckite, also were precipitated in veins and as spherical clusters in Allamoore rocks late during the alkalic metasomatism.
As the out of sequence Streeruwitz thrust was emplaced over the foreland, it truncated all early structures within the Allamoore Formation and the CMG (Fig. 23C). Folds related to the Streeruwitz thrust (F5, this study), other kinematic indicators, and mapping by Reynolds (1988) all show that motion on the Streeruwitz thrust had a north-northeast– to northeast-directed transport. F1–F4 structures were truncated at a high-angle by the Streeruwitz and associated imbricate thrusts, indicating a change in the overall tectonic transport direction. Motion on the Streeruwitz thrust was at a high angle to the northwest-directed transport in the hinterland (Grimes and Mosher, 2003). Elston and Clough (1993) noted a 30° clockwise rotation after deposition of the Hazel Formation, based on paleomagnetic data. In Bristol and Mosher (1989), a progressive clockwise rotation of fold phases in the northwest Van Horn Mountains was noted. Both Glahn (1997) and Leaf (1999) noted a change in direction of principal stress in foreland rocks. Thus several lines of evidence indicate that there was a clockwise rotation of shortening directions that happened late during the emplacement of the Streeruwitz thrust.
Breccias along the Streeruwitz thrust are cemented dominantly by iron-rich carbonates and quartz, with some albite. Breccia clasts indicate a ductile to brittle transitional history. The minerals cementing the breccias show features characteristic of brittle deformation, indicating continued deformation. The fluids that formed the cements may have been a mixture of meteoric water with connate alkali fluids already present within the rocks.
The final phase of deformation in the foreland folded the Streeruwitz thrust sheet and adjacent rocks into complex domes and basins (Fig. 23D). We propose that reactivation and movement along preexisting strike-slip faults in the foreland caused the formation of domes and basins after thrusting ceased, through transpressive inversion, as proposed by Allen et al. (2001) for southern Kazakstan. Glahn (1997, p. 76) noted two sets of transcurrent faults (northeast-striking faults with general sinistral motion, and northwest-striking faults with general dextral motion) both sets postdating “major thrust sheet emplacement and subsequent folding.” In addition, Kwon (1990) and Soegaard et al. (1993) proposed that many of the high-angle transcurrent faults have been reactivated numerous times and some show both dextral and sinistral motion. Cooling of the CMG through 300 °C is determined to have taken place between 1000 and 980 Ma (Grimes and Copeland, 2004) as a result of exhumation along the Streeruwitz thrust. Thus, transpression must have occurred after 980 Ma.
Evolution of the Southern Margin of Laurentia
Our results show that previously proposed models (e.g., Mosher, 1998; Bickford et al., 2000) must be reconsidered. In the Llano uplift of central Texas, the main collision-related deformation occurred between ca. 1150 and 1120 Ma, and syntectonic to posttectonic plutonism continued until ca. 1070 Ma (Mosher, 1998, and references therein; Mosher et al., 2008b). Deformation in the west Texas Van Horn exposures occurred from ca. 1060 to 980 Ma, thus postdating deformation in the Llano uplift by at least 60 m.y. (Grimes and Copeland, 2004). In the Llano uplift, kinematic and structural observations are consistent with a model that associates deformation and metamorphism with a southern-colliding continent (Mosher, 1998; Mosher et al., 2008b), and are not consistent with large zone of dextral transcurrent motion (Bickford et al., 2000; see Reese and Mosher, 2004). The complex structural evolution with changing directions of tectonic transport, documented in this study for the Van Horn exposures, is also inconsistent with dextral transcurrent motion as proposed by Bickford et al. (2000).
Any tectonic model of this region must account for the difference in timing of deformation between the central Texas Llano uplift (1150–1115 Ma) and west Texas Van Horn exposures (1050–980 Ma) and different tectonic transport directions between the two areas. Such models must explain the temporal variation in tectonic transport from northwest-, to north-northeast–, to northeast-directed motion, followed by continued transpression, as seen in the Van Horn exposures. The simplest model consistent with these constraints is that a different continental block or island arc collided in what is now the west Texas area ∼60 m.y. after a southern continent collided in the Llano uplift area. It is difficult, however, to explain the observed changes in tectonic transport observed in the Van Horn exposures with collision of a single, separate block in that area. Thus we propose a modification to the indenter model as outlined in the following.
The Grenville-age orogenic events as recorded in the Llano uplift of central Texas are well explained by collision of a southern continental block and an exotic arc terrane with the Laurentian continent between 1150 and 1115 Ma (Fig. 24; see Mosher et al., 2008b). We propose that subduction along the southern margin of Laurentia, that resulted in collision of a southern continent in what is now central Texas, continued along the rest of the plate boundary at least as far as present-day west Texas. The southern continental block underwent clockwise rotation and eventual collision along the rest of its margin as this narrow basin closed in a “zipper-like” fashion (Fig. 24A). This rotation may have been enhanced by the slab breakoff and intrusion of late syntectonic to posttectonic granites (1119–1070 Ma) in the Llano area (proposed in Mosher et al., 2008b). The rotation of this large-scale continental block resulted in the change in tectonic transport directions seen in the Van Horn exposures. During the initial closure, northwest transport and dextral transpression occurred in the CMG (ca. 1060–1035 Ma). As the intervening basin between the irregular margins of the two colliding continents closed, tectonic transport changed to north-northeast and then northeast directed (ca. 1035–980 Ma), forming the structures in the foreland. The interaction between these two plates as they collided reactivated preexisting faults and caused transpression and formation of domes and basins in the foreland after 980 Ma. An additional possibility is that an island arc (or small continental block) was present between the two colliding continents and collided, first causing the northwest transport. Then final collision between the two continents resulted in north-northeast to northeast transport. Such an island arc was proposed in the west Texas area on the basis of the small exposures in Chihuahua, Mexico (Mosher, 1998). Interaction between a small intermediate block and the two larger continents would also help explain the later reactivation of preexisting faults with multiple senses of shear and formation of the domes and basins.
Reconstructions of Rodinia have changed over the past few decades (summarized in the review by Evans, 2013), and still rely heavily on paleomagnetic data. Integration of geochronologic, structural, and metamorphic work in Grenville-age orogenic belts is essential in developing valid reconstructions. Future Rodinia reconstructions must satisfy the timing and kinematic constraints identified in this study for Van Horn exposures in the west Texas area, as well as for the Llano uplift in the central Texas area. In the Llano uplift, metamorphism (and associated deformation) did not occur until ca. 1150–1120 Ma. The earliest medium-T eclogites are indicative of subduction of continental crust. Subsequent regional amphibolite facies metamorphism and deformation with northeast tectonic transport were followed by intrusion of syntectonic to posttectonic plutons (1119–1070 Ma) with a juvenile signature (Mosher et al., 2008b). In west Texas, metamorphism and deformation did not begin until ca. 1060 Ma and were initially expressed as dextral transpression and northwest-directed tectonic transport. After ca. 1035 Ma, transport directions changed to north-northeast, then northeast in the foreland, with final uplift and exhumation occurring until ca. 980 Ma. Successful Rodinia reconstructions must satisfy constraints on the timing of deformation and direction of tectonic transport documented along the southern margin of Laurentia.
The Grenville foreland exposed in west Texas underwent transpressional deformation that can be separated into four distinct phases; fluids with an evolving chemistry were focused along faults and/or shear zones throughout the deformation. Early polyphase ductile folding and foliation development were concurrent with greenschist facies metamorphism and a major influx of silica-rich fluids that resulted in extensive metasomatism and the formation of economic talc bodies. Observed structures are most compatible with northward tectonic transport. These structures were deformed by west- to west-northwest–striking, dextral transcurrent shear (fault) zones forming vertical sheath-like folds, and were cut at high angle by the out of sequence Streeruwitz and associated imbricate thrusts with north-northeast to northeast transport directions. Early in the thrust history, deformation was more ductile and alkali-rich fluids caused further metasomatism. Later brittle deformation along the Streeruwitz thrust was associated with an influx of Fe- and Mg-rich carbonate and silica-rich fluids. During the last deformation phase, the thrusts and all previous structures were deformed by complex dome and basin formation, most likely as a result of movement on preexisting transcurrent faults. The kinematic and structural analysis of the foreland, in comparison with the hinterland (CMG) and the Llano uplift of central Texas, indicates that Grenville orogenesis along the southern margin of Laurentia involved arc-continent and continent-continent collision with a southern indenter that subsequently underwent clockwise rotation, causing orogenesis in west Texas ∼60 m.y. later.
We thank Steve Whitmeyer, Tim Wawrzyniec, and Raymond Russo for their insightful reviews of this paper, and Mark Helper and Rich Kyle for their help with previous versions. We also thank Zemex Industrial Minerals for granting access to the talc mines and Steve Cox and Jonathon Gant for help in the field. This project was made possible by funding from a Geological Society of America Penrose grant.