ABSTRACT Paleocene Lower Wilcox Group sedimentation rates are three times the Cenozoic average for the Gulf of Mexico region and are attributed to Laramide tectonism within the Laramide–Rocky Mountains region. These increased rates likely represent the erosion of easily weathered Phanerozoic strata that blanketed the Laramide-age basement-cored uplifts. Geologic observations and U-Pb geochronology are not sufficient to fully address this hypothesis alone, so we conducted 439 Lu-Hf isotopic analyses on detrital zircons from eight samples from the San Juan Basin and five samples from the Gulf of Mexico Basin. Focusing on the zircons younger than 300 Ma allowed us to make direct comparisons to the eight principal components that comprise the North American Cordilleran magmatic arc: (1) Coast Mountains batholith; (2) North Cascades Range; (3) Idaho batholith; (4) Sierra Nevada batholith; (5) Laramide porphyry copper province; (6) Transverse Ranges; (7) Peninsular Ranges; and (8) Sierra Madre Occidental. The εHf ( t ) results range from +8.9 to –27.0 for the San Juan Basin samples and from +13.0 to –26.6 for the Gulf of Mexico samples. Using the San Juan Basin samples as a proxy for the eroded Mesozoic cover that was shed from the Laramide uplifts, we show that much of the sediment entering the Gulf of Mexico through the Houston and Mississippi embayments during the late Paleocene was derived from reworked cover from the greater Laramide–Rocky Mountains region. However, the Gulf of Mexico samples also include a distinct juvenile suite (εHf [ t ] ranging from +13 to +5) of zircons ranging in age from ca. 220 to 55 Ma that we traced to the Coast Mountains batholith in British Columbia, Canada. This transcontinental connection indicates an extension to the headwaters of the previously defined paleo-Mississippi drainage basin from ca. 58 to 56 Ma. Therefore, we propose a through-going fluvial system (referred to here as the “Coast Mountains River”) that was routed from the Coast Mountains batholith to the Gulf of Mexico. This expands the previously defined paleo-Mississippi drainage basin area by an estimated 280,000 km 2 . Our comprehensive Hf isotopic compilation of the North American Cordilleran magmatic arc also provides a benchmark εHf ( t ) versus U-Pb age plot, which can be used to determine provenance of detrital zircons (85–50 Ma) at the scale of specific region(s) within the Cordillera based on their εHf ( t ) values.

ABSTRACT We describe the time-space evolution of a segment of the Laramide arc in east-central Arizona that is associated with porphyry copper mineralization, as constrained by U-Pb zircon geochronology conducted by laser ablation–multicollector–inductively coupled plasma–mass spectrometry. Mid-Cenozoic normal faulting dismembered and tilted many of the plutons and the associated porphyry copper deposits and produced a wide range in depths of exposure. The study area reconstructs to a 75-km-long slice along the arc, with exposures from <1 to >10 km depth. The copper deposits are related to granodioritic to granitic plutons that exhibit variable magmatic sources and locally severe degrees of zircon inheritance. U-Pb zircon ages of plutons in the study area range from 75 to 61 Ma, with dioritic rocks at the older end of the range. The age range of magmatism and mineralization in a cluster of deposits near the Schultze Granite, including the Globe-Miami, Pinto Valley, and Resolution deposits, is from ca. 69–61 Ma. To the south in the Tortilla and Dripping Spring Mountains, the porphyry systems range from ca. 74 Ma at Kelvin-Riverside to ca. 69 Ma at Ray and ca. 65 Ma at Christmas. At several localities where geologic constraints exist, mineralizing plutons were emplaced following Laramide shortening. The ages of the inherited zircon cores correspond fairly closely to the ages of basement rocks in the immediate vicinity of sample sites, implying that similar basement ages and lithologies contributed to the source areas of magmas that produced Laramide porphyry deposits. The U-Pb results on hypabyssal rocks are typically 1–5 m.y. older than previous K-Ar ages, and U-Pb ages on more deeply emplaced plutonic rocks are as much as 5–10 m.y. older. These results are consistent with predictions from thermal modeling and suggest that temporal evolution of the entire Laramide arc needs revision. For this segment of the arc, magmatism was stagnant for ~15 m.y., with minimal migration over time and mineralization occurring episodically over most of that lifespan. There is no simple geographic progression in ages along or across the strike of the arc. Thus, it is difficult to call upon time-specific far-field or plate margin triggers for magmatism or mineralization. The intrusive flux of the Laramide arc appears to be similar to that of the Sierra Nevada arc during the Mesozoic during its “background” periods, rather than during episodes of flare-up. The wide compositional diversity of the Laramide arc is more akin to northeastern Nevada during the onset of extension in the mid-Cenozoic than to the Mesozoic of the Sierra Nevada.

ABSTRACT The Santa Catalina and Rincon Mountains north and east of Tucson, Arizona, form one of the largest core complexes on Earth. Both ranges consist primarily of Eocene leucogranites that intrude Proterozoic and late Cretaceous granitoids, and two Oligocene plutons. Mylonitic fabrics are well developed on the southern flank of the Santa Catalina Mountains and the southwestern flank of the Rincon Mountains. The corrugated form of the two ranges reflects the grooved form of the ca. 15–30 Ma Catalina–San Pedro detachment fault exposed primarily at the foot of the ranges. Normal displacement on two younger high-angle normal faults is responsible for much of the substantial relief of the ranges. This field guide is focused on fault rocks and mylonitic fabrics in the footwalls of the detachment fault and the high-angle Pirate normal fault, and includes description and analysis of shear-zone kinematics and processes, U-Pb geochronology of leucogranites, and core-complex geomorphology.

The crystalline basement of the Central Andes between 21°S and 26°S consists of a variety of Neoproterozoic–Paleozoic arc-type and basinal assemblages. We characterize these assemblages through analysis of U-Th-Pb ages of zircons sampled from 16 different plutonic suites and from 21 different sandstones in northern Argentina and Chile. The ages of igneous zircons show that magmatism occurred in three main phases: ca. 550 Ma (late Neoproterozoic); 490–464 Ma (Late Cambrian to Middle Ordovician); and 318–264 Ma (late Carboniferous–Late Permian). Detrital zircon ages are mainly 600–450 Ma from Paleozoic strata all across the orogen, reflecting derivation primarily from local Neoproterozoic, Cambrian, and Ordovician magmatic constructs. These relations suggest that crystalline basement in this portion of the Andes was assembled within a broad extensional convergent margin system during early Paleozoic time. Because similar arc and basinal assemblages characterize much of the Terra Australis orogen, it is not possible to constrain the degree of tectonic mobility within this convergent system.

The Lhasa and Qiangtang terranes of Tibet collided following Late Jurassic–Early Cretaceous consumption of oceanic lithosphere along the intervening Bangong suture zone. This continental collision led to the development of the south-directed, northern Lhasa thrust belt that is exposed ~1200 km along strike in central Tibet. We conducted geologic mapping and stratigraphic and geothermochronologic studies in the Duba region of the northern Lhasa terrane, located ~250 km northwest of the city of Lhasa. In the Duba region, granites were emplaced into the mid-crust between 139 and 121 Ma and subsequently exhumed and juxtaposed against Cretaceous strata between 105 and 90 Ma in the footwall of an interpreted passive roof thrust system. We suggest that this structural style dominates the Cretaceous–early Cenozoic evolution of the northern Lhasa thrust belt and provides an explanation for the scarcity of basement rock exposures in the Lhasa terrane despite >50% upper crustal shortening. Furthermore, we highlight similarities between the collision-related northern Lhasa and Tethyan Himalayan thrust belts, both of which are bound by sutures and associated with underthrusting of lower plate lithosphere.

The Coast orogen of western coastal British Columbia and southeastern Alaska is one of the largest batholithic belts in the world. This paper addresses the structure and composition of the crust in the central part of this orogen, as well as the history of its development since the mid-Cretaceous. The core of the orogen consists of two belts of metamorphic and plutonic rocks: the western metamorphic and thick-skinned thrust belt comprising 105–90-Ma plutons and their metamorphic country rocks, and the Coast Plutonic Complex on the east, with large volumes of mainly Paleogene magmatic rocks and their high-temperature gneissic host rocks. These two belts are separated by the Coast shear zone, which forms the western boundary of a Paleogene magmatic arc. This shear zone is subvertical, up to 5 km wide, and has been seismically imaged to extend to and offset the Moho. Lithologic units west of the Coast shear zone record contractional deformation and crustal thickening by thrusting and magma emplacement in the mid-Cretaceous. To the east, the Coast Plutonic Complex records regional contraction that evolves to regional extension and coeval uplift and exhumation after ca. 65 Ma. Igneous activity in the Complex formed a Paleogene batholith and gave rise to high crustal temperatures, abundant migmatite and, as a result, considerable strain localization during deformation. In both belts, during each stage of the orogeny, crustal-scale deformation enabled and assisted magma transport and emplacement. In turn, the presence of magma, as well as its thermal effects in the crust, facilitated the deformation. After 50 Ma, the style of crustal evolution changed to one dominated by periods of extension oriented approximately perpendicular to the orogen. The extension resulted in tilting of large and small crustal blocks as well as intra-plate type magmatic activity across the orogen. Seismic-reflection and refraction studies show that the crust of this orogen is unusually thin, probably due to the periods of orogen-perpendicular stretching. Magmatic activity west of the Coast shear zone in the Late Oligocene and Miocene was related to one period of orogen-parallel transtension along the margin. Small-scale, mafic, mantle-derived volcanic activity continues in the region today. The change from convergence to translation and extension is related to a major plate reorganization in the Pacific that led to a change from subduction of an oceanic plate to northwestward translation of the Pacific plate along the northwest coast of North America. Although it has been proposed that this orogen is the site of major (up to 4000 km) pre-Eocene northward terrane translation, there is little evidence for such large-scale displacement or for the kind of discontinuity in the geological record that such displacement would entail.

The tectonic significance of early Paleozoic convergent-margin rocks of the Alexander and Sierran-Klamath terranes is poorly understood. New phengite 40 Ar/ 39 Ar and Rb-Sr results from the schist of Skookum Gulch of the Yreka subterrane in the Klamath Mountains (454 ± 10 Ma) confirm that blueschists are the oldest known subduction-zone rocks of the western North American Cordillera. The blueschists are juxtaposed with kilometer-scale tectonic blocks of ca. 565 Ma tonalite. Detrital zircons from the blueschists require close proximity to a diverse source of cratonal or derivative supracrustal rocks and preclude formation within an isolated intra-oceanic setting. This strong cratonal provenance (mostly 1.0–2.0 Ga, with resolved concentrations of 1.49–1.61 Ga zircon) is also exhibited by adjacent Early Devonian lower greenschist units of the Yreka subterrane (Duzel phyllite and Moffett Creek Formation). Additional results from temporally equivalent arc-derived sedimentary units (Sissel Gulch graywacke and Gazelle Formation) yield strongly unimodal zircon age distributions of early Paleozoic zircon. The results indicate that the Yreka subterrane formed at an Ordovician–Silurian–Early Devonian convergent margin near a Mesoproterozoic-Paleoproterozoic craton and Ediacaran crust. Appreciable 1.49–1.61 Ga zircon within the Yreka subterrane is compatible with a recent biogeographic analysis that indicates a non-Laurentian origin for the eastern Klamath terrane. Additional new data reveal that key early Paleozoic convergent-margin rocks within the northern Sierran-Klamath and Alexander terranes share similar arc and cratonal provenance, including 1.49–1.61 Ga zircon. We hypothesize that the rocks from all three areas are dispersed tectonic fragments that were derived from the same convergent margin and were independently transported to western North America. Of the orogenic source regions indicated by previous paleomagnetic and biogeographic analysis, the detrital zircon provenance favors western Baltica over eastern Australia.

U-Pb isotopic dating of detrital zircons from a conglomeratic barite sandstone in the Sonora allochthon and a calciclastic sandstone in the Mina México foredeep of the Minas de Barita area reveals two main age groups in the Upper Devonian part of the Los Pozos Formation, 1.73–1.65 Ga and 1.44–1.42 Ga; and three main age groups in the Lower Permian part of the Mina México Formation, 1.93–1.91 Ga, 1.45–1.42 Ga, and 1.1–1.0 Ga. Small numbers of zircons with ages of 2.72–2.65 Ga, 1.30–1.24 Ga, ca. 2.46 Ga, ca. 1.83 Ga, and ca. 0.53 Ga are also present in the Los Pozos sandstone. Detrital zircons ranging in age from 1.73 to 1.65 Ga are considered to have been derived from the Yavapai, Mojave, and Mazatzal Provinces and their transition zones of the southwestern United States and northwestern Mexico. The 1.45–1.30 Ga detrital zircons were probably derived from scattered granite bodies within the Mojave and Mazatzal basement rocks in the southwestern United States and northwestern Mexico, and possibly from the Southern and Eastern Granite-Rhyolite Provinces of the southern United States. The 1.24–1.0 Ga detrital zircons are believed to have been derived from the Grenville (Llano) Province to the east and northeast or from Grenville-age intrusions or anatectites to the north. Several detrital zircon ages ranging from 2.72 to 1.91 Ga were probably derived originally from the Archean Wyoming Province and Early Paleoproterozoic rocks of the Lake Superior region. These older detrital zircons most likely have been recycled one or more times into the Paleozoic sandstones of central Sonora. The 0.53 Ga zircon is believed to have been derived from a Lower Cambrian granitoid or metamorphic rock northeast of central Sonora, possibly in New Mexico and Colorado, or Oklahoma. Detrital zircon geochronology suggests that most of the detritus in both samples was derived from Laurentia to the north, whereas some detritus in the Permian synorogenic foredeep sequence was derived from the evolving accretionary wedge to the south. Compositional and sedimentological differences between the continental-rise Los Pozos conglomeratic barite sandstone and the foredeep Mina México calciclastic sandstone imply different depositional and tectonic settings.

The Tlikakila complex is a northeast-striking ∼5-km-wide and ∼75-km-long belt of lower greenschist-facies sedimentary and igneous rocks in the Lake Clark region of south-central Alaska. It forms the only exposures of pre-Cretaceous rocks between the Peninsular terrane and the Farewell terrane. Protoliths include basalt, gabbro, ultramafic rocks, limestone, chert, mudstone, chertpebble conglomerate, and minor quartz sandstone. Geochemical analyses of igneous rocks indicate primitive island arc compositions. Rare earth element (REE) patterns of the volcanic rocks and gabbro are flat with most elements between 5 and 15 times chondrite values. Initial 87 Sr/ 86 Sr isotope ratios range from 0.7042 to 0.7065. ε Nd ranges from + 9.3 to +2.6. The complex is older than 192 Ma based on a 40 Ar/ 39 Ar date of white mica from a pegmatite vein in a metasedimentary rock and younger than ca. 293 Ma based on the youngest zircon in a chertpebble conglomerate. Detrital zircon ages suggest source rocks included the Yukon-Tanana and Wrangellia-Alexander terranes. Previously reported Norian conodonts in nearby correlative limestone indicate at least some of the complex is Late Triassic. Metamorphism reached peak temperatures of 350–450 °C based on mineral assemblages in metapelite and metabasite. Three 40 Ar/ 39 Ar dates of biotite from metapelite indicate metamorphism at 177 ± 1 Ma. We interpret the Tlikakila complex as a dismembered suprasubduction-zone ophiolite that originated near a trench above a north-dipping subduction zone in Late Triassic time. This subduction eventually created the Talkeetna arc. A second north-dipping subduction zone closed the intervening basin between the Talkeetna arc and the southern Alaska continental margin. Metamorphism and deformation of the Tlikakila complex was coeval with either collision of the arc or shallowing of this slab at ca. 177 Ma.

Six samples collected from pre-, syn-, and post-Talkeetna arc units in south-central Alaska were dated using single-grain zircon LA-MC-ICP-MS geochronology to assess the age of arc volcanism and the presence and age of any inherited components in the arc. The oldest dated sample comes from a volcanic breccia at the base of the Talkeetna Formation on the Alaska Peninsula and indicates that initial arc volcanism began by 207 ± 5 Ma. A sedimentary rock overlying the volcanic section in the Talkeetna Mountains has a maximum depositional age of <167 Ma. This is in agreement with biochronologic ages for the top of the Talkeetna Formation, suggesting that the Talkeetna arc was active for ca. 40 m.y. Three samples from interplutonic screens and roof pendants in the Jurassic batholith on the Alaska Peninsula provide information about the tectonic setting of Talkeetna arc magmatism. All three samples contain Paleozoic to Proterozoic zircons and require that arc magmas on the Alaska Peninsula intruded into detritus that contained older continental zircons. This finding is distinct from observations from eastern exposures of the arc in the Chugach and Talkeetna Mountains, where there is only limited evidence for pre-Paleozoic zircons, and it suggests that there were along-strike variations in the tectonic setting of the arc.

The Kahiltna assemblage of southern Alaska crops out in an 800-km-long belt that forms the core of much of the rugged Alaska Range. New sedimentologic, provenance, and geologic mapping data suggest that the Kahiltna assemblage exposed in the western Alaska Range represents a late Early Cretaceous to Late Cretaceous marine basin that formed in response to oblique collision between a composite island-arc terrane and the Mesozoic continental margin of North America. The Kahiltna assemblage in the study area crops out in two belts located north and south of the Denali fault system. Measured stratigraphic sections show that the Kahiltna assemblage in the southern outcrop belt has a minimum thickness of 5560 m and consists of eight siliciclastic lithofacies that represent tabular and weakly channelized mixed sand-mud submarine-fan systems that developed in a base-of-slope environment of deposition. Our analysis of the Kahiltna assemblage located north of the Denali fault indicates the presence of similar lithofacies along with additional strata that we interpret to represent outer-shelf and/or upper-slope (slope apron) depositional environments. Geologic mapping for this study identified the depositional basement of both outcrop belts as Upper Triassic to Lower Jurassic marine-volcanic and volcaniclastic strata that form the upper part of the Mystic subterrane. Detrital zircon data constrain the depositional age of most of the Kahiltna assemblage in the study area to Early Cretaceous time (Aptian or Albian) or later and suggest a significantly younger timing of basin development than previously recognized. Compositional data indicate that sandstone and conglomerate of the Kahiltna assemblage were derived from both Mesozoic continental margin and composite island-arc terrane sources. Modal sandstone compositions (n = 41) are consistent with a mixed arc and recycled orogen provenance (Q 23 F 9 L 68 ; Qm 11 F 9 Lt 80 ). Detrital zircons from sandstone collected in the lower part of the Kahiltna assemblage yield Precambrian (32%), Paleozoic (12%), and Mesozoic (56%) U-Pb ages. Concordant ages are consistent with the age distributions of Proterozoic, Devonian, Mississippian, and Triassic-Jurassic plutonic rocks of the former continental margin that formed the northern boundary of the Kahiltna basin. Plutons of the Talkeetna and Chisana arcs, part of the composite island-arc terrane located south of the basin, also probably contributed to the abundance of detrital zircons with ages between 200 and 163 Ma and between 124 and 106 Ma, respectively. Our new findings indicate that by Early Cretaceous time, the North American continental margin and composite island-arc terrane were in close enough proximity for both to contribute sediment to the Kahiltna basin. Stratigraphic, structural, and geochronologic relationships presented here, combined with previous regional studies of Mesozoic strata in the suture zone, suggest that the Kahiltna assemblage is the product of oblique island-arc terrane collision. Oblique collision resulted in the juxtaposition of continental margin and oceanic strata within thrust sheets along the closing suture zone. Dominantly west- and southwest-directed submarine-fan systems transported detritus axially away from the closing suture zone and into the along-strike marine basin represented by most of the Kahiltna assemblage exposed in the western Alaska Range. Comparisons with along-strike uplifted Mesozoic marine basins suggest westward time-transgressive closure of a suture zone that extends from British Columbia to southwestern Alaska.

Upper Jurassic-Lower Cretaceous sedimentary strata of the Nutzotin basin, the Nutzotin Mountains sequence, crop out in the Nutzotin and Mentasta Mountains of the eastern Alaska Range. These strata represent one of the best-exposed and least-metamorphosed examples of a basin that is interpreted to have formed during collision of an allochthonous volcanic arc (i.e., the Wrangellia terrane) with a continental margin. New stratigraphic, geologic mapping, and provenance data indicate that the Nutzotin basin formed as a retroarc foreland basin along the northern margin (present coordinates) of the Wrangellia terrane. Coeval with basin development along the northern margin, sedimentary basins and plutons located along the southern margin of the Wrangellia terrane were being incorporated into a regional fold-and-thrust belt. This fold-and-thrust belt, located south of the Nutzotin basin, exposed multiple structural levels of the Wrangellia terrane that were eroded and provided sediment that was transported northward and deposited in the Nutzotin basin. New sedimentologic and stratigraphic data from the ∼3 km thick (minimum thickness) Nutzotin Mountains sequence define a three-part stratigraphy. The lower part consists of Upper Jurassic (Oxfordian to Tithonian) conglomerate with outsized limestone clasts (>10 m in diameter) and interbedded sandstone and shale that grade basinward into mainly black shale with minor micritic limestone and isolated lenses of conglomerate. The middle part of the stratigraphy consists of Upper Jurassic (Tithonian) to Lower Cretaceous (Valanginian) normal-graded sandstone and shale interbedded with massive tabular sandstone and lenticular conglomerate. The upper part of the stratigraphy consists of Upper Jurassic (Tithonian) to Lower Cretaceous (Valanginian) mudstone with distinctive fossil-rich horizons and minor interbedded sandstone. The overall stratigraphy of the Nutzotin Mountains sequence represents a general upward-shallowing and upward-coarsening package that represents a general transition from distal mud-rich submarine-fan strata to more proximal sand-rich submarine-fan strata that are in turn overlain by marine shelf strata. Feldspathic sandstone compositions (Q 6 F 67 L 27 ), eastward and northeastward directed paleocurrent indicators, diagnostic clasts in conglomerate, and detrital zircon U-Pb ages of 151–147 Ma (n = 8) and 159–156 Ma (n = 2) indicate that sediment in the Nutzotin basin was derived primarily from the Wrangellia terrane and the Chitina and Chisana arcs that intrude the Wrangellia terrane. The stages of deformation documented in the Nutzotin Mountains sequence provide insight into the growth of collisional continental margins by the tectonic incorporation of basinal strata. Our data show that strata of the Nutzotin basin have been deformed into an accretionary wedge by north-dipping thrust faults and related overturned folds above a north-dipping décollement. Displacement on this décollement was the product of northward underthrusting of basinal strata beneath the former continental margin and resulted in southward tectonic transport of distal basinal strata of the Nutzotin Mountains sequence strata over both more proximal basinal strata and the Wrangellia terrane. Previously published K-Ar ages from plutons that cross-cut both the décollement and folded Nutzotin Mountains sequence strata indicate that contractional deformation ended between 117 and 105 Ma. Regionally, the Nutzotin Mountains sequence represents part of a series of Mesozoic sedimentary basins located along the inboard margin of the Wrangellia composite terrane that have similar depositional styles and were all subsequently incorporated into accretionary wedges that dip toward the former continental margin. These deformed strata define a continental-scale suture zone that extends along the northwestern Cordillera for over 2000 km.

Mesozoic strata of the northwestern Talkeetna Mountains are located in a regional suture zone between the allochthonous Wrangellia composite terrane and the former Mesozoic continental margin of North America (i.e., the Yukon-Tanana terrane). New geologic mapping, measured stratigraphic sections, and provenance data define a distinct three-part stratigraphy for these strata. The lowermost unit is greater than 290 m thick and consists of Upper Triassic–Lower Jurassic mafic lavas, fossiliferous limestone, and a volcaniclastic unit that collectively we informally refer to as the Honolulu Pass formation. The uppermost 75 m of the Honolulu Pass formation represent a condensed stratigraphic interval that records limited sedimentation over a period of up to ca. 25 m.y. during Early Jurassic time. The contact between the Honolulu Pass formation and the overlying Upper Jurassic–Lower Cretaceous clastic marine strata of the Kahiltna assemblage represents a ca. 20 m.y. depositional hiatus that spans the Middle Jurassic and part of Late Jurassic time. The Kahiltna assemblage may to be up to 3000 m thick and contains detrital zircons that have a robust U-Pb peak probability age of 119.2 Ma (i.e., minimum crystallization age/maximum depositional age). These data suggest that the upper age of the Kahiltna assemblage may be a minimum of 10–15 m.y. younger than the previously reported upper age of Valanginian. Sandstone composition (Q-43% F-30% L-27%—Lv-71% Lm-18% Ls-11%) and U-Pb detrital zircon ages suggest that the Kahiltna assemblage received igneous detritus mainly from the active Chisana arc, remnant Chitina and Talkeetna arcs, and Permian–Triassic plutons (Alexander terrane) of the Wrangellia composite terrane. Other sources of detritus for the Kahiltna assemblage were Upper Triassic–Lower Jurassic plutons of the Taylor Mountains batholith and Devonian–Mississippian plutons; both of these source areas are part of the Yukon-Tanana terrane. The Kahiltna assemblage is overlain by previously unrecognized nonmarine strata informally referred to here as the Caribou Pass formation. This unit is at least 250 m thick and has been tentatively assigned an Albian–Cenomanian-to-younger age based on limited palynomorphs and fossil leaves. Sandstone composition (Q-65% F-9% L-26%—Lv-28% Lm-52% Ls-20%) from this unit suggests a quartz-rich metamorphic source terrane that we interpret as having been the Yukon-Tanana terrane. Collectively, provenance data indicate that there was a fundamental shift from mainly arc-related sediment derivation from sources located south of the study area during Jurassic–Early Cretaceous (Aptian) time (Kahiltna assemblage) to mainly continental margin-derived sediment from sources located north and east of the study area by Albian–Cenomanian time (Caribou Pass formation). We interpret the three-part stratigraphy defined for the northwestern Talkeetna Mountains to represent pre- (the Honolulu Pass formation), syn- (the Kahiltna assemblage), and post- (the Caribou Pass formation) collision of the Wrangellia composite terrane with the Mesozoic continental margin. A similar Mesozoic stratigraphy appears to exist in other parts of south-central and southwestern Alaska along the suture zone based on previous regional mapping studies. New geologic mapping utilizing the three-part stratigraphy interprets the northwestern Talkeetna Mountains as consisting of two northwest-verging thrust sheets. Our structural interpretation is that of more localized thrust-fault imbrication of the three-part stratigraphy in contrast to previous interpretations of nappe emplacement or terrane translation that require large-scale displacements.

The El Antimonio Group is herein proposed as a new lithostratigraphic unit that encompasses the Antimonio, Río Asunción, and Sierra de Santa Rosa Formations in a revised nomenclature from Lucas and Estep (1999b). The type section for the Antimonio, Río Asunción, and the lower part of the Sierra de Santa Rosa Formations is located in the Sierra del Álamo, whereas the representative upper part of the Sierra de Santa Rosa Formation is located in the mountains of same name in northwestern Sonora. The ∼4.5-km-thick sedimentary succession of this group is abundantly fossiliferous, and its biostratigraphic age is constrained between the Late Permian and Early Jurassic. The 3.4-km-thick section that crops out in the Sierra del Álamo is divided into 14 unconformity-bounded sequences that are tens to hundreds of meters thick and grade from the base upward from a fluvial to shallow marine conglomerate to open marine shale. The El Antimonio succession is correlated with several other Triassic and Jurassic sections that are known in Sonora, all of which are located south of the proposed trace of the Mojave-Sonora megashear. The closest Triassic and Lower Jurassic sections that are located north of the Mojave-Sonora megashear that we correlate with the El Antimonio are known in southern Nevada and southeastern California and include the Moenkopi, Virgin Limestone, Union Wash, Silverlake, and Fairview Valley Formations and the Kings sequence. On the basis of these proposed correlations, we suggest that the El Antimonio Group was deposited in an evolving shallow shelf (Upper Permian–Triassic) to fore-arc basin (Lower Jurassic) that was originally positioned adjacent to southern California and later translated to its present position, along with the Caborca block, by left-lateral Jurassic displacement of the Mojave-Sonora megashear. In this proposed paleogeography, a lower Mesozoic magmatic arc that accumulated volcanic, volcaniclastic, and shallow marine sedimentation in the Mojave Desert and along the California-Nevada border separated the El Antimonio basin from a shallow shelf that developed to the north. New U-Pb geochronology on detrital zircon and Sm/Nd isotope and petrographic data from terrigenous samples of the El Antimonio Group may help to elucidate its provenance and to support this paleogeography. Zircon grains from samples of the lower, middle, and upper parts of the El Antimonio Group yielded ages that cluster around 1.8, 1.6–1.7, 1.4, and 1.00–1.18 Ga and 340, 270–240, and 190 Ma. The Pro-terozoic zircons are interpreted to indicate provenance from the basement provinces of the southwestern United States, although a reworked source for these grains is also possible as they are present in the Cordilleran miogeocline and off-shelf assemblages of Nevada and California and in Proterozoic and Paleozoic strata in Sonora. The closest known sources for the Permian and Lower Triassic zircons are plutons and volcanic rocks that formed a lower Mesozoic magmatic arc extending from southeastern to northern California and western Nevada. Probable sources for the single zircon grain dated at 340 Ma are the Sierra-Klamath terranes, according to interpretation by other authors of grains of similar age in rocks of Nevada. Grains dated around 190 Ma in the youngest sample most probably reveal provenance from the Lower Jurassic magmatic arc of southeastern California or southern Arizona. The Sm/Nd isotopic data from three samples of the lower, middle, and upper parts of the El Antimonio Group indicate a progressive decrease in model ages, from the base upward (T DM = 1.9–1.8 to T DM = 1.13 Ga) of this succession, indicating a most probable derivation from the Yavapai and Grenville provinces in the southwestern United States. Sandstone and conglomerate clast composition in the El Antimonio Group indicate mixed sources of provenance from sedimentary and vulcanoplutonic origin. These most probably correspond to the Proterozoic and Paleozoic sedimentary successions of southwestern North America and to the Triassic-Jurassic magmatic arc of this same region, respectively.

Full article available in PDF version.

Full article available in PDF version.

Full article available in PDF version.

Full article available in PDF version.

Full article available in PDF version.

Full article available in PDF version.