The Sangre de Cristo Mountains of southern Colorado and northern New Mexico, USA, contain an unusual combination of thick- and thin-skinned contractional structures involving both basement and cover rocks in the Laramide Rocky Mountain foreland. These structures are truncated by down-faulted extensional basins to the east and west. Together with synorogenic sediments, these structures preserve a record of the rise of the Ancestral Rocky Mountains, the Laramide orogeny, and Rio Grande rifting. Laramide structures within the mountains provide clues to processes that link the three events and to necessary conditions for thin-skinned and thick-skinned contractional structures to form together in continental interiors.

To examine the full variety of structural styles, a portion of the northern Sangre de Cristo fold- and-thrust belt in Colorado was described and interpreted using geologic maps and structural cross-sections. Stratigraphic relations of the Ancestral Rocky Mountain highlands and basin fill were reconstructed from existing maps. These relations allow identification of faults inherited from the Ancestral Rocky Mountains, differentiation of thrust sheets, and in some cases, estimation of the magnitude of displacement. To examine relations between Laramide thrusts and Rio Grande rifting, kinematic data were collected from a thrust fault adjacent to rift faults.

Three thrust fault styles were recognized: thin-skinned basement, thin-skinned cover rocks, and thick-skinned basement. Thin-skinned thrusts arising from a hinterland beneath the present San Luis Valley carried sheets of Proterozoic basement rocks northeast over a Laramide foreland. These basement thrusts are interpreted to be faults of the Ancestral Rocky Mountains that reactivated during the Laramide orogeny. The Laramide foreland consists of thin-skinned thrusts and folds in sedimentary cover rocks as young as 49 Ma. Both thin-skinned thrusts in basement and cover rocks are bounded by thick-skinned basement thrusts that moved intermittently throughout the Laramide orogeny.

We infer that thin-skinned thrusts form in continental interiors where deformation is focused in weak strata of thick basin fill and in fluid-reaction weakened preexisting faults in basement rocks. Both conditions are met in the Sangre de Cristo Mountains. Basement thrusts adjacent to the San Luis Valley contain evidence of plastic contractional microstructures overprinted by extensional microstructures that may record the transition from Laramide contraction to Rio Grande extension of the crust.

The style (Harding and Lowell, 1979) of contractional deformation in orogenic fold-and-thrust belts is commonly described as “thick-skinned” or “thin-skinned” (Butler and Mazzoli, 2006; LaCombe and Bellahsen, 2016; Pfiffner, 2006, 2017). Geographic distribution and variation in structural style differs among orogenic belts. Styles of orogens formed by subduction of oceanic plates beneath continental plates range from thick-skinned, basement-involved thrusts in hinterlands to thin-skinned, thrusted and folded sedimentary cover rocks in forelands. Examples include the Sevier fold-and-thrust belt of the North American Cordilleran orogen (Yonkee and Weil, 2015) and the Malargüe fold-and-thrust belt of the Andean orogen (Giambiagi et al., 2008). In the Malargüe belt, thin-skinned thrusts formed in cover rocks when former rift faults were reactivated as basement-involved thrusts. In contrast to subduction--related North American Cordilleran and Andean orogens, the outer Albanides of the continental-collision Mediterranean Alpine system contain ramps and thin-skinned detachments in cover rocks and thick-skinned thrusts arising from the basement (Mazzoli et al., 2022).

In front of the North American Cordilleran and Andean orogens, thick-skinned thrusts cut and uplift basement of the Laramide (~70–50 Ma) Rocky Mountain foreland of the conterminous United States (Yonkee and Mitra, 1993; Mitra, 1993) and the ~10 Ma–present Sierras Pampeanas of Argentina (Jordan and Allmendinger, 1986). Thick-skinned thrusts of both forelands are interpreted to have formed along preexisting faults during flat- (or shallow) slab subduction of oceanic ridges (Yonkee and Weil, 2015; LaCombe and Bellahsen, 2016; Liu and Currie, 2022). Jones et al. (2011) discuss other processes that could influence both tectonism and magmatism above shallow subducting slabs. Worldwide, ~10% of modern convergent plate margins display evidence of flat-slab subduction (Gutscher et al., 2000), but not all of these are accompanied by thick-skinned structure. The thick-skinned structure of the Rocky Mountain foreland has been described as composed of basement-cored arches separated from basins by thrusts that originate in the crust ~30 km below the surface and propagate upward into folded cover rocks (Erslev et al., 2022).

Thin- and thick-skinned thrusts form by different mechanisms (Yonkee and Mitra, 1993; Butler and Mazzoli, 2006; LaCombe and Bellahsen, 2016). In the brittle crust above ~10 km, both thick- and thin-skinned thrusts can form by simple shear, and are commonly localized along preexisting strength heterogeneities and, in some cases, assisted by fluid-mediated alteration and associated mineral reaction weakening. Thick-skinned thrusts originate at crustal depths where plastic, distributed deformation is dominant. A depth of ~10 km is commonly assumed for the transition from brittle plastic deformation (e.g., Yonkee and Mitra, 1993; Verwater et al., 2021), but might have been as deep as 20 km when the Archean crust of the Wyoming craton, located adjacent to Proterozoic basement farther south in Colorado, USA, cooled during flat subduction (Yonkee and Weil, 2015).

Here, we integrate descriptions of three structural styles of contractional deformation present in the Laramide Rocky Mountain foreland more than 1000 km inboard from the North American plate margin, in the Sangre de Cristo Mountains of Colorado: thin-skinned basement, thin-skinned cover rock, and thick-skinned basement deformation. The description is organized in the context of structural styles involving thin-skinned and thick-skinned deformation observed globally. The Sangre de Cristo Mountains offer an important, yet uncommonly reported, example of diverse contractional structural styles in a foreland setting involving thin-skinned basement and thin-skinned cover rocks in addition to thick-skinned basement deformation. Finally, we explore possible explanations for the occurrence of all three styles together in the foreland.

The basement-involved thrusts described here also play a key role in tectonic inheritance. The Sangre de Cristo Mountains contain structures and orogenic sediments of the Pennsylvanian and Permian Ancestral Rocky Mountains, the Late Cretaceous to Eocene Laramide orogeny, and Miocene and younger Rio Grande rifting. Tectonic inheritance between the Ancestral Rocky Mountains and Laramide contraction (Hoy and Ridgway, 2002) and between Laramide contraction and Rio Grande rifting (Kellogg, 1999; Kluth, 2007; Morgan et al., 2010; Sitar et al., 2022) has been proposed. Using criteria for the reactivation of tectonic features (Holdsworth et al., 1997), we examine evidence for tectonic inheritance.

The Sangre de Cristo Mountains are one of the major physiographic and structural geologic features of southern Colorado and northern New Mexico, USA (Fig. 1A). The northern third of the Mountains, herein called the “northern Sangre de Cristo Mountains,” extends from the Arkansas River near Salida, Colorado, to the headwaters of the Huerfano River north of the town of Blanca (Fig. 1B).

The geologic history of the northern Sangre de Cristo Mountains (McCalpin, 1982; Lindsey et al., 1983, 1986a; Hoy and Ridgway, 2002, 2003; Jones and Connelly, 2006; Lindsey, 2010) can be summarized as follows: (1) magmatism and metamorphism of island-arc terranes during the Proterozoic, followed by a long hiatus in the geologic record; (2) submergence beneath shallow epicontinental seas from the Ordovician through Mississippian; (3) faulting and uplift of the Ancestral Rocky Mountains, accompanied by the filling of adjacent basins with coarse clastic sediment in the Pennsylvanian and Permian; (4) renewed submergence beneath the western interior seaway in the Jurassic through most of the Cretaceous; (5) contractional faulting and folding during the Laramide orogeny from Late Cretaceous into the Eocene; (6) deposition of volcanic rocks erupted from the San Juan volcanic field to the west during the Oligocene; (7) Rio Grande rifting, accompanied by normal faulting, footwall uplift, basin fill sedimentation, shallow intrusion, and volcanism both within and on the flanks of the rift from early Miocene to the present; and (8) extensive glaciation during the Pleistocene.

The northern Sangre de Cristo Mountains (Fig. 2) consist mostly of Proterozoic igneous- and amphibolite-facies metamorphic rocks (Johnson et al., 1987; Jones and Connelly, 2006) and locally metamorphosed Pennsylvanian and Permian clastic sedimentary rocks (Lindsey et al., 1986a, 1986b). Thin (~300 m) sequences of Ordovician, Devonian, and Mississippian marine sedimentary rocks occur locally in the western part of the range. Jurassic and Cretaceous sedimentary rocks, deposited in the western interior seaway, crop out along the eastern side of the range, especially in Huerfano Park and the Raton Basin (Johnson, 1959), and in a small area on the downthrown side of the Sangre de Cristo fault south of Crestone (Watkins, 1996). These exposures confirm that the western interior seaway covered the area now occupied by the Sangre de Cristo Mountains. A few Oligocene and Miocene volcanic and sedimentary rocks are scattered around the periphery, and small intrusions of igneous rock occur within the mountains.

The northern Sangre de Cristo Mountains comprise a narrow horst bounded on the west by the downfaulted San Luis Valley and on the east by the Wet Mountain Valley and Huerfano Park, a northern extension of the Raton Basin (Figs. 1 and 2). The western topographic and structural boundary of the range is sharply delineated by the active Sangre de Cristo normal fault. The eastern boundary is more complex and subdued, and marked by a largely concealed, inactive normal fault that follows the older Alvarado reverse fault. Southward, topographic relief is enhanced by the contrast between easily eroded sedimentary rocks in Huerfano Park and erosion-resistant Proterozoic igneous and metamorphic rocks in the range.

Maps and Sections

A simplified geologic map was prepared from existing geologic maps (Fig. 2). The simplified map shows major geologic units, faults and folds, and lines of structural cross-sections; some minor structures and Quaternary deposits in stream valleys have been omitted for clarity. Table S1 in the Supplemental Material1 provides the precise locations of cross sections and the data sources used in the construction of cross sections and maps. Minor revisions of geologic mapping are shown in various figures. Measurements of mylonitic foliation and fault striae orientations from a thin-skinned, basement-involved thrust fault are included in Table S2 (see footnote 1).

Ten sections were drawn through the study area to identify structural styles and to explore the relative timing and style of contractional deformation (Fig. 2). Sections A–A′ through I–I′ were drawn perpendicular to the commonly observed northwest–southeast strike of major contractional structures, from southwest to northeast. A section from Mount Zwischen to Twin Peaks (MZ–TP) was drawn approximately parallel to the strike of structures, from north to south, in the interior of the range. The structure-parallel MZ–TP section reveals relations among structures in adjacent cross-strike sections.

The first step in constructing the sections involved projecting formations and structures into the subsurface using surface map and orientation data (e.g., Lindsey et al., 1986c) following the method for kink-style folds (Wojtal, Chapter 13 in Marshak and Mitra, 1988). Projections to depth were guided by levels of detachment observed in lower Paleozoic carbonate and shale, in the shaley lower and upper parts of the Minturn and Madera formations, and in Cretaceous shale formations. In some places, bed length and volume (Marshak and Woodward, Chapter 14 in Marshak and Mitra, 1988), facies contrasts, down-structure views (Mackin, 1950), and possible rotation of faults and fold limbs were considered. Locally, concealed structures were verified by comparison with a reconnaissance aeromagnetic survey of the Sangre de Cristo Wilderness (U.S. Geological Survey, 1983). The sections are provisional, however, because gaps remain in geologic map coverage and detailed geophysical surveys have not been made. Sections could not be balanced with confidence because suitable pin lines and stratigraphic contacts are not exposed, but we consider the sections to be conceptually reasonable given the data available.

For sections B and H, the line of section was extended southwest from the range front to show subsurface structure along the eastern side of the San Luis Valley. Drilling (Watkins, 1996), interpretation of seismic lines (Kluth and Schaftenaar, 1994), and modeling of detailed magnetic and gravity data (Grauch et al., 2013) guided subsurface interpretation of normal faults that down-drop the San Luis Valley and separate it from the Sangre de Cristo Mountains (Fig. 2).

Reconstruction of Paleozoic Stratigraphic Relations

Reconstruction of stratigraphic relations among Paleozoic formations in the range is key to distinguishing individual thrust sheets and estimating their horizontal separation. Thrust sheets representing Ancestral Rocky Mountain highlands can be distinguished from those representing basin fill, and faults that separate the two likely originated from the Ancestral Rocky Mountains, even if subjected to later reactivation.

For this study, two transects were selected for reconstruction of Paleozoic stratigraphy (Figs. 3A3C). Transect A crosses the Sangre de Cristo Mountains north of the Great Sand Dunes, at about the latitude of the village of Crestone. Transect B crosses the range east of the Great Sand Dunes and extends into Huerfano Park. Each transect represents only the facies that are now preserved in the range. An alternative to this method is structural restoration (line-length balancing, Lindsey et al., 1983; restoration, Hoy and Ridgway, 2002), which yields estimates of 8–10 km of shortening (40–50% of the original width) within the range near transect A. Structural restoration is a useful check but does not yield the stratigraphic detail sometimes needed to place thrust sheets in their original depositional context, such as highland margin, basin margin, or basin center.

The horizontal scale of the reconstructed sections in transects A and B is only approximate. Detailed reconstructions of local facies relationships are few (e.g., Tischler, 1963; Lindsey et al., 1986b). Transect A was constructed from mapping and sections measured in the hanging wall of the Spread Eagle Peak thrust, where the structural strike crosses the original depositional strike at a low-angle, in places as little as 10–20° (Lindsey and Schaefer, 1984; Lindsey et al., 1985a; Lindsey and Clark, 1995). The location and nature of highland and basin-margin faults in transect A relies on interpolation of stratigraphic relationships in the hanging wall of the Sand Creek (formerly Huckleberry Mountain of Lindsey et al., 1983, and Hoy and Ridgway, 2002) thrust and the absence of Pennsylvanian–Permian strata in test holes drilled in the eastern half of the San Luis Valley (Gries, 1985; Kluth and Schaftenaar, 1994). Gaps in bedrock exposure preclude complete and accurate regional palinspastic restoration, but the reconstructions presented here permit thrust sheets to be distinguished and horizontal separation to be estimated.

The stratigraphic section in transect A (Fig. 3C) is divided into the middle Pennsylvanian Minturn Formation (sandstone, shale, limestone, and minor conglomerate; ~1.5–2 km in thickness) and the Pennsylvanian and Permian Sangre de Cristo Formation (conglomerate, sandstone, shale, and minor limestone; 2 km or more in thickness). Both formations were deposited in the central Colorado trough (Fig. 3A; Lindsey et al., 1986b; Hoy and Ridgway, 2002, 2003), but the Minturn Formation was deposited closer to the basin axis, and the Sangre de Cristo Formation was deposited along the western margin as well as in the basin. The near-source Crestone Conglomerate Member of the Sangre de Cristo Formation is much coarser grained than most of the Minturn Formation, which contains thick, shaley intervals and limestone with marine fossils (Lindsey et al., 1986b). Cambrian through Mississippian formations were deposited outside the Transcontinental arch (Fig. 3B), but, in the region of transect A, only Ordovician through Mississippian formations were deposited.

Marker beds and distinctive sedimentary facies provide local details that are important for recognizing individual thrust sheets. In transect A, the upper 500 m of the Minturn Formation contains limestone beds that can be used as local markers. Likewise, intervals of turbidite sandstone can be traced in the middle of the Minturn. On the western margin of the trough, north of the village of Crestone, the lower part of the Minturn contains as much as 300 m of quartz pebble redbeds (sandstone and shale; Lindsey et al., 1985b). East of Crestone Needle, the Minturn Formation contains near-source alluvial deposits of coarse conglomerate interbedded with marine limestone and shale (Lindsey and Clark, 1995; Hoy and Ridgway, 2003). In the trough, the Minturn overlies less than 300 m of Ordovician, Devonian, and Mississippian sandstone; carbonate rock; and shale (Lindsey, 2010). In the overlying Sangre de Cristo Formation, as much as 2 km of the Crestone Conglomerate Member overlies and interfingers sandstone along the western side of the central Colorado trough (Lindsey and Schaefer, 1984; Lindsey et al., 1986b). Farther south, in the Deadman–Sand Creek thrust sheet (Lindsey et al., 1986b, 1986c; Johnson et al., 1989) as much as 2 km of the Crestone Conglomerate Member rests directly on Proterozoic basement rocks of the Uncompahgre highland, an element of the Pennsylvanian–Permian Ancestral Rocky Mountains; this relationship is used for the western part of the reconstructed section. A bounding fault (or faults) separated the highland from the basin fill, but only a single exposure of an undisturbed bounding fault has been described (Hoy and Ridgway, 2002); two faults are arbitrarily shown in Figure 3C.

Reconstruction of stratigraphic facies within the Sangre de Cristo Formation indicates that the alluvial fans that deposited the Crestone Conglomerate Member had a radius of ~10–20 km from fan head to toe (Lindsey and Schaefer, 1984; Lindsey et al., 1986b). Between the present-day latitudes of Villa Grove and Mosca Pass, three fans of approximately the same size, distinguished by distinctive clast lithology, were deposited along the western margin of the trough. The presence of coarse conglomerate in the Minturn Formation east of Crestone Needle indicates that the Crestone Conglomerate Member fans began to develop during deposition of the Minturn Formation.

Transect B (Fig. 3C) is constructed from geologic maps of the Sangre de Cristo Mountains south of Medano Pass (Burford, 1960; Johnson et al., 1989; Johnson and Bruce, 1991; Lindsey et al., 2012) and maps and stratigraphic studies of Huerfano Park (Johnson, 1959; Rhodes, 1964; Smith, 1961; Tischler, 1963). In some reports (e.g., Johnson, 1959), the Minturn Formation (also known as the Pennsylvanian Madera Formation) is described but not named. Transect B facies were deposited in a transitional basin between the central Colorado and Taos troughs (Casey, 1980). For simplicity, we refer to this basin as the northern Taos trough. Stratigraphic names for transect B refer to the modified nomenclature of northern New Mexico introduced by Read and Wood (1947).

In transect B, the Madera Formation (~1.6 km in maximum thickness, approximately equivalent to the Minturn Formation) was deposited directly on Proterozoic rocks. No Cambrian, Ordovician, Devonian, or Mississippian formations are present beneath the Madera Formation in the vicinity of transect B in the northern Sangre de Cristo Mountains (Fig. 3B; Lindsey, 2010). At its base, the Madera Formation contains ~300 m of quartz pebble redbeds, which are identical to those of the lower Minturn Formation in transect A. The quartz pebble redbeds mapped as Minturn Formation east of the Great Sand Dunes and around Blanca Peak (Johnson and Bruce, 1991; Lindsey et al., 2012) underlie Madera Formation in Huerfano Park (Smith, 1961; Tischler, 1963; Rhodes, 1964) and, for the present reconstruction, are considered part of the Madera Formation. The remainder of the Madera Formation consists, from base to top, of an informal gray limestone member (200–300 m thick), an informal arkose member (700–900 m thick), and the Whiskey Creek Pass Limestone Member (50–100 m thick; Brill, 1952; Bolyard, 1959; Rhodes, 1964; Tischler, 1963; Wallace, 1996).

The highland basin-bounding fault evidently lies west of the section in transect B. Approximately 2 km of Sangre de Cristo Formation overlies the Madera Formation. No coarse alluvial fan facies comparable to that of the Crestone Conglomerate Member have been mapped above the Madera Formation. The Mosca Creek thrust sheet, which represents the westernmost part of the transect, cuts out the uppermost part of the quartz pebble redbeds in the lower part of the Madera Formation. In the subsurface of Huerfano Park, the Madera and Sangre de Cristo (identified in unpublished well logs as “Sangre de Cristo Formation”) formations have not been divided.

Thin-Skinned Basement Thrusts

Four basement thrusts interpreted as thin-skinned carry Proterozoic igneous and metamorphic rocks over Paleozoic sedimentary rocks in the range (Fig. 2). These four, the Sand Creek, Deadman Creek, Medano, and Mosca Pass thrusts, are described in order below. The first two, the Sand Creek and Deadman Creek thrusts (Fig. 4), may be parts of the same thrust. The Medano thrust cuts the Sand Creek thrust and probably lies on top of it. The regionally extensive Mosca Creek thrust overlies the Medano thrust and is discussed last.

Sand Creek Thrust

The Sand Creek thrust underlies a large sheet of basement rock that extends southeast of Crestone, Colorado, for ~10 km across the mountains (Figs. 2 and 4). Based on similar hanging wall–footwall stratigraphic relationships, the Sand Creek thrust is interpreted as the eastern continuation of the Deadman Creek thrust, and is exposed in a window on the southwestern side of the mountains. Together, they form the sole of the Deadman–Sand Creek sheet (Fig. 5). The Sand Creek thrust probably descends to near sea level, where it is cut by younger (post-Deadman) faults. Before it was cut by younger faults, the Sand Creek thrust probably continued as the Deadman Creek thrust on the western side of the range (Fig. 5, section B–B′).

A second interpretation, which does not link the Sand Creek with the Deadman Creek thrust, projects the Sand Creek thrust at 30° west to a depth near 5 km below sea level at the western side of the range. Under this interpretation, suggested by Hoy and Ridgway (2002), the hanging wall of the thrust is a block of Proterozoic basement typical of thick-skinned deformation. Given the lack of subsurface information, a thick-skinned interpretation seems plausible, except that it leaves the Deadman Creek thrust on lower Paleozoic rocks in a hindward position in the hanging wall instead of the footwall of the Sand Creek thrust. The footwalls of the Sand Creek and Deadman Creek thrusts represent basin, not highland, settings.

The hanging wall of the Sand Creek thrust consists of Proterozoic igneous and metamorphic rocks overlain by the Crestone Conglomerate Member of the Pennsylvanian and Permian Sangre de Cristo Formation (Figs. 6A and 6B). The Crestone Conglomerate Member was deposited on Proterozoic rocks on the flank of the late Paleozoic Uncompahgre highland, probably more than 15 km west of their current position (Fig. 3C). In contrast, the footwall of the Sand Creek thrust consists of the Minturn and Sangre de Cristo formations, ~4 km in thickness, which were deposited in the central Colorado trough. Ordovician to Mississippian formations are inferred to underlie Pennsylvanian and Permian formations beneath the Sand Creek thrust.

The hanging wall of the Sand Creek thrust is complexly faulted by reverse faults, thrusts, and backthrusts. The Little Sand Creek thrust rises off the Sand Creek thrust; its northern extent has been overridden by backthrusts west of the Sand Creek thrust (Figs. 2 and 4; and Fig. 5, sections B–B′ and C–C′). Together, the backthrusts form a stack of interleaved Paleoproterozoic igneous and metamorphic rocks (gneiss), Mesoproterozoic Music Mountain pluton (quartz monzonite; Jones and Connelly, 2006), and the Pennsylvanian and Permian Crestone Conglomerate Member between the leading edge of the Sand Creek thrust and the Little Sand Creek thrust. In sections D–D′ and E–E′ (Fig. 5), a single backthrust occurs behind the frontal ramp of the Sand Creek thrust.

Southeast of Sand Creek, the hanging wall of the Sand Creek thrust contains the Sand Creek syncline (Figs. 4 and 7; Fig. 5, sections D–D′ and E–E′). The syncline axial trace is oriented northwest–southeast, parallel to the Sand Creek thrust, which is consistent with nearly perpendicular, northeast-directed, subhorizontal shortening. The syncline is a broad open fold containing Proterozoic rocks overlain by as much as 2 km of Crestone Conglomerate Member. Where it is well-exposed in the western limb, the contact is clearly depositional (Fig. 6B). The contact between conglomerate and Proterozoic rocks on the eastern limb of the syncline has always been mapped as a fault (Volckmann, 1965; Burford, 1960; Lindsey et al., 1986c; Johnson et al., 1989), but it is reinterpreted here as a depositional contact. The contact in the eastern limb is not well exposed enough to make a definitive interpretation, but locally sheared rocks along the contact indicate possible flexural slip (Hoy and Ridgway, 2002). Field checking near section E–E′ (Figs. 5 and 7), east of Medano Pass, revealed no evidence of faulting, such as networks of slip surfaces or breccias at or near the contact.

The northwest-striking segment of the Sand Creek thrust, which extends from a point about 2 km south of Marble Mountain to about 3 km southeast of the village of Crestone (Figs. 2 and 4), is interpreted as an oblique ramp. The thrust segment is straight and upright at the level of exposure, with Proterozoic rocks on the southwestern side and more than 4 km of Minturn and Sangre de Cristo formations on the northeastern side. The upright orientation of the thrust is evident where it crosses ridgelines with more than 0.6 km of relief near the range crest (Fig. 4).

On the ridge southwest of Crestone Peak, the hanging wall of the thrust contains a small occurrence of Crestone Conglomerate Member in depositional contact with underlying Paleoproterozoic gneiss. This occurrence is notable because it demonstrates the widespread stratigraphic continuity of the Crestone Conglomerate Member on Proterozoic rocks in the hanging wall of the Sand Creek thrust. The stratigraphy of the Sand Creek hanging wall is representative of the Ancestral Rocky Mountains highland.

South of the northwest-striking segment, a footwall ramp on the Minturn and Sangre de Cristo formations (B–B′ and C–C′; Fig. 5) is interpreted to extend about 3 km west from the surface trace of the Sand Creek thrust. On the ridge southeast of Marble Mountain, where the thrust strike turns southward, it dips 40° southwest at the surface.

Deadman Creek Thrust

The Deadman Creek thrust (Fig. 5) is exposed in a window on the southwestern side of the range, from Deadman Creek south to the Great Sand Dunes (Clement, 1952; Lindsey et al., 1986c; Johnson et al., 1987; Caine et al., 2013). At Deadman Creek, the hanging wall of the Deadman Creek thrust consists of Paleoproterozoic gneiss and granitoid; the footwall consists of Ordovician through Mississippian sedimentary rocks but is composed mostly of Ordovician Harding Sandstone (quartzite; Fig. 8A). In the thrust footwall, the Harding Sandstone lies in depositional contact on variably foliated granitoids like the Mesoproterozoic granitoids documented by Jones and Connelly (2006) elsewhere in the range (Fig. 8B). A sample of granitoid collected near the Harding unconformity shown in Figure 8B yielded a zircon U-Pb date of 1440 ± 20 Ma (Holm-Denoma et al., 2019) and the chemical composition of quartz monzonite, which supports possible correlation with the Music Mountain pluton in the hanging wall of the Sand Creek thrust 10 km east (Fig 4). After the Deadman Creek thrust formed, it was folded and faulted (by post-Deadman faults) along a northwest-plunging anticline (Fig. 2; Lindsey et al., 1986c; Caine et al., 2013; Weigel, 2014).

Ordovician Harding Sandstone and Fremont Dolomite are variably metamorphosed up to lowest greenschist facies, thinned, and locally removed by the thrust, leaving only isolated slices of these formations. Fault rocks in the Deadman Creek thrust, immediately above Harding and Fremont rocks, are characterized by a thick and complex zone of chlorite-quartz mylonite that is well-developed in the hanging wall (Caine et al., 2013). Analysis of shear bands, striated quartz slip surfaces, and small asymmetric folds in the Deadman Creek mylonites along the gently inclined western limb of the anticline indicates progressive deformation and movement of the hanging wall toward the northeast (Fig. 9; Caine et al., 2013). These kinematic indicators are consistent with the direction of overturning of the folded Deadman thrust farther south (Fig. 5, section B–B′). Hanging-wall fault rocks at Deadman Creek involved both plastic and brittle deformation mechanisms. Outcrop and thin-section observations show both top-to-the-northeast and top-to-the-southwest shear sense indicators in retrograded quartz + feldspar + muscovite + chlorite greenschist-facies protomylonites, mylonites, ultramylonites, and discrete zones of phyllonite (Figs. 9 and 10). Evidence for dynamic recrystallization is common in quartz-rich lithons, where grains with undulose extinction and deformation lamellae are largely replaced by smaller, near equigranular subgrains with irregular boundaries and elongate-shape–preferred orientation parallel to foliation. Feldspars, primarily microcline, generally do not show these textures but are separated by small offsets forming incipient boudins. Chlorite is a well-developed synkinematic mineral phase that is integral to defining the mylonitic foliations.

Medano Thrust

The Medano thrust, also known as the East-side boundary fault of Lindsey et al. (2012), extends southeast from Medano Creek across the range crest and then veers south along the eastern side of the range from the head of North Greaser Creek to the head of Pantleon Creek (Fig. 7). At North Fork Greaser Creek, the hanging wall of Paleoproterozoic gneiss overrides the Sand Creek thrust and uppermost part of the Madera Formation (Tischler, 1961). Farther south, at Pantleon Creek, the hanging wall overrides limestone and sandstone of the overturned uppermost Madera and lowermost Sangre de Cristo Formations (Burford, 1960). At the latter locality, the thrust dips less than 20° west. Intersecting zones of interpreted en echelon tension fractures in footwall limestone (like those described by Lindsey, 1998, from the Culebra Range) are kinematically consistent with subhorizontal, northeast-directed shortening. From Pantleon Creek south, the thrust is concealed beneath Tertiary volcanic rocks and younger alluvium. The northwestern extent of the Medano thrust has not been mapped with confidence, as indicated by the dashed and queried segment (Fig. 7).

The gentle western dip of the Medano thrust at Pantleon Creek is consistent with a magnetic low representing nonmagnetic Paleozoic sedimentary rock beneath the hanging wall (U.S. Geological Survey, 1983) and suggests that the Medano thrust may extend south beneath Tertiary volcanic rocks and Quaternary sediments and project westward into Proterozoic rocks beneath the Mosca Creek thrust.

Mosca Creek Thrust

The Mosca Creek thrust is exposed in four areas: in a window at Mosca Creek, in lateral ramps north and south of Mosca Creek, and in a klippe and window east of Blanca Peak (Figs. 11 and 12). The thrust is the southernmost basement thrust identified in the Sangre de Cristo Mountains east of the Great Sand Dunes (Fig. 2). Hanging-wall rocks consist of metamorphosed Paleoproterozoic plutons of quartz diorite and trondhjemite (Johnson and Bruce, 1991; Bruce and Johnson, 1991) and Paleoproterozoic igneous and metamorphic rocks (gneiss and schist). Footwall rocks everywhere consist of Paleoproterozoic rocks overlain by quartz pebble redbeds of the lower part of the Madera (Minturn equivalent) Formation.

East of the Great Sand Dunes along Mosca Creek, the thrust is exposed in a window that extends 5 km across the range (Fig. 12; Aughenbaugh, 1958; Lindsey et al., 2012; Webster, 2000). The exposed part of the thrust surface is broadly anticlinal and verges east; it rises from ~2.5 km above sea level at the western range front to more than 3.1 km at the crest of the range, then descends into the subsurface. The thrust does not cut down section but follows the top of contorted Madera quartz pebble redbeds in the footwall. The anticlinal form of the thrust probably reflects contractional thickening by reverse faulting, folding, and imbrication of quartz pebble redbeds in the footwall (G–G′ in Fig. 11). Some reverse faults within the Mosca Creek window face in opposite directions, producing a bulge filled with redbeds in the core of the anticline (Lindsey et al., 2012). Tear faults in the footwall (Webster, 2000) may indicate variable horizontal displacement above another thrust, which is possibly the southern extension of the Medano thrust.

The northern segment of the Mosca Creek thrust is exposed 3–5 km north of Mosca Pass (Fig. 12). This segment of the thrust strikes northwest for ~3 km across the range crest but has not been traced farther northwest. The thrust segment is buried beneath Tertiary and Quaternary alluvium southeast of the range front. The hanging wall of the thrust is Paleoproterozoic gneiss. The footwall of the thrust contains ~10–20 m of Madera quartz pebble redbeds in depositional contact with Paleoproterozoic gneiss; evidently, the rest of the Madera has been removed by the thrust. The linear trace of the thrust in steep terrain indicates that the northern segment may be a lateral ramp.

The southern segment of the Mosca Creek thrust strikes east–west from California Peak to the western side of the range by ~6 km (Figs. 2 and 12). The footwall consists of quartz pebble redbeds of the Madera Formation in depositional contact with Paleoproterozoic rocks. Previously, the latter contact was mapped as a thrust (Johnson and Bruce, 1991), but reexamination near the range front 5 km west of California Peak revealed an unsheared depositional contact. A depositional contact at the base of the Madera has also been described in the cirque on the southern side of California Peak (Rhodes, 1964). The southern segment is interpreted as a lateral ramp of the Mosca Creek thrust (Fig. 12).

The Mosca Creek thrust is also exposed in a klippe and window east of the Huerfano River (Figs. 11, section I–I′, and 12), 1–5 km east of Blanca Peak (Kasabach and Robinson, 1959; Johnson and Bruce, 1991). The hanging wall is composed of Paleoproterozoic rocks, and the footwall is composed of quartz pebble redbeds of the Madera Formation in depositional contact with Paleoproterozoic rocks. Preservation of the thrust east of Blanca Peak is due, in part, to down-faulting in a graben as well as the high-elevation of this part of the range. The thrust sheet was interpreted by Kasabach and Robinson (1959) to have moved northeast (60° from north), then back westerly, based on running the hand over “chatter marks.” However, if the chatter marks are Riedel fractures, then all movement could be interpreted as northeast.

Thin-Skinned Foreland Structures

A complex network of thrusts, reverse faults, and associated folds in Paleozoic and Mesozoic cover rocks lies north and east of the Sand Creek, Deadman, Medano, and Mosca Creek thrusts (Fig. 2). From Marble Mountain to Huerfano Park, foreland thrusts are thin-skinned detachments in sedimentary rocks. Dip angles of foreland thrusts vary from steep to almost horizontal. The Marble Mountain, Beck Mountain, and underlying thrusts are interpreted to sole in shaley strata in Ordovician–Mississippian sedimentary rocks or the lower part of the Minturn Formation (Fig. 5). Farther south in Huerfano Park, the Greaser Creek and J M thrusts are nearly horizontal and bring Paleozoic rocks over Cretaceous shales. The latter two thrusts emanate from the Sangre de Cristo Mountains thrust, which extends southward along the western side of Huerfano Park. In contrast to both plastic shear zones and brittle faults on the western side of the range, plastic deformation structures in the foreland faults were not observed on the eastern side of the range. At Marble Mountain, maximum burial temperatures did not exceed 50–70 °C, based on the conodont alteration index there (Lindsey et al., 1986a).

Marble Mountain Thrust

The Marble Mountain thrust is well-exposed on the eastern side of Marble Mountain. There, it dips ~45° west and brings west-dipping beds of the lower part of the Minturn Formation over nearly horizontal sandstone beds of the Sangre de Cristo Formation (Fig. 13). Northwest along strike, the fault dips steeply west (~75°) at the line of section (Fig. 14A). The steep dip of the northwestern segment of the fault is interpreted as a ramp that flattens east into a now-eroded roof thrust. Pennsylvanian and Permian sedimentary rocks of the hanging wall have been folded into a syncline. The hindward side of the Marble Mountain thrust is truncated by the oblique ramp of the Sand Creek thrust (Figs. 4 and 14A; Lindsey et al., 1986c). Truncation and folding of the hanging wall indicate that the Marble Mountain thrust developed in multiple stages during movement of the Sand Creek thrust (Fig. 14B). The Sand Creek thrust may have originated from an Ancestral Rocky Mountains basin-bounding fault (Figs. 14B and 14C).

The Marble Mountain thrust is interpreted to sole into sedimentary rocks at depth, probably in shale in Ordovician–Mississippian sedimentary rocks or the lower part of the Minturn Formation (Fig. 14A). On the eastern side of Marble Mountain, directly north of the point where the thrust joins the Sand Creek thrust, the Marble Mountain thrust cuts the lower part of the Minturn Formation. Ordovician–Mississippian sedimentary rocks and the lower part of the Minturn Formation contain shale beds that could localize displacement; these beds also serve as the main zone of detachment for the Crestone thrust (Lindsey et al., 1985b), which is located only ~12 km northwest of Marble Mountain (Fig. 4).

The hanging wall of the Marble Mountain thrust consists entirely of proximal alluvial and marine beds of the Minturn Formation unconformably overlain by the Crestone Conglomerate Member of the Sangre de Cristo Formation (Lindsey and Clark, 1995; Lindsey et al., 1986b). In contrast, the footwall contains 2 km of sandstone in the Sangre de Cristo Formation. The Crestone Conglomerate Member is not present in the footwall. The reason for this stratigraphic disparity is that the hanging wall contains sedimentary facies deposited much closer to their source than does the footwall (Fig. 14C; see Fig. 3C, transect A). The hanging wall is represented by facies deposited in the basin close to the highland, whereas the footwall is represented by facies deposited in the central Colorado trough. Based on estimates of approximate dimensions of alluvial fans in the Crestone Conglomerate Member farther north in the range (Lindsey and Schaefer, 1984), as much as 10 km separated the original locations of the hanging wall and the footwall in the vicinity of the thrust (Fig. 14C).

Thrust and Reverse Faults East of the Sand Creek Thrust

The Beck Mountain thrust is the principal structure in the footwall of the Sand Creek thrust east of Marble Mountain and southward. The thrust folds and cuts the Sangre de Cristo Formation into a fault-bounded anticline-syncline pair at the surface (Figs. 4 and 5, sections B–B′ and C–C′). The thrust dips as little as 25° west near the surface but probably has only a few hundred meters of horizontal separation; it is interpreted to be rooted in a detachment horizon in the lower part of the Minturn Formation near sea level, where it may join the Sand Creek thrust. The footwall contains two splays (designated collectively as the Loco Hill thrust) that dip west at ~30° and enclose the east-verging, overturned Loco Hill syncline (Lindsey et al., 1983). The splays cut the limbs of the syncline, which is composed of the uppermost part of the Sangre de Cristo, the Middle Jurassic Entrada, and the Upper Jurassic Morrison formations.

South of Loco Hill to Huerfano Park, the Beck Mountain thrust steepens to ~60° west at the surface (Fig. 5, sections D–D′ and E–E′). Topographic relief and exposures of the thrust are insufficient to accurately determine the attitude of the thrust at the surface, so the dip is interpreted from its nearly straight trace as approximately conformable to dips of 50–60° in adjacent strata. In the northern part of the Medano Pass quadrangle (Fig. 7; Johnson et al., 1989), west-dipping strata in the hanging wall shallow in dip westward from 50° to 35° west, in agreement with the interpreted broad synclinal form of the hanging wall (Fig. 5).

Greaser Creek and J M Thrusts

Lobate sheets of sedimentary rocks ride on the nearly flat-lying Greaser Creek and J M thrusts along the western side of Huerfano Park (Fig. 2). First mapped by Burbank and Goddard (1937; see also Briggs and Goddard, 1956) and later by Johnson (1959), these thrust sheets overlie and cut Cretaceous, Paleocene, and Eocene formations (Fig. 11, section F–F′). Sections drawn by each of the previous authors differ in detail from one another and from F–F′, but the overall picture is the same. The Greaser Creek thrust extends east in the footwall of the Sangre de Cristo thrust fault (Johnson, 1959), imbricates Pennsylvanian and Permian formations, and cuts up-section in a footwall ramp. Complexly folded and faulted Pennsylvanian and Permian Sangre de Cristo Formation and Mesozoic formations underlie the northern side of the thrust. About 5 km of horizontal separation is interpreted from section F–F′. To the east, the Greaser Creek sheet overrides the J M sheet, which at the surface consists entirely of Cretaceous formations folded into east-verging anticlines and synclines (Fig. 2). The J M thrust clearly defines the northern and eastern sides of the J M sheet (Johnson, 1959) but is difficult to trace around its southern side. On the eastern side, the J M sheet abuts the upturned Paleocene Poison Canyon and Eocene Cuchara formations.

Projection of formations and structures in the footwall of the Greaser Creek thrust depends on perspective (Fig. 11, section F–F′). Looking south at the northern side of the thrust, the thrust clearly overrides folded and faulted Pennsylvanian and Permian formations as well as Mesozoic rocks. The folded and faulted footwall structure does not emerge from the southern side of the thrust, however. Instead, the entire southern side of the Greaser Creek sheet overlies Upper Cretaceous Pierre Shale (as shown in map and section by Johnson, 1959). Section F–F′ passes through the middle of the Greaser Creek sheet and is drawn to show some of the formations and structures beneath the northern side. In the footwall south of the J M thrust, the Poison Canyon, Cuchara, and Eocene Huerfano formations are folded into an anticline-syncline pair (Johnson, 1959; Fig. 2) with moderately to steeply dipping limbs (Briggs and Goddard, 1956; Johnson, 1959; Fig. 15; locations in Fig. 1B).

Sangre de Cristo Mountains Thrust

The Sangre de Cristo Mountains thrust of Johnson (1959) extends along the western side of Huerfano Park (Fig. 2). Various segments have been designated as the Sharpsdale fault and the Smith Reservoir thrust by Smith (1961), but to sidestep this confusion, we adopt the earlier, albeit cumbersome name (Figs. 2 and 11) from Johnson (1959). The thrust transfers horizontal slip from the Beck Mountain and subjacent thrusts to a single fault that provided the root of the Greaser Creek and J M thrusts. Southward, the thrust is down-dropped on the west by the Wilson Reservoir normal fault (Fig. 11, sections G–G″ and H–H″). The down-dropped thrust probably extends east from the Medano thrust, at or below the contact between Proterozoic basement and Pennsylvanian cover rocks. Most importantly, the Sangre de Cristo Mountains thrust truncates faults in the Culebra block (Figs. 2 and 11, section G–G″).

Thick-Skinned Basement Thrusts

Thick-skinned thrusts cut and uplift blocks of Proterozoic basement rocks. They occur along both sides of the northern Sangre de Cristo Mountains. On the east, the Alvarado fault forms the western boundary of the Wet Mountain block (Wet Mountain Valley in Fig. 2). The Chama fault system and the Frontal thrust define the eastern side of the Culebra block. West of the mountains, the Crestone thrust, and the post-Deadman Creek faults define the eastern sides of blocks that are mostly concealed beneath the San Luis Valley (Fig. 2).

Alvarado Fault

The Alvarado fault is a west-verging reverse fault that juxtaposes Proterozoic gneiss above sandstone of the Pennsylvanian and Permian Sangre de Cristo Formation (Lindsey et al., 1983). The fault marks the Laramide structural boundary between the Wet Mountain Valley and the Sangre de Cristo Mountains. The fault has not been traced south of the Wet Mountain Valley but may extend southeast to join the faulted southwestern boundary of the Wet Mountains east of Huerfano Park (Johnson, 1959). Along the eastern side of the Sangre de Cristo Mountains, west-verging anticlines in the Minturn and Sangre de Cristo formations overlie a few splays in the footwall of the fault (Fig. 5).

A mostly concealed normal fault (or faults) that down-drops the western side of the Wet Mountain Valley follows the strike of the Alvarado fault (Fig. 2). Normal faults exposed at Goat Creek (Fig. 1B), northwest of Westcliffe, down-drop Tertiary volcanic rocks into the valley (Scott and Taylor, 1975).

Faults of the Culebra Block

The Culebra block extends from the Huerfano River (Figs. 1B and 2) south into New Mexico and is the dominant structural element with positive relief (elevation) in the central Sangre de Cristo Mountains (also known as Culebra Range; Lindsey, 1998). The block is barely discernable on geologic maps and sections north of the river (Fig. 11, section F–F′), but the Proterozoic core of the Culebra block is exposed south of the river (Figs. 2 and 11, sections G–G″ and Hº–H″) and probably accounts for the strong magnetic anomaly there (U.S. Geological Survey, 1983). The northern end of the Culebra block is truncated by the Sangre de Cristo Mountains thrust.

South of the Greaser Creek sheet, the Sangre de Cristo Mountains thrust and the Wilson Reservoir normal fault delineate the western margin of the Culebra block; several reverse faults, the Chama fault system, and the Frontal fault comprise the eastern side of the block. Based on its consistent strike adjacent to steeply dipping strata, the Frontal fault probably extends into Proterozoic basement rocks and has a more upright orientation (Fig. 11, sections G–G″ and Hº–H″) than was interpreted by Johnson (1959) and Smith (1961). Near the Huerfano River, the Chama fault branches southward into a complex system of folds and anastomosing thrusts and reverse faults, which is assigned to three thrust plates (sheets of sedimentary rocks) by Schavran (1985). Structures in the Chama fault system are well-defined by offsets of the Whiskey Creek Pass Limestone Member of the Madera Formation (Figs. 16A and 16B), which were mapped and described to the south by Wallace (1996).

The Wilson Reservoir normal fault (Figs. 11 and 12, sections G–G″ and Hº–H″), which is interpreted as a Rio Grande rift fault, cuts the Rito Oso and Sangre de Cristo Mountains thrusts and down-drops a half-graben of Pennsylvanian and Permian sedimentary rocks and Tertiary volcanic rocks against the western side of the Culebra block. In the subsurface, the down-dropped thrust may join the Medano thrust to the west. Net vertical separation of the Sangre de Cristo Mountains thrust and the Wilson Reservoir normal fault is down-to-the-west, which leaves the Proterozoic-cored Culebra block in a structurally high position.

The Chama fault system consists of at least two (possibly three) folded and faulted thrust sheets of the Madera and Sangre de Cristo formations that overlie Proterozoic basement rocks (Figs. 16A and 16B). The upper sheet (upper plate of Schavran, 1985) contains the faulted Sharpsdale anticline, the Manzanares Creek syncline, an out-of-syncline reverse fault, and a west-dipping section of Madera Formation that comprises the exposed leading edge of the sheet. The upper thrust sheet rests on the middle sheet (middle plate of Schavran, 1985), which is composed of folded and imbricated Whiskey Creek Pass Limestone Member and Sangre de Cristo Formation. The middle sheet rides on a detachment in the lower part of the Sangre de Cristo Formation that may emerge near the base of the Sangre de Cristo Formation in the Chama Valley (Fig. 16A). The entire middle sheet has been folded into an anticline-syncline pair with up-limb backthrusts. Rocks beneath the middle sheet have been folded and faulted, which indicates that these rocks may comprise a lower thrust sheet. By analogy with the lowest stratigraphic level of detachment farther north in the Sangre de Cristo Mountains (Fig. 5), the base of the lower thrust sheet may lie slightly above the contact between Proterozoic basement rocks and the Madera Formation (Fig. 16B). Aggregate horizontal separation in the Chama fault system is unknown but may be ~8 km if the system has undergone 50% shortening. Although the Chama fault system is a thin-skinned structure involving only cover rocks, it is located above and between reverse faults that comprise the eastern side of the Culebra block (Fig. 16B), much like faults in the Frontal thrust block farther south (cf. Lindsey, 1998).

East of the Chama fault system (Fig. 11, sections G–G″ and Hº–H″), two reverse faults of only a few hundred meters of stratigraphic separation divide the footwall of the Chama fault system into distinct blocks. North of the Huerfano River, the westernmost block has been rotated down to the west, whereas the next block east is horizontal to gently folded. South of the river, the easternmost anticline passes into the Frontal (reverse) fault, which marks the boundary between the Culebra block and the Huerfano Park portion of the Raton Basin. The footwall of the Frontal fault contains folded Cretaceous and Tertiary sedimentary rocks.

Crestone Thrust

The Crestone thrust is well-exposed northeast of Crestone village, where it brings Paleoproterozoic quartz monzonite (Jones and Connelly, 2006) and Ordovician to Mississippian sedimentary rocks over the lower part of the Minturn Formation (Lindsey et al., 1985b). Near Crestone, Ordovician Harding Sandstone is in depositional contact with Paleoproterozoic gneiss in the hanging wall. Northeast of Crestone, the Crestone thrust dips as low as 35° west, but it is an upright reverse fault for much of its southern extent. South of Crestone, the thrust is truncated by the Sand Creek thrust (Fig. 4; Lindsey et al., 1986c). The Crestone thrust defines the eastern boundary of an uplifted basement block.

Based on its cutoff by the Sand Creek thrust, the Crestone thrust predates the Sand Creek thrust. This cutoff may have taken place during the Laramide orogeny, when the Ancestral Rocky Mountains Sand Creek thrust was reactivated. Although both structures were interpreted previously as reactivated Ancestral Rocky Mountain faults (Lindsey et al., 1986b; Hoy and Ridgway, 2002), displacement along the Crestone thrust may be mostly, if not entirely, Laramide. The presence of Ordovician rocks in the hanging wall, and the small stratigraphic separation between the hanging wall and the footwall of the Crestone thrust (only a few hundred meters; Lindsey et al., 1985b), are in contrast to the lack of lower Paleozoic rocks in the hanging wall and large stratigraphic separation (as much as 4 km, Fig. 5; Lindsey et al., 1986c) of the Sand Creek thrust.

Post-Deadman Faults

Post-Deadman reverse and thrust faults cut the limbs of the Deadman Creek anticline along the western side of the range (Fig. 17). The faults mark the eastern boundary of a proposed block uplift of Proterozoic rocks, a small remnant of which is exposed along the range front. The remainder of the proposed block is concealed beneath the present San Luis Valley. Movement along some post-Deadman faults may account for the presence of plastic deformation features in the Deadman Creek thrust and shear zones along the range front west of the thrust.

The Deadman Creek thrust, a thin-skinned fault bringing basement rocks over Ordovician to Mississippian rocks, has been folded into an anticline. The anticline varies from an open, gently plunging fold at the northern end of exposure to a strongly overturned east-verging fold with limbs sheared by low-angle, post-Deadman faults to the south. Post-Deadman faults follow the limbs of the overturned anticline and extend into Proterozoic rocks at the southern end of the exposure. This north-to-south progression of increased overturning of the anticlinal axis and decreased dip angle of post-Deadman faults may indicate decreasing dip of the faults at depth.

The hanging wall of the westernmost post-Deadman fault contains numerous unmapped shear zones and reverse faults, which resemble the Independence Mine shear zone (Sitar et al., 2022) north of Deadman Creek.

Evidence for Ancestral Rocky Mountains Tectonism

The thickness (2 km and greater) and coarseness (to large boulder size) of the Crestone Conglomerate Member of the Pennsylvanian and Permian Sangre de Cristo Formation in the northern Sangre de Cristo Mountains indicate erosion of a highland source. Paleocurrent directions measured from crossbedding in these same formations show that the source was located west of the present range (Lindsey et al., 1986b). Fission-track cooling ages from a transect in the mountains west of Westcliffe, Colorado, indicate that the source was being uplifted to upper crustal levels during the Pennsylvanian and Permian (Lindsey et al., 1986a). Zircon fission-track ages from sandstone in the Minturn Formation and the lower part of the Sangre de Cristo Formation are Neoproterozoic and Cambrian (612–521 Ma). Zircon fission-track ages from sandstone and a syenite boulder in the upper part of the Sangre de Cristo Formation are 307–265 Ma. Together, the cooling ages define an unroofing sequence from erosion of an Ancestral Rocky Mountains source west of the present Sangre de Cristo Mountains at the present latitude of Westcliffe.

Some workers (DeVoto and Peel, 1972; Hoy and Ridgway, 2002) cite unconformities in the Pennsylvanian and Permian Sangre de Cristo Formation in the Sangre de Cristo Mountains as evidence that some of the folded and faulted structure of the range developed during the Pennsylvanian and Permian Ancestral Rocky Mountains orogeny. Unconformable overlap of Pennsylvanian and Permian strata over slightly older folded and faulted strata is perhaps the most definitive geologic evidence of Ancestral Rocky Mountains deformation. Comparable examples include the Wichita and Arbuckle Mountains of Oklahoma, USA (Denison, 1989) and the Sacramento Mountains of New Mexico (Pray, 1961), where angular unconformities in folded and faulted Pennsylvanian and Permian strata are plainly visible. In northern New Mexico, Ancestral Rocky Mountains deformation has been inferred from disconformities and lateral changes in the facies and thickness of Pennsylvanian and Permian clastic rocks (Baltz, 1999).

Faults along the eastern side of the Uncompahgre highland may have separated the highland from the sedimentary fill of the central Colorado and Taos troughs but, except for the Picuris-Pecos fault in northern New Mexico (Cather et al., 2011; Sanders et al., 2006), such faults have been obscured by younger faulting. In the Sangre de Cristo Mountains, evidence for an Ancestral Rocky Mountains (Pennsylvanian) fault has been described at one locality near the eastern front of the Deadman–Sand Creek sheet (Fig. 4) and inferred from unconformities in the Sangre de Cristo Formation northeast of Crestone (Hoy and Ridgway, 2002). Along Sand Creek south of Marble Mountain, the Pennsylvanian and Permian Crestone Conglomerate Member unconformably overlies a fault separating conglomerate from Proterozoic rocks. This fault projects southeast, beneath the axial trace of the younger Sand Creek syncline, and could be the remnant of an Ancestral Rocky Mountains fault (Fig. 4). The fault dips steeply west and brings Proterozoic rocks (on the western side) over the Crestone Conglomerate Member (to the eastern side). As described by Hoy and Ridgway (2002, their fig. 10B), the fault is overlapped by continuous beds of conglomerate lying unconformably on Proterozoic rocks on the upthrown side and conglomerate on the downthrown side.

The Ancestral Rocky Mountains fault on upper Sand Creek does not follow other mapped faults, but instead underlies the Sand Creek syncline (Fig. 7). The syncline contains only near-source Crestone Conglomerate Member in depositional contact with Proterozoic rocks and no intervening Paleozoic formations (Figs. 57; Lindsey et al., 1986c; Johnson et al., 1989), which indicates that these rocks belong to an Ancestral Rocky Mountains highland (Fig. 3C). East of the syncline, the Sand Creek thrust (formerly Huckleberry Mountain thrust of Lindsey et al., 1983, and Hoy and Ridgway, 2002) separates rocks of the highland from those deposited in the central Colorado trough (Fig. 7). Thus, the Ancestral Rocky Mountains fault described by Hoy and Ridgway (2002) was located within the Uncompahgre highland, probably near but west of the highland-basin boundary during the Pennsylvanian and Permian. The boundary is now obscured by probable later movement of the Sand Creek thrust.

The limbs of the Sand Creek syncline (Fig. 5, section D–D′, and Fig. 7) contain contrasting thicknesses of the Crestone Conglomerate Member that may reflect faulting during deposition of the conglomerate. Between Sand and Medano creeks, approximate measurements of thickness from geologic maps (Lindsey et al., 1986c; Johnson et al., 1989) indicate as much as 2 km of the conglomerate member in the eastern limb but no more than 0.8 km in the western limb. The axial trace of the syncline is close to the Ancestral Rockies fault described by Hoy and Ridgway (2002), which suggests that the difference in thickness, which holds for the full length of the syncline, may reflect the location of a concealed fault, coincident with the axial trace of the syncline.

Unconformities within the Pennsylvanian and Permian Sangre de Cristo Formation in the western limb of the Gibson Peak syncline (Hoy and Ridgway, 2002), in the footwall of the Crestone thrust, provide a second line of evidence for synorogenic Ancestral Rocky Mountains tectonism. These features are beyond the present area of investigation, though we note that the Crestone thrust, proposed as an Ancestral Rocky Mountains fault that drove synorogenic contraction of the Gibson Peak syncline, lacks the stratigraphic separation that may be expected of a major Ancestral Rocky Mountains basin-bounding fault.

Evidence for Laramide Tectonism

Notwithstanding evidence for faulting and erosion of highlands during Pennsylvanian and Permian Ancestral Rocky Mountains tectonism (Lindsey et al., 1986b; Hoy and Ridgway, 2002), most of the contractional deformation of rocks in the Sangre de Cristo Mountains has long been assigned to the Laramide orogeny (Late Cretaceous to Eocene; Burbank and Goddard, 1937; Lindsey et al., 1983; Lindsey, 2010).

The Sand Creek thrust truncates and deflects structures and formations in the hanging wall of the Marble Mountain thrust, including the entire thickness of the Minturn and Sangre de Cristo formations (Fig. 4). The Marble Mountain thrust and the Gibson Peak syncline are deflected southeast from the contiguous Spread Eagle Peak thrust, and the hanging wall of the Marble Mountain thrust is cut by small displacement faults subparallel to the Sand Creek thrust. All of these features reflect movement of the Sand Creek thrust after development of the Marble Mountain thrust. In the footwall of the Marble Mountain and Sand Creek thrusts, the Loco Hill thrust, and its adjacent syncline (Fig. 4), cut and fold Jurassic rocks (Fig. 5, section C–C′; Lindsey et al., 1983). These relations indicate that the Sand Creek thrust moved after the Ancestral Rocky Mountain orogeny and as illustrated by the Loco Hill structures, during the Laramide orogeny. Nevertheless, the Sand Creek thrust separates an Ancestral Rocky Mountains highland terrane from basin fill of the Central Colorado trough, which indicates that it likely began as an Ancestral Rocky Mountains fault (Fig. 14B).

Dynamically recrystallized fabrics, retrograde mineral assemblages, and kinematic indicators from the Deadman Creek thrust and other faults at the western foot of the Sangre de Cristo Mountains indicate a transition from Laramide contraction to Rio Grande rift extension (e.g., Figs. 9 and 10). Exposures of the Deadman Creek thrust provide evidence of a fluid-mediated sequence of progressive subhorizontal, northeast-directed contraction followed by subhorizontal, southwestern extension at depths consistent with at least greenschist-facies metamorphic conditions, all of which are compatible with known Laramide shortening directions and style followed by the initiation of Rio Grande rift extension (e.g., Erslev and Koenig, 2009; Weil and Yonkee, 2012; Yonkee and Weil, 2015).

In Huerfano Park, the foreland Greaser Creek and J M thrusts cut and override Jurassic and Cretaceous formations along the western side of the park (Fig. 11, section F–F″; Briggs and Goddard, 1956; Johnson, 1959). Beneath and in front of the thrusts, beds as young as the Eocene Huerfano Formation are upturned. Vertebrate remains from two levels of the Huerfano Formation are correlated with late Wasatchian and early Bridgerian land mammal ages, respectively (Robinson, 1966; Lichtig and Lucas, 2015). The middle Eocene Bridgerian is dated at 49–45.5 Ma (Murphey and Evanoff, 2011). Thus, deformation of the Huerfano Formation is consistent with Laramide contraction occurring as late as 49 Ma.

Structural Styles and Tectonic Evolution

Three structural styles (assemblages of related geologic structures, differentiated by basement involvement; see Harding and Lowell, 1979) comprise the major contractional elements of the Sangre de Cristo Mountains: (1) thrust sheets of basement rocks, (2) sheets of folded and thrusted sedimentary rocks that do not involve basement, and (3) blocks bounded by basement-involved thrust and reverse faults. The first two structural styles are thin-skinned and are uncommon in the Laramide foreland of Colorado and Wyoming, USA, but the latter typically defines the arches that comprise major ranges in the foreland (Erslev, 1993; Erslev et al., 2022).

Three basement thrust sheets (style 1)—the Deadman–Sand Creek, Mosca Creek, and Medano sheets—extend from a Laramide hinterland rejuvenated from the Ancestral Rocky Mountains Uncompahgre highland (Fig. 18). A complex system of folded and thrusted Paleozoic and Mesozoic rocks (style 2) comprises the Laramide foreland east of the basement thrust sheets. The eastern side of the foreland is bounded by basement-involved thrust faults of the Laramide Wet Mountain and Culebra blocks (style 3). On the western side of the range, the Crestone and post-Deadman faults reveal the edges of two Laramide blocks (style 3).

Basement thrust sheets (style 1) in the Sangre de Cristo Mountains may be as much as 4 km in thickness (Figs. 5 and 11) and, as illustrated by the Medano thrust (Fig. 11), may also be transitional with thick blocks of faulted basement (style 3). Distinguishing basement thrust sheets from thick basement blocks is difficult without data on subsurface structure, but sheet-like geometry is favored by distinctive stratigraphic similarities in hanging walls and footwalls, especially as exposed in windows of the Deadman Creek and Mosca Creek thrusts, and over areas exceeding 10 km in the direction of thrust movement. Such thin-skinned basement thrusts are known from orogenic belts globally. In the Moine thrust belt of northwestern Scotland, the Glencoul, Ben More, and other thrusts in the brittle regime bring thin (~1 km) sheets of Archean Lewisian gneiss over a foreland of Cambrian and Ordovician formations in normal contact with Lewisian gneiss and Neoproterozoic Torridonian Sandstone (Butler et al., 2006; Fox and Searle, 2021). Mylonite zones in hanging walls are thin (~1 m; Butler et al., 2006). Such brittle regime, low-angle basement thrusts likely represent low-taper wedges of high-strength crust riding on weak detachments (Suppe, 2007).

Similar diversity of structural styles occurs at two other localities in the Laramide foreland of North America. In the Madison Range of southwestern Montana, the Scarface thrust brought a thin sheet of Archean gneiss (style 1) over folded and thrusted Paleozoic cover rocks (style 2; Schmidt et al., 1993a; Tysdal, 1986). The thin-skinned Scarface thrust (style 1) is interpreted as the forward extension of a thick-skinned basement thrust (style 3; Schmidt et al., 1993a). In a second example, from the Fra Cristobal Range of New Mexico, sheets of folded and thrusted sedimentary rocks (style 2), extend forward from thick-skinned basement thrusts (style 3; Nelson, 1993). Such basement faults in the foreland were interpreted to have formed in the shallow brittle regime along preexisting shear zones (Chase et al., 1993; Schmidt et al., 1993b).

Globally, styles 2 and 3 occur together in the Malargüe fold-and-thrust belt in the Andes Mountains of Argentina (Giambiagi et al., 2008), in the Mediterranean Alpine orogen in northern Italy (Giudicarie belt; Verwater et al., 2021), and in many other orogenic belts (LaCombe and Bellahsen, 2016; Pfiffner, 2017). The Mediterranean localities do not have flat-slab or shallow subduction-related forelands, but they may offer insight into the relations between structural styles and their varying kinematics. First, in these multi-style localities, thin-skinned thrusts in cover rocks are rooted in deep, thick-skinned basement thrusts. These examples imply that the thin-skinned thrust system—both basement- and cover-involved—of the Sangre de Cristo Mountains has a deep crustal root that is designated the “Laramide hinterland” (Fig. 18). Second, in the Giudicarie belt of northern Italy, displacement is in the Lombardian Basin, where the thin-skinned Tremosine-Tignale fault follows a long detachment in thick basin fill (Verwater et al., 2021). The Lombardian Basin example may be comparable to the extensive development of thin-skinned structures in the Sangre de Cristo Mountains, where thrusts are observed to have been localized along lithologically weak zones in the thick and heterogenous fill of Ancestral Rocky Mountains basins. The Sangre de Cristo Mountains contain some of the thickest (Nelson and Lucas, 2011) and most stratigraphically variable (Lindsey et al., 1986b) basin fill in the Rocky Mountain foreland.

The sequence of thrusting in the Sangre de Cristo Mountains is difficult to establish; most structures may have developed almost simultaneously (Fig. 18). We propose that the thin-skinned basement thrust sheets now spanning much of the width of the present northern Sangre de Cristo Mountains formed by Laramide reactivation of optimally oriented Ancestral Rockies faults near the highland-trough boundary. Final emplacement of basement sheets probably occurred after the development of thin-skinned faults and folds in the foreland (Figs. 4 and 7). In Huerfano Park, thin-skinned thrusting of the foreland continued into the Eocene. Simultaneously, blocks began to rise along basement-involved thrusts. Uplift and erosion of the Wet Mountain (Briggs and Goddard, 1956; Rasmussen and Foreman, 2017) and Culebra (Lindsey, 1998; Bush et al., 2016) basement blocks began in the Late Cretaceous and culminated in the Eocene. These blocks probably functioned as a buttress to thin-skinned deformation from the west. The Crestone block rose before the Sand Creek thrust developed, possibly during the Pennsylvanian to Permian (Hoy and Ridgway, 2002) or the Late Cretaceous. Uplift of the post-Deadman block postdates the Deadman Creek thrust, and probably the Sand Creek thrust, if the latter two are contiguous.

The Missing Erosional Record of the Laramide Orogeny

The record of unroofing the Sangre de Cristo Mountains is not found in Huerfano Park, in contrast to farther south along the western side of the Raton Basin and north along the range front and northern Wet Mountains. Although once considered to have been derived from the Laramide Sangre de Cristo Mountains (e.g., Briggs and Goddard, 1956), the Paleocene Poison Canyon and Eocene Cuchara formations in Huerfano Park are now considered to have been derived from the Wet Mountains north of Huerfano Park (Rasmussen and Foreman, 2017; Rasmussen et al., 2020; Zhu and Fan, 2018). The apparent lack of alluvial sediment derived from Laramide thrust sheets in the northern Sangre de Cristo Mountains contrasts with the western margin of the Raton Basin to the south, where Late Cretaceous to Eocene alluvial sediment accumulated in front of the upthrust Culebra range (Lindsey, 1998; Bush et al., 2016). Two possible explanations, or a combination of both, may explain the contrast.

Explanation 1

Horizontal-versus-vertical separation along thin-skinned thrusts in the northern Sangre de Cristo Mountains during the Laramide orogeny differs from that farther south in the central Culebra Range (Applegate and Rose, 1985; Lindsey, 1998). In the former, contraction produced low-angle thrusts, whereas farther south, contraction was mainly along high-angle, basement-involved reverse faults and thrusts. Estimated separations are 10+ km horizontal and ~1–4 km vertical in the north but 2–5 km horizontal and ~10 km vertical farther south along the eastern margin of the Culebra Range (compare Figs. 5 and 11 with sections by Lindsey, 1998). This contrast means that Laramide rock uplift and associated erosion was greater in the south than in the north. Lower rock uplift in the northern Sangre de Cristo Mountains could explain the lack of Laramide sediments derived from thrust sheets west of Huerfano Park, which is consistent with dominantly thin-skinned deformation. Major footwall uplift of the northern Sangre de Cristo Mountains was probably initiated by Rio Grande rift extension at ca. 25 Ma in the Miocene (age is from Ricketts et al., 2015b).

North of the study area, a widespread erosion surface had developed by the late Eocene throughout the Front Range and Wet Mountains (Taylor, 1975). This surface undoubtedly extended farther west, into the area north of the study area, where thin-skinned structures exit the western side of the northern Sangre de Cristo Mountains. At Goat Creek (Fig. 1B) on the eastern side of the Sangre de Cristo Mountains, and on the plateau of the northern Wet Mountains, easterly flowing streams cut paleo valleys in the surface at 35–29 Ma (Scott and Taylor, 1975; Taylor, 1975). These streams may have shed sediment from the thrust system eastward into the Denver Basin, but perhaps not southeast into the Huerfano and Raton basins.

The contrast in vertical separation between the northern (present study area) and central Sangre de Cristo Mountains (Culebra Range) may be explored by considering alternative scenarios for erosion of thrust wedges (Konstantinovskaia and Malavielle, 2005). Their experiments suggest that erosion is generally higher for wedges with high-basal friction, such as high-angle thrust blocks in basement rocks of the Culebra Range, than in wedges with low-basal friction, such as thin-skinned thrust sheets riding on weak sedimentary rock in the study area.

Explanation 2

Thrust sheets in the northern Sangre de Cristo Mountains did achieve sufficient rock uplift, but the detritus shed from them was diluted with detritus shed from the Wet Mountain block. Streams draining the Laramide Sangre de Cristo Mountains may have been diverted by the Wet Mountain block during the Paleocene and Eocene. The Laramide Wet Mountain block abutted the foreland of the northern Sangre de Cristo Mountains along the west-verging Alvarado fault (Lindsey et al., 1983), and possibly other faults northeast of Huerfano Park (Briggs and Goddard, 1956; Johnson, 1959). As such, the block included both the Proterozoic terrane of the present-day Wet Mountains and the now-downfaulted terrane beneath the Wet Mountain Valley. Streams draining both the Laramide Wet Mountains and Sangre de Cristo Mountains would have traversed extensive tracts of Proterozoic rocks. The principal differences between the two sources would have been (1) in the northern Wet Mountains, small alkalic intrusive complexes of early Paleozoic age, and (2) in the Sangre de Cristo Mountains, a foreland terrane of Paleozoic and Mesozoic sedimentary rocks. Zircon ages indicate that alkalic intrusive rocks of the Wet Mountains provided a unique source for Paleocene and Eocene formations in Huerfano Park (Rasmussen and Foreman, 2017; Rasmussen et al., 2020; Zhu and Fan, 2018). Other zircon ages from these formations are not unique to the Wet Mountains, however, and may have been derived from the Sangre de Cristo Mountains as well as the Wet Mountains. A drainage system where streams from two sources joined to flow southward into the northwestern Raton Basin (present-day Huerfano Park) is plausible (Fig. 18).

Transition from Laramide Contraction to Rio Grande Rift Extension

The Deadman Creek and post-Deadman thrusts near the Laramide hinterland were reactivated during early Rio Grande rifting. Greenschist-facies bedrock shear zones with contractional, low-angle, plastic deformation microstructures were overprinted by small strain extensional, likely plastic, microstructures such as those found at the Deadman thrust (Fig. 9), the Independence Mine fault (Sitar et al., 2022), and other subsidiary shear zones. One shear zone, also with top-to-the southwest shear sense indicative of post-Laramide extensional deformation, cuts a diorite stock dated at ca. 26 Ma (Sitar et al., 2022). At Deadman Creek, greenschist-facies, synkinematic chlorite in quartz-feldspar ultramylonite is consistent with deformation temperatures on the order of ~350–450 °C and shear zone-hosted fluid involvement that likely caused reaction weakening (e.g., Wibberley, 2005). At other localities along the western front of the mountains, temperatures of 350–650 °C have been inferred from metamorphic mineral assemblages (Malavarca et al., 2023). The age of contact metamorphism estimated from small intrusions north of Crestone is 33–35 Ma (Malavarca et al., 2023). Although crustal depths of ~10 km may be indicated for plastic deformation (e.g., Yonkee and Mitra, 1993), high-regional heat flow associated with rifting and local igneous intrusions probably drove metamorphism and plastic deformation at much shallower depths along the western side of the northern Sangre de Cristo Mountains.

Chlorite-rich, quartz and feldspar protomylonites to ultramylonites at Deadman Creek show that internal competency contrasts and discrete - muscovite-rich phyllonites are in the weakest fault-related rocks. Within all mylonites, but particularly in the phyllonites, subhorizontally foliated top-to-the northeast shear-sense indicators (contractional) are locally cut and deflected by top-to-the southwest shear-sense indicators (extensional). Restricted to the western part of the range, these crosscutting relations are indicative of the initiation of distributed extensional reactivation during the transition from Laramide to Rio Grande rift orogenesis. The deformation and mineral assemblages are also consistent with metamorphic temperature and age reported by Sitar et al. (2022) and Malavarca et al. (2023).

A discrete large displacement, low-angle normal fault system has not yet been recognized in Proterozoic basement rocks of the Sangre de Cristo Mountains. However, Benson and Jones (1990) and Watkins (1996) interpret discrete zones of clay-rich cataclasites as low-angle normal faults and associated structures along the western front of the Sangre de Cristo Mountains at the San Luis Mine and at the mouth of Deadman Creek, respectively. Hudson and Moscati (2019) interpreted fault kinematic and magnetic anisotropy data to indicate lateral injection of a ca. 30 Ma granitoid sheet near the southwestern foot of Blanca Peak (Fig. 2); they interpret subsequent brittle deformation as the transition from Laramide contraction to Rio Grande extension. At regional scales along the Rio Grande Rift from Colorado to New Mexico, Fletcher et al. (2006) and Ricketts et al. (2015a) discuss a variety of low-angle extensional structures and their possible role in incipient core-complex extension.

Regionally, low-angle structures are cut by a zone of northwest–-northeast-striking, high-angle synthetic and antithetic normal faults with respect to the mountain-front Sangre de Cristo normal fault, which accommodates surface uplift of the range (McCalpin, 1982; Caine et al., 2013; Grauch et al., 2013; Drenth et al., 2017; Hudson and Moscati, 2019). Although many of the high--angle faults are new structures that diminish in number northeastward into the range, some are also reactivated along high-angle reverse faults, such as those found in association with the Deadman Creek thrust (cf. Fletcher et al., 2006). The overall structural relief of the Sangre de Cristo Mountains from the deepest buried basement contact with overlying rift-fill sediments to the range crest is on the order of 8.2 km (Kluth and Schaftenaar, 1994). Vertical displacement is distributed among several branches of the Sangre de Cristo fault; the high-angle normal fault with the largest displacement lies west of the Sangre de Cristo Range front (Drenth et al., 2017; Grauch et al., 2013). This zone of high-angle normal faulting likely produced the footwall uplift exposing Sangre de Cristo thrust system.

We conclude by summarizing our findings on five key topics:

  1. Age of deformation: Thin-skinned thrusts in Paleozoic and Mesozoic sedimentary rocks of the foreland are clearly Laramide, involving formations as young as 49 Ma. Thin-skinned basement thrusts that carry remnants of Ancestral Rocky Mountains highlands are interpreted as faults of the Ancestral Rocky Mountains that reactivated during the Laramide orogeny, because they override Laramide foreland structures. Thick-skinned basement thrusts were active throughout the Laramide orogeny. Except for one possibility (the Crestone thrust), there is no evidence that these were reactivated from faults of the Ancestral Rocky Mountains. Coeval thin- and thick-skinned deformation indicates that both formed together in response to the regional stress field of the Laramide foreland.

  2. Structural style: The northern Sangre de Cristo Mountains contain a system of thin-skinned thrusts in Proterozoic basement rocks and an eastern foreland of thin-skinned thrusts in Paleozoic and Mesozoic sedimentary rocks. These two structural styles are rarely found in the rest of the Laramide Rocky Mountain foreland. The root zone for this system of thin-skinned thrusts lies beneath the present San Luis Valley. Additionally, around the periphery of the mountains, four thick-skinned thrust systems delineate partial boundaries of uplifted basement blocks. These thick-skinned thrusts resemble those in the rest of the Laramide Rocky Mountain foreland. The unusual development of thin-skinned styles in crystalline basement and sedimentary rock cover in the northern Sangre de Cristo Mountains required two pre-existing conditions to localize deformation: (1) thick basin fill with stratigraphic variation in rock competency and (2) gently inclined weak zones in crystalline basement rocks. Thick basin fill was provided by >2 km of interbedded shaley and sandy sediment shed from the Ancestral Rocky Mountains. Ancestral Rocky Mountain faults that cut basement at angles typical of reverse or thrust faults and layer-parallel weak zones in basin fill provided pathways for detachment. Forelands formed by flat-slab or shallow subduction will lack thin-skinned structural styles if these two conditions are not present.

  3. Topographic response: Absence of a record for erosion of the thin-skinned tract in the study area contrasts with areas to the south and north. To the south, the sedimentary fill of the Raton Basin records erosion of the uplifted Culebra range. North of the study area, where thin-skinned deformation exits the western side of the range, paleo valleys on the eastern side indicate erosion of the range. We infer that rock uplift in the thin-skinned tract was not accompanied by erosion (explanation one in the discussion). Major footwall uplift did not occur until Rio Grande rifting. Lack of a record of erosion in adjacent basins, or in the form of paleo-valleys carved into the exhumed landscape, may signal thin-skinned deformation.

  4. Deformation conditions and the transition from contraction to extension: Most faults in the sedimentary cover rocks in the study area formed in the brittle regime, which is consistent with thin-skinned deformation in other orogenic belts. However, the exposed hindward part of at least one basement thrust (Deadman Creek) and post-Deadman faults and shear zones that cut it reveal that plastically deformed mylonite formed at temperatures of 350 °C to 450 °C, which is consistent with estimates of temperatures based on metamorphic mineral assemblages that may represent conditions in the root zone of the Laramide hinterland. If metamorphic mineral assemblages are representative of the root zone, fluid-assisted reactions likely weakened rock in shear zones, localizing strain and initiating the formation of basement slivers. Although there currently are no Laramide dates on syntectonic mineral phases within plastic shear zones, microstructural observations of overprinting kinematics do provide indirect clues leading to a better understanding of the broader tectonic framework. Plastic microstructures showing top-to-northeast and top-to-southwest shear sense in mylonite shear zones also may record the transition from Laramide contraction to Rio Grande Rift extension.

  5. Implications for contractional orogenesis: The diversity of contractional structural styles mapped in the Sangre de Cristo Mountains of Colorado is uncommon in continental forelands inboard of orogenic belts. Conditions that may contribute to the presence and recognition of these styles that are more commonly found alone include transmission of orogenic stress deep into continental interiors; interaction of thick, basement-involved wedges with thin-skinned wedges; thick sequences of mechanically diverse cover rocks; long-lived shear zones weakened by fluid-mediated alteration; and erosion exposing upper crustal styles of deformation. The Sangre de Cristo Mountains provide an important example of the conditions required for mixed styles of contractional deformation in continental crust.

1Supplemental Material. Tables showing locations of cross-sections and data sources (S1) and measurements of foliations and fault slip data (S2). Please visit https://doi.org/10.1130/GEOS.S.25387897 to access the supplemental material, and contact editing@geosociety.org with any questions.
Science Editor: David E. Fastovsky
Associate Editor: Valerio Acocella

Tien Grauch provided inspiration and many conversations in our utilization of available aeromagnetic data in the construction of cross sections, geology, and evolution of the Rio Grande rift. Chris Holm-Denoma and Laura Pianowski provided zircon U-Pb laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses, data, and interpretative dates. Terry Klein, Ed DeWitt, and Carma San Juan compiled a previous version of the simplified geologic map (Lindsey, 2010). We thank the employees of the Great Sand Dunes National Park and Preserve, Mosca, Colorado, USA, especially Andrew Valdez and Fred Bunch, for their assistance and hospitality during fieldwork. The National Park Service provided camp accommodations for D.A. Lindsey. Robert Webster and Jacob Weigel provided copies of their M.S. theses. Discussions with Mark Hudson, John Singleton, and faculty and students at Colorado State and Washington and Lee universities were invaluable. Joye Fehringer, University of Nebraska Foundation, helped locate a copy of Russell Smith’s Ph.D. thesis. The library staff at the University of Michigan loaned an original copy of Smith’s thesis as well as photocopies of M.S. theses in their collection. Brandon Lutz, Jay Chapman, two anonymous reviewers, and the Geosphere editorial staff offered many helpful suggestions for improving the manuscript. This work was supported in part by a U.S. Geological Survey Bradley Scholar grant to D.A. Lindsey and by the U.S. Geological Survey Mineral Resources Program. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Gold Open Access: This paper is published under the terms of the CC-BY-NC license.