Load-Induced Subsidence of the Ancestral Rocky Mountains Recorded by Preservation of Permian Landscapes

Home , Midcontinent Rift System, Ouachita orogeny, Paradox Basin, Yeso Group

Load-induced subsidence of the Ancestral Rocky Mountains recorded by preservation of Permian landscapes

Gerilyn S. Soreghan1, G. Randy Keller1, M. Charles Gilbert1, Clement G. Chase2, and Dustin E. Sweet3 1Conoco-Phillips School of Geology & Geophysics, University of Oklahoma, Norman, Oklahoma 73019, USA 2Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA 3Department of Geosciences, Texas Tech University, Lubbock, Texas 79409, USA

ABSTRACT to buckling and thrust formation with the coincide spatially with much older structures application of suffi cient compressive stress, linked to the Precambrian–Cambrian rifting of The Ancestral Rocky Mountains (ARM) and subsidence of topography formed by the Rodinian supercontinent (Ham et al., 1964; formed a system of highlands and adja- buckling upon relaxation of the high com- Perry, 1989; Fig. 3). cent basins that developed during Penn- pressional stresses. We therefore infer that The ARM form a classic example of intra- sylvanian–earliest Permian deformation of the core ARM highlands subsided owing to plate orogeny and remain enigmatic, although interior western North America. The cause the presence of a high-density upper crustal several authors have linked the orogenesis to of this intracratonic deformation remains root, and that this subsidence began in the far-fi eld effects of the Marathon-Ouachita con- debated, although many have linked it to far- Early Permian owing to relaxation of the vergent margin (e.g., Kluth and Coney, 1981; fi eld compression associated with the Car- in-plane compressional stresses that had Kluth, 1986; Algeo, 1992; Dickinson and Law- boniferous–Permian Ouachita-Marathon accompanied the last phase of the Ouachita- ton, 2003). New data and reanalysis of exist- orogeny of southern North America. The Marathon orogeny of southern and south- ing data indicate that even the termination of ultimate disappearance of the ARM uplifts western Laurentia. Our results highlight the the ARM orogeny is enigmatic. It has been long has long been attributed to erosional bevel- importance of tectonic inheritance in intra- accepted that the ARM highlands continued to ing presumed to have prevailed into the Tri- plate orogenesis and epeirogenesis, including rise from middle Pennsylvanian through at least assic–Jurassic. New observations, however, its potential role in hastening the reduction Early Permian time, and that subsequent ero- indicate an abrupt and unusual termination of regional elevation, and enabling the ulti- sional beveling associated with isostatic adjust- for the largest of the ARM uplifts. Field evi- mate preservation of paleolandscapes. ment over tens of millions of years ultimately dence from paleohighlands in the central obliterated the mountains by Triassic–Jurassic ARM of Oklahoma and Colorado indicates INTRODUCTION time (e.g., Lee, 1918; Mallory, 1972; Blakey, that Lower Permian strata onlap Pennsyl- 2008); however, we present new observations vanian-aged faults and bury as much as The Pennsylvanian–Permian Ancestral Rocky of signifi cant preserved paleorelief on top of 1000 m of relief atop the paleohighlands. In Mountains (ARM) of the west-central U.S. ARM uplifts that challenge this view. This parts of Oklahoma and Colorado, late Ceno- (Fig. 1) formed a collection of largely crystal- paleorelief preservation is remarkable because zoic partial exhumation of these paleohigh- line basement-cored highlands that shed debris it archives landscapes of great antiquity, and lands has revealed landscapes dating from into adjacent basins in western equatorial appears to record subsidence of highland and Permian time. These relationships suggest Pangea far from any recognized plate bound- adjacent regions not previously recognized. cessation of uplift followed by active sub- ary (e.g., Kluth and Coney, 1981; Kluth, 1986). Here we combine geologic mapping, strati- sidence of a broad region that encompassed The term “Ancestral Rockies” arose nearly a graphic, petrologic, structural, and geophysi- both basins and uplifted crustal blocks and century ago, in recognition of the thick, coarse- cal data from some of the largest-magnitude that commenced in Early Permian time, grained strata that wedge toward Precambrian ARM highlands and intervening regions to directly following the Pennsylvanian tectonic basement regions of the modern Rockies (Lee, document an episode of widespread subsidence apogee of the ARM. Independent from these 1918; Melton, 1925). Many of these paleo- that followed the tectonic apogee of the ARM geological observations, geophysical data highlands are bounded by high-angle, Pennsyl- orogeny. We then integrate these observations reveal a regional-scale mafi c load under- vanian-aged faults refl ecting signifi cant (several with documentation of a high-density crustal pinning these paleohighlands, emplaced kilometer) dip-slip offset, as well as lateral dis- load underpinning the core ARM, and model during Cambrian rifting associated with the placements (e.g., McConnell, 1989; Thomas, the possible effects of this load in light of the southern Oklahoma aulacogen. Geophysical 2007; Keller and Stephenson, 2007). The changing stress fi elds associated with ARM modeling of the effects of such a load in the core ARM uplifts are characterized by large orogenesis. Our analysis indicates that tectonic presence of a horizontal stress fi eld, such as structural displacements and thick (≥2 km), inheritance such as ancient mass loads in the that implied by ARM orogenesis, indicates proximally conglomeratic mantles, and extend crust or lithosphere should be considered as a that the amplitude of fl exurally supported beyond the immediate Rocky Mountains region previously unrecognized means to hasten the features is modulated nonlinearly. This leads into Oklahoma (Fig. 2). Here, ARM structures demise of orogenic highlands.

Geosphere; June 2012; v. 8; no. 3; p. 654–668; doi: 10.1130/GES00681.1; 11 ﬁ gures; 1 supplemental ﬁ le.

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Uncompahgre Front Range Wichita Notably, Larson et al. (1985) suggested that Sangre de the SOA extended to the Uncompahgre uplift Cristo Fm (E) Upper Post Oak/ Cutler Fountain Hennessey region of Colorado on the basis of distributed, Figure 1. A schematic view of the Pennsyl- Fm (W) Fm Fm but limited, Cambrian maﬁ c intrusives. Recent

vanian–Permian Ancestral Rocky Mountains E. Permian 1000 m geophysical studies corroborate this inference (ARM) system, highlighting locations noted (e.g., Smith, 2002; Casillas, 2004; Rumpel in text (modifi ed from G. Soreghan et al., et al., 2005; Keller and Stephenson, 2007; 2008). Black rectangles denote areas shown U/CF UPF MVF Pardo, 2009; details in the following). in detail in Figures 4 and 7. ARM uplifts Sangre de Lower Extensive petroleum exploration of the south- coded as major are those marked by >1000 m Cristo/ Fountain Granite L. Pennsylvanian Cutler Fm Fm Wash Fm ern Oklahoma region provides good constraints of adjacent Pennsylvanian strata (see Fig. 2). on the postrift thermal subsidence history, which Inset at top depicts deformed late Pennsyl- Un co 40° m FF includes ~3 km of predominantly Ordovician P Front Range- vanian (syntectonic) and onlapping Early ara p ? do a carbonate strata preserved in uplifted blocks x h Apishapa Permian (post-tectonic) stratigraphic rela- B SC a g s r and within the axis of the proto-Anadarko basin i e tions in the ancestral Uncompahgre, Front n (Johnson et al., 1988). Following thermal sub- Range–Apishapa, and Wichita uplifts; the Wichita A na sidence and associated Ordovician sedimenta- da thick horizontal line schematically depicts BD rko Bas tion in the wake of Cambrian rifting, subsidence the transition between syntectonic and post- in within the SOA region and greater interior North tectonic strata (sources: data in Fig. 2 and America slowed considerably. In southern Okla- DeVoto, 1980; Hoy and Ridgway, 2002; homa, a relatively thin Silurian–lower Missis- Sweet and Soreghan, 2010). Stratal names sippian carbonate and shale section records this are shown for both the eastern (E) and west- ARM Highlands e interval of tectonic quiescence (Feinstein, 1981). major r u ern (W) regions of the Uncompahgre uplift. t minor u This period was followed by a Mississippian– Other abbreviations denote outcrop areas of S n well in Fig. 2 o 30° Pennsylvanian subsidence event heralding the the Fountain Formation (FF) and Sangre de O th uac ara beginning of the present Anadarko basin and Cristo Formation (SC), and the subsurface 110° hita M accompanying the Ouachita orogeny (Garner location of Bravo Dome (BD), regions also 120°W 100°W 80°W and Turcotte, 1984; Arbenz, 1989). mentioned in the text and fi gures. U/CF— Uncompahgre and Crestone faults, UPF— 40°N Late Paleozoic Ancestral Rocky Mountains ancestral Ute Pass fault, MVF—Mountain View fault. By latest Mississippian and Pennsylvanian 30°N time, the Ancestral Rocky Mountains orogeny commenced, as recorded by uplift of various highlands and major subsidence and sediment accumulation within highland-adjacent basins GEOLOGIC SETTING: EARLY AND Hoffman et al., 1974; Kruger and Keller, 1986; (e.g., Kluth and Coney, 1981; Kluth, 1986). The LATE PALEOZOIC GEOLOGIC Perry, 1989; Keller and Stephenson, 2007; core ARM uplifts exhibiting the largest-magni- EVENTS OF THE NORTH AMERICAN Fig. 3). The SOA is a classic example of an tude fault displacements across basin-bounding MIDCONTINENT intracontinental failed rift that was later tec- faults, and thickest mantles of locally derived, tonically inverted, and geologic studies indi- coarse-grained conglomerate, occur in Colorado Major tectonic events that affected the North cate that a combination of thrusting and lateral and Oklahoma, most notably in the Wichita- American midcontinent through Phanerozoic movements occurred during its formation (e.g., Anadarko and Uncompahgre-Paradox systems time include early Paleozoic (Early Cambrian) Ham et al., 1964; Brewer et al., 1983; Thomas, (Figs. 1 and 2). rifting associated with the breakup of the 2011). Knowledge of the Cambrian extension Within southern Oklahoma, the Wichita Rodinian supercontinent, and late Paleozoic is based on many studies of the bimodal igne- uplift–Anadarko basin system formed as a result (Pennsylvanian–Permian) compression associ- ous rocks exposed in the Wichita uplift of Okla- of late Mississippian–Pennsylvanian ARM com- ated with the assembly of the Pangean super- homa (summarized in Gilbert, 1983), regional pression that inverted the failed Cambrian rift continent (references following). Although the relationships (Keller et al., 1983), and postrift (Larson et al., 1985; Gilbert, 1992). The inver- latter event forms the focus of this paper, the subsidence (Hoffman et al., 1974). Cambrian sion structures are unusually large, with at least early Paleozoic events imparted a tectonic fabric igneous activity resulted in intrusion of a volu- 12 km of vertical separation between the Cam- that possibly infl uenced later deformation. minous gabbroic complex (Glen Mountains brian basement exposed in the Wichita Moun- Layered Complex) and associated shallow tains and that present in the subsurface of the Early Paleozoic Southern intrusives (Hogan and Gilbert, 1997). As much adjacent Anadarko basin (Perry, 1989; Keller Oklahoma Aulacogen as 40,000 km3 of metaluminous silicic mag- and Stephenson, 2007). This 12 km displacement mas were generated ca. 530 Ma, producing the results from the dual-phase history of the SOA- A series of igneous rocks and associated Carlton Rhyolite Group (and intrusive Wichita Anadarko system, wherein the basin contains crustal-scale structures extending at least Granite Group; Ham et al., 1964). This assem- 4–5 km of rift-related lower Paleozoic section through Oklahoma and the Texas panhandle blage of Lower Cambrian granite, rhyolite, and and an additional 7 km attributable to fl exurally mark the trend of the so-called southern Okla- gabbro forms the basement of southwestern induced subsidence related to Mississippian– homa aulacogen (SOA) (e.g., Shatski, 1946; Oklahoma and the Texas panhandle (Fig. 3). Pennsylvanian compressional deformation

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0 110° 105° ° 95° 100 0

0 0

40° 1000

1000 0

0 0 2000 40° 0

00 1000 50 0 0

0 500

500

2 0

0 Oz 0 0 ark Dome

1000

0 0 20001500 35°

0 3000 0

50 0 0 0 0 35°

500 50 0 WU 0 0

0 500 0 0

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0 1500 Major ARM Uplift 0 500 Isopachs 0 Minor ARM Uplift Dashed in areas of poor control; 0 30° dotted where projected. Positive region Thickness in meters

km 0 0 0 50 100 150 200 250 0 Ouachita Orogenic Belt 30°

110 ° 105° 100° 95°

Figure 2. Isopach map showing preserved Pennsylvanian strata of the region shown in Figure 1. ARM—Ancestral Rocky Mountains. Note the large (>1000 m) thicknesses of Pennsylvanian strata adjacent to the Uncompahgre, Front Range–Apishapa, and Wichita uplifts (WU; see Fig. 1). Bold black lines are faults. Modiﬁ ed from McKee and Crosby (1975).

during the ARM orogeny (Johnson, 1989; Perry, basinward of the Wichita uplift) of Oklahoma tical displacement documented from drilling 1989). The length of the SOA (~1500 km; Keller and the Texas panhandle reaches thicknesses and seismic data bordering the Uncompahgre and Stephenson, 2007) approximates that of the of 2–3 km and exhibits a well-known reverse uplift (Frahme and Vaughn, 1983; White and Main Ethiopia and Kenya rifts combined, and stratigraphy representing the active unrooﬁ ng Jacobson, 1983). Adjacent basins such as the the signature transects the prevailing northeast- of the Wichita uplift during Pennsylvanian time Paradox basin accumulated several kilometers southwest grain of the Mesoproterozoic base- (Edwards, 1959; Johnson et al., 1988). Minimal of syntectonic carbonate-clastic strata, and thick ment of North America (Karlstrom and Bowring, post-Paleozoic deformation in the craton region conglomeratic aprons mantling several uplifts 1998). Pennsylvanian strata are not well exposed of the southern mid-continent has enabled nearly (Mallory, 1972; DeVoto, 1980). in this region, but intense petroleum explora- pristine preservation of this system. Therefore, Judging by sedimentation rates and structural- tion has revealed the presence of thick uplift- the Anadarko basin area archives a complete biostratigraphic data, deformation that produced adjacent conglomeratic units (e.g., Tomlinson record of early Paleozoic extensional to late the Ancestral Rocky Mountains peaked in middle and McBee, 1959; Dutton, 1982; Fig. 2). For Paleozoic compressional deformation. Pennsylvanian to earliest Permian time, albeit example, the so-called Granite Wash within the Similar relationships exist in Colorado. ARM with some spatial variation, such as Permian subsurface along the Frontal fault zone (i.e., tectonism resulted in as much as 8 km of ver- ages of deformation in the Marathon region of

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The rise and demise of the Ancestral Rocky Mountains N

70°

40°

30° 50° c avapai-

n dashed Grenville Front Grenville

80°

Front ed crustal density Grenville Superior Province

500 km

EGR 90° rift Reelfoot

MCR MCR Early Paleozoic continental margin Province Superior SGR ′ A

c intrusive complex, underlain by the Central Basin Platform, an A AGM 100° tzal WU

Trans- a aulacogen Orogen Hudson PMIC

ed from Barnes et al., 1999). Inset at left shows a simpliﬁ ed from Province

WM Yavapai-Maza section; detailed version of the central portion this cross a more ; see Fig. 10 for ′ FM SU

110° Southern Oklahom Province Wyoming ed from Keller and Stephenson, 2007). Keller ed from

UU ; modiﬁ –3 ′ A × 10 3 3.3

Ouachita thrust front MCR

Proterozoic 120°W MOHO Observed Gabbro Carlton Rhyolite 2.72 Calculated COCORP Anadarko Basin 2.5 Rift Fill 2.63 A UP 3.0 LP G Distance (km) 2.95 Seismic refraction control numbers are densities (in kg/m numbers are Figure 3. Regional tectonic index map of southern Laurentia showing features of interest in this study. Proterozoic terranes (Y Proterozoic in this study. of interest showing features 3. Regional tectonic index map of southern Laurentia Figure rifts and mafi Proterozoic SGR—Southern granite-rhyolite province). EGR—Eastern granite-rhyolite province; Mazatzal Province, (MCR—Midcontinent Rift System; PMIC—Pecos mafi intrusives in green AGM—Abilene gravity minimum (ca. 1.4 Ga?). Cambrian Ancestral Rocky Mountains [ARM] uplift; FM—Franklin outcrops). The Mountains. WM—Wet uplift. WU—Wichita uplift; SU—Sierra Grande UU—Uncompahgre ARM features: rifts in magenta. denoted by the orange and gree and the early Paleozoic continental margin are location of the Ouachita thrust front approximate lines, respectively (from Keller and Stephenson, 2007; modifi Keller (from lines, respectively model across the Wichita-Anadarko system (line A–A system (line Wichita-Anadarko the model across Uplift 100 200 300 400 Wichita Crust 2.7 Crust 3.0 Proterozoic 2.6 Upper Lower 0 0 Hardeman Basin A

0 0

30 20 40 10 Depth (km) (km) Depth Bouguer Gravity (mGal) (mGal) –40 Gravity –80 Bouguer

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southwest Texas (Kluth and Coney, 1981; Kluth, paleorelief on the Cambrian basement (Fig. 4). ment demonstrate that the Wichita Mountains 1986; Algeo, 1992; Trexler et al., 2004; Poole At the surface, stream drainages are visible as observed today represent a late Paleozoic et al., 2005). Owing in part to this timing, which today carved into both Cambrian igneous base- (Early Permian) landscape undergoing exhuma- coincides with that of the Ouachita-Marathon ment and Cambrian–Ordovician carbonates, but tion for the fi rst time since the Early Permian. orogeny, Kluth and Coney (1981), Kluth (1986), these drainages commonly do not propagate That is, the paleomountains are being progres- Dickinson and Lawton (2003), and others linked headward into the superjacent Permian strata, sively exposed as erosion removes the friable the intraplate deformation of the ARM to far- despite the less competent nature of the latter. Lower Permian mudstone units that mantle fi eld effects of the Ouachita-Marathon orogeny, Rather, drainages appear to have been beheaded the paleolandscape (Fig. 4). The granitic hills a model that seems particularly applicable for (crosscut) by horizontal Lower Permian strata of the Wichita Mountains display minimal the Wichita-Anadarko system of Oklahoma. In (Gilbert, 1982; Donovan, 1986). Numerous evidence for modern erosion; for example, this model, the ARM intracratonic deformation shallow wells drilled in the 1950s (Ham et al., alluvial fan mantles of granitic material do not stems from activation of preexisting weaknesses 1964) reveal a carapace of Permian strata as occur. Rather, the crosscutting relationships of by propagation of far-fi eld stresses associated much as 1 km thick onlapping the basement of Lower Permian strata across drainages carved either with (south-dipping) subduction of prom- the Wichita highland, thus revealing the magni- into Cambrian basement date the landscape to ontories or wrenching of Laurentia as eastern tude of the onlap. the pre–Early Permian. This profound noncon- parts of the Pangean suture locked (Kluth and Seismic data corroborate and expand upon formity has long been recognized (Ham et al., Coney, 1981; Kluth, 1986; Algeo, 1992; Dick- these surface and well-bore observations. The 1964; Johnson et al., 1988), but its tectonic inson and Lawton, 2003; Poole et al., 2005). Mountain View thrust fault, the main fault of the signifi cance has largely escaped notice. Taken Marshak et al. (2000) reinforced the role of pre- Frontal fault zone, marks the boundary between together with the subsurface data, these rela- existing weaknesses by suggesting that faults the uplift and adjacent basin (Brewer et al., tionships document Early Permian subsidence associated with the ARM orogeny were formed 1983; McConnell, 1989). This fault zone is well that (1) abruptly postdates compressional uplift, initially by crustal rupturing during Protero- imaged seismically (Fig. 5), and demonstrates and (2) extends beyond the foredeep of the zoic rifting. Others suggested a connection to profound displacement in the pre-Permian sec- Anadarko basin and onto the Wichita uplift; that events (convergence or megashear activity) tion, and an onlap by minimally deformed Early is, the core of the uplift subsided along with the along southwestern or western Laurentia (e.g., Permian strata that extend from the basin onto fl anking basinal regions. Ye et al., 1996; Trexler et al., 2004; Cashman the uplift. These relationships have long been et al., 2011), most notably for features in the far- recognized and cited as evidence for a Penn- Permian Onlap in the Uncompahgre Uplift, western ARM system. sylvanian age for the deformation; however, the Colorado magnitude of the onlap (~1 km; Figs. 4 and 5) PERMIAN HISTORY OF THE CORE and the extension of the onlap from the basin In contrast to the minimally disturbed record ARM: POSTOROGENIC SUBSIDENCE? up onto and across the uplift have not been of Permian burial in Oklahoma, the ARM paleo- highlighted . uplifts of Colorado exhibit a complex history The subsidence history of the proximal affected by Mesozoic burial, reactivation of Permian Onlap of the Wichita Uplift, Anadarko basin (Fig. 6) shows that subsidence uplifts during Cretaceous–Paleogene (Laramide) Oklahoma associated with the late Paleozoic basin his- tectonism, and signifi cant Neogene exhumation tory continued into Permian time. This history associated with landscape evolution and cli- Lower Cambrian magmatic rocks in the depicts a rapid late Mississippian through late mate change in the Cenozoic Rocky Mountains Wichita Mountains of southwest Oklahoma Pennsylvanian subsidence event, a less rapid but (Eaton, 2008). Of the Colorado ARM uplifts, form the largest surface exposure of the SOA. signifi cant Early Permian event, and the even- however, the Uncompahgre system of western These units, uplifted and eroded during ARM tual cessation of subsidence by middle Permian Colorado is comparatively well preserved, with tectonism, now protrude through a mantle of time. This subsidence analysis employs a a carapace of relatively undeformed Mesozoic Lower Permian redbeds (Fig. 4) that otherwise composite stratigraphic section from wells in strata (Williams, 1964). This perhaps refl ects extend across the region. Relative to the Rocky the foredeep of the basin, and includes a rare its location within the larger Colorado Plateau, Mountain region in general, Oklahoma has foredeep well with a complete log through the which was relatively undisturbed by Laramide remained less disturbed by post-Paleozoic tec- Permian section, thus capturing a subsidence tectonism (Marshak et al., 2000). The (ancient) tonism. As demonstrated by apatite fi ssion-track event that is equivalent in age to the onlapping Uncompahgre uplift was a large northwest- and auxiliary data from the Wichita Mountains Permian strata previously documented (Fig. 6; southeast–trending feature that formed during and neighboring Anadarko basin, the region Supplemental File1). the ARM orogeny and was separated from the records ~800 m to 1.5 km (1–3 km inferred by The surface observations of onlap of the adjacent Paradox basin to the southwest by a Lee and Deming, 1999) of Permian–Jurassic Lower Permian strata onto Cambrian base- seismically imaged subsurface reverse fault sys- burial before denudation that began in the Late tem that exhibits as much as 8 km of up-to-the Jurassic, and ≤1.5 km of denudation in the late 1Supplemental File. PDF fi le of information on the northeast vertical displacement (Fig. 1; Frahme Mesozoic–Paleogene in response to tectonic construction of the subsidence plot. The supplemen- and Vaughn, 1983; White and Jacobson, 1983; tal fi le includes data sources for stratal thicknesses and/or climatic infl uences (Schmoker, 1986; and ages used, including well locations; procedure Trudgill and Paz, 2009). The modern Uncom- Cardott, 1989; Carter et al., 1998; Winkler et al., and equations used for subsidence calculations; pahgre Plateau composes only a part of the 1999; Hemmerich and Kelley, 2000; Eaton, lithologic porosity assumptions and sources; map ancient highland and consists of a Precambrian 2008), but no reactivation of old uplifts, as locations of wells used for the subsidence plot; and basement core mantled by Mesozoic strata, cited references. If you are viewing the PDF of this occurred in the Rocky Mountains. paper or reading it offl ine, please visit http://dx.doi except where breached by Unaweep Canyon, a Geologic relationships in the Wichita Moun- .org/10.1130/GES00681.S1 or the full-text article on large gorge in the southwestern plateau (Fig. 7) tains demonstrate that Permian strata onlap www.gsapubs.org to view the Supplemental File. that exposes the Precambrian crystalline core

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t a A e r P strata st Faul - e r Pre-PermianP

View FFZ Caddo County Comanche County arates the Anadarko arates the Lawton

cian i p Mountain v o ououp ′ d r r O - le G k 98°30 ianian-Ordov r bucbuckle Group r trata in the Wichita Mountains, Wichita trata in the A A ambramb C

Fault Moun- Wichita orms in the western km Canyon Permian

BCCF 25 km 510

Creek 015

Blue MF ′ 98°45 ) extends across the center and north side of the Wichita uplift. Wichita and north side of the the center ) extends across ′ Igneous Rocks Igneous (undifferentiated) Fault Early Cambrian le A–A le AA

Meers Permian Redbeds Kiowa County Tillman County ′ 99°00 Carbonate Lower Paleozoic Location of photo Rhyolite Cambrian ed from Ham et al., 1964). Cross section (proﬁ Ham et al., 1964). Cross ed from Permian Redbeds W ′ 99°15 Granite Cambrian Altus Cambrian Granite Jackson County Gabbro Cambrian N ′ ′ ′ 35°00 1 34°30 T S 34°45 Figure 4. Map, cross section, and photo illustrating regional structure and burial of paleorelief and faults by Lower Permian s and faults by Lower and burial of paleorelief structure section, and photo illustrating regional 4. Map, cross Figure Oklahoma (map and cross section modiﬁ Oklahoma (map and cross basin and the uplift, is buried by Permian strata (seismically imaged in Fig. 5). Photo shows partially emergent paleolandf FFZ—Frontal fault zone (includes Mountain View fault), MF—Meers fault, BCCF—Blue Creek Canyon fault. The Frontal fault zone sep The Frontal Canyon fault. fault), MF—Meers fault, BCCF—Blue Creek View fault zone (includes Mountain FFZ—Frontal and is onlapped by Permian strata. through, Cambrian igneous basement (unfaulted) protrudes tains, where

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Frontal Thrust North South Wichita Uplift Zone Anadarko Basin

0.2

0.4

Permian

0.6 ~2 km TWTT (s)

0.8 Wolfcampian

1.0 ~40 km

Figure 5. Seismic image of the Wichita Frontal fault zone and Permian overlap succession (line location is shown in Fig. 10). TWTT—two- way traveltime. The blue lines mark the approximate top and bottom of the Lower Permian (Wolfcampian) section. Note the presence of ~600 m of Permian strata preserved on top of the uplift and ~1500 m in the proximal basin in this view; these values do not include the Permian subsequently eroded during ongoing Cenozoic exhumation. This image was provided by Petroleum Geo-Services from their three- dimensional Wichita Mountain Front reﬂ ection surveys (modiﬁ ed from Rondot, 2009).

of the plateau. Clastic detritus eroded from the highland during Carboniferous–Permian tec- Geologic Time (Ma) tonism accumulated in the adjacent Paradox basin 500 450 400 350 300 250 to form the Cutler Formation (Wengerd, 1962; Mallory, 1972; Campbell, 1980; Mack and Ordovician Silurian Devonian Mississippian Penn. Permian 0 Rasmussen, 1984; Condon, 1997; Dubiel et al., Carboniferous Anadarko Basin Subsidence Permian Epeirogenic Subsidence 2009; Soreghan et al., 2009a). The contact between the Cutler Formation Cambrian Rift 2000 and Precambrian basement along the southwestern front of the modern Uncompahgre Pla- teau is well exposed near Gateway, Colorado 4000 (Fig. 7). In this location, Cater (1955) mapped the contact as a depositional onlap; Frahme Tectonic Subsidence (Lone Star) and Vaughn’s (1983) analysis of seismic data Tectonic Subsidence (Weller) 6000 to the northwest revealed a zone of reverse Decompacted Subsidence (Lone Star) faults in the subsurface. Recent detailed map- Decompacted Subsidence (Weller) ping (Moore et al., 2008; Eccles et al., 2010; 8000 Fig. 7) conﬁ rms Cater’s (1955) depiction of a

300 Depth (m) 0 m OK Permian (Cutler) onlap contact, and expands 10000 the recognized extent of the onlap onto Pre- cambrian basement. Furthermore, these map- Lone Star ping results indicate little deformation during 6000 m Rogers #1 12000 the time recorded by the Cutler Formation now exposed at the surface, a point also emphasized D Weller U #51-11 by Cater (1970). These relationships indicate D 14000 U that motion on the subsurface Uncompahgre 50 km Wichita Uplift fault largely ceased before deposition of the youngest (Permian) Cutler strata, as Cater (1970) originally suggested. Figure 6. Paleozoic subsidence history of the Anadarko basin, illustrating the dual history At this location, the post-tectonic Permian of Cambrian rifting and thermal subsidence followed by late Paleozoic compressional sub- Cutler Formation buries ~520 m of paleorelief sidence. This analysis was constructed by compositing the Cambrian through Pennsylvanian on Precambrian basement, observable in outcrop (Penn.) record from the Lone Star Rogers #1 well and the Permian record from the Weller as documented on published maps (Cater, 1955; #51–11 well, both located in the foredeep of the Anadarko basin (Supplemental File [see Moore et al., 2008; Fig. 7). Furthermore, the footnote 1]). Note the signifi cant subsidence through the Early Permian (see text). U— Cutler Formation here projects into Unaweep upthrown; D—downthrown. The amount of Permian (decompacted) sediment accumula- Canyon, a hypothesized exhumed landscape with tion here in the foredeep of the basin is ~2.5 km. (For details of plot construction, see foot- remnant Pennsylvanian–Permian fi ll (Soreghan note 1.) et al., 2007, 2009b). The age of the canyon fi ll is inferred from the combined evidence for its

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exclusively Precambrian provenance, Pennsyl- pahgre uplift is also consistent with Cater’s PERMIAN ONLAP AND PROVENANCE vanian–Permian palynomorph content, and (1970) observation that well data near the Gate- RELATIONSHIPS IN THE GREATER shallow (late Paleozoic) paleomagnetic inclina- way area show > 2000 m of Cutler strata on top ARM REGION tions (detailed in Soreghan et al., 2007, 2009b). of Precambrian basement, and thus subsidence A pre-Mesozoic age for the Precambrian (inner) of the upthrown block. Structural relationships analogous to those gorge of Unaweep Canyon is also inferred from These data indicate that the paleo canyon was documented for the Uncompahgre and Wichita geomorphologic relationships wherein Meso- backfi lled by the end of deposition of the upper- systems also exist in the intervening ARM zoic strata crosscut tributary valleys carved in most Cutler Formation in Early Permian time. regions (Fig. 1). In southern Colorado, Penn- Precambrian basement (Soreghan et al., 2007), The Triassic Moenkopi Formation between sylvanian–Permian strata bury ARM faults and analogous to the crosscutting relationships the Permian Cutler and Triassic Chinle units adjacent basement by 800–1000 m (DeVoto, noted for the Wichita Mountains. Acceptance in the proximal Paradox basin thins to almost 1980; Hoy and Ridgway, 2002). Farther to the of the antiquity of Unaweep Canyon implies nothing toward the highland with a slight north, 315 m of the (Lower Permian) upper the preservation of at least 970 m of paleo- angular (<5°) unconformity between the Cut- Fountain Formation record sedimentologic, relief, as measured between the Precambrian– ler and Moenkopi units (Cater, 1955). These structural, and stratigraphic relationships that Mesozoic nonconformity contact on top of the relationships could refl ect gradual erosional indicate that these strata postdate movement Uncompahgre Plateau and the nonconformity beveling of a highland that persisted to Trias- of the local basin-bounding fault, thus requir- contact between the inferred Pennsylvanian– sic time; however, any such beveling should ing a regional mechanism for accommodation Permian canyon fi ll and Precambrian basement have produced a coarse clastic apron, yet no (Sweet and Soreghan, 2010; Fig 1). Postoro- encountered in a corehole in the base of the known strata exist that record this later (post- genic burial of the piedmont of an orogen (e.g., canyon (Fig. 7). Cutler) history. Rather, the coarse-grained, Tertiary strata of the modern Front Range) can The onlap of the uppermost (exposed) locally derived conglomeratic aprons persist simply refl ect strong erosion in the hinterland. Permian Cutler Formation onto Precambrian only into earliest Permian time. Alternatively, However, preservation of substantial paleo- basement of the (paleo) Uncompahgre uplift these relationships could refl ect reduction (via relief in the hinterland, rather than piedmont, records Permian burial of the Uncompahgre subsidence) of the highland to a low-elevation of both the Wichita and Uncompahgre systems highland. Cater (1970, p. 68) fi rst reached this surface, against which the Moenkopi Forma- precludes an explanation linked to denuda- conclusion, noting, “After the [Uncompahgre] tion onlapped, and the ultimate disappearance tion of the highland. Rather, the relationships highland attained its maximum height and of the uplift as a signifi cant eroding source by documented here demonstrate Permian burial while the Cutler was being deposited, the high- middle Permian time. Taken together, the geo- extending across the ARM highlands and land began sinking—at least along its southwest logic relationships are most consistent with the recording ~1 km of accommodation space fl ank” (brackets are ours). The inferred Penn- interpretation that the proximal Cutler Forma- (conservatively ignoring compaction effects). sylvanian–Permian age of Unaweep Canyon is tion records cessation of ARM uplift in the This implies that uplift of the mountains in consistent with this conclusion, and increases region, followed by ~1 km of subsidence of this compressional orogen ceased, and the the recognized magnitude of the onlap, from the greater region, encompassing both the core highlands, even beyond faulted fl anking ≥500 m to nearly 1000 m. proximal Paradox basin and the adjacent regions, underwent subsidence beginning in Passive post-tectonic erosional beveling of Uncompahgre highland. Early Permian time. the highland was once thought to have produced the depositional onlap along the margin of the uplift. However, the burial extends on top of the paleo-uplift, well beyond the fl anking Figure 7 (on following page). Map and cross-section data illustrating regional structure and regions, indicating that the highland must have burial of paleorelief and faults by Permian strata in the Uncompahgre uplift (Colorado, CO) been buried by at least 970 m of sediment to pre- (see Fig. 1 for additional location information). AZ—Arizona; NM—New Mexico; UT— serve the observed paleorelief. Accumulation of Utah. (A) Simplifi ed geologic map (inset) and digital elevation model of a part of the Uncom- this stratal thickness on top of the highland and pahgre Plateau (CO), focused on Unaweep Canyon; inset (lower right) on the geologic map its paleorelief in the absence of subsidence is shows the regional location on the Colorado and Uncompahgre Plateaus, and location of diffi cult to conceive for the active margin of a the cross-section X–X′ (detailed in B). The Pennsylvanian–Permian Cutler Formation (blue) compressional orogen. The observations sug- onlaps Precambrian basement of Unaweep Canyon and projects into the canyon. Geologic gest that the highland subsided, and the onlap- map data from Cater (1955), Moore et al. (2008), Eccles et al. (2010), and G. Soreghan (our ping Permian strata record burial of the uplift data). (B) Subsurface profi le across the Uncompahgre front in Utah (along cross-section and of the fault along which the highland had X–X′ in A), showing Permian onlap onto Precambrian basement (modifi ed from Moore been uplifted. That is, the uplift and surround- et al., 2008). SL—sea level. (C) Transverse cross section across Unaweep Canyon, showing ing regions underwent subsidence together. the paleorelief on the nonconformity surface. (See text for detailed discussion.) (D) Detailed The thickness of the proximally exposed Cutler geologic map of the area shown in box in A. This map highlights the onlap relationship of Formation, 965 m as indicated by the sequen- the Permian Cutler Formation onto the Permian paleorelief of the Precambrian basement, tial measured sections of the Cutler Formation originally documented by Cater (1955). The minimum amount of paleorelief buried here, in its most proximal location against the uplift observable in outcrop, is 370 m, measured between the highest elevations of the Cutler out- (Soreghan et al., 2009a), provides a minimum lier and the lowest elevation of the Precambrian onlap contact. However, these contacts are amount of subsidence on the Uncompahgre separated by a fault estimated (from Cater, 1955) to exhibit ~115–150 m of (down-to-the- highland, and closely approximates the 970 m north) offset. Palinspastically restoring this offset increases the observed paleorelief here of preserved paleorelief within Unaweep Can- to ~520 m. Note that this is a minimum, because this onlap relationship continues for an yon. The inference of subsidence of the Uncom- unknown extent into the subsurface. U—upthrown; D—downthrown.

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A)A 108°55′W Quaternary strata 108°35′W Mesozoic strata 39°

Lower Permian Gunniso Permian-Pennsylvanian 38°50′N PC basement

D) River Uncompahgre UT Plateau Colorado CO Plateau X’ X •

AZ 4 km NM 38°40′N

Y’ Inset Geologic Map

Y core

38.7°

Gateway

109° 5 km 108.5°

B ′ C X 1 km X Mesozoic strata Y Y′

SL Cutler 2633 m Formation ? ? ? Mobil McCormick Federal C-1 corehole km

Honaker Trail Formation Paradox salt 500 m 500 m Mississippian-Cambrian strata 1665 m –6 no vertical exaggeration ′ D 1980 Z Z Z 1900

Precambrian basement 1830 1700 (m)

D U Ute Creek Fault 1680

est Creek W Z′ Cutler Formation 1 km

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Provenance data for Pennsylvanian–Permian Paradox Basin Bravo Dome Anadarko Basin strata ﬂ anking many ARM uplifts shed addi- Dechelly,Dechelly, WhiteWhite HHorse-Cloudorse-Cloud ChiefChief tional light on the timing of active uplift and OOrganrgan RRock,ock, ? YesoYeso GGrouproup ElEl RenoReno GroupGroup erosion. For example, the well-documented MesaMesa FFormationsormations Permian TubbTubb Wellington—HennesseyWellington—Hennessey Pennsylvanian–Lower Permian conglomeratic Cutler WWolfcampianolfcampian Formation AAbobo strata that mantle many of the core ARM uplifts LowerLower undifferentiated Pennsylvanian of Colorado and Oklahoma are the hallmarks of CutlerCutler BBedseds undifferentiated

ARM tectonism, cited for nearly a century as the Hermosa Group pC Granite Wash basis for recognition of the ARM system (e.g., Pennsylvanian Permian Pennsylvanian Permian Lee, 1918; Melton, 1925). However, these conglomeratic units, which record local highland Distal (mixed) provenance Local (ARM) provenance erosion, persist only into the Lower Permian section. Their subsequent disappearance aligns Figure 8. Schematic provenance relations in the Paradox and Anadarko basins, and inter- with detrital zircon provenance results from vening Bravo Dome area (see Fig. 1 for general locations). Dark blue denotes synorogenic sandstone and siltstone of the greater region. If strata, light blue denotes postorogenic onlap; the color gradation between these depicts the the ARM uplifts persisted as signifi cant sedi- provisional nature of the age control. Fine stippled pattern signifi es generally fi ne-grained ment sources into Triassic–Jurassic time, then (mudstone to sandstone) redbed units; conglomeratic pattern denotes coarse clastic wedges Mesozoic strata in ARM-proximal regions along active basin margins. Dark brown denotes proximal Ancestral Rocky Mountains should exhibit a signifi cant provenance signa- (ARM) sources, whereas light brown bars denote regional sources, as inferred from pub- ture refl ecting an ARM source, but they do not lished detrital zircon data. Sources for provenance data: Paradox basin—M. Soreghan (Dickinson and Gehrels, 2009, 2010). Rather, et al. (2002), Dickinson and Gehrels (2003); Bravo Dome region—M. Soreghan et al. (2008); these Mesozoic strata refl ect a dominant source Anadarko basin—Giles et al. (2009), Templet and Soreghan (2010). from eastern Laurentia (e.g., Grenville basement exposed in the Appalachian-Ouachita system), and lack a signifi cant signature from the crys- ing (Fig. 9). In southwestern Oklahoma, both netic data, indicate that a large (>1000 km2) talline basement (either the Yavapai-Mazatzal COCORP seismic refl ection profi les (Brewer part of the Wet Mountains is cored by Cam- or Wichita provinces) coring the ARM uplifts, et al., 1983) and a large refraction, wide-angle brian mafi c igneous rocks, an inference con- excepting a minor and inferred recycled popu- refl ection experiment (Chang et al., 1989; Keller sistent with seismic refraction data (Rumpel lation (Dickinson and Gehrels, 2009, 2010). and Baldridge, 1995; Rondot, 2009) were inte- et al., 2005). We thus infer that the northwest- Gehrels et al. (2011) documented this prove- grated with geologic, drilling, gravity, and mag- trending gravity anomalies that begin in north- nance shift in Paleozoic strata of the Grand Can- netic data to produce the crustal model shown eastern Texas, are most prominent in southern yon, which exhibit an evolution from a signifi - in Figure 10 (simplifi ed in Fig. 3). Notably, the Oklahoma, and extend to the Uncompahgre cant ARM source in the Pennsylvanian–earliest mafi c complex exposed in the Wichita highland highland of Colorado, represent a high-density Permian to an Appalachian-Ouachita source in and associated with a >100 mGal gravity high upper crustal load, the total length of which is the later Permian. This shift is also captured occurs almost entirely within the upper plate of ~1500 km. The potential effects of this load in Lower Permian siltstone units of New Mexico a large thrust zone manifested near the surface have not been previously addressed. and Oklahoma (M. Soreghan et al., 2002, 2008; as the Mountain View fault (Fig. 10). Regional Templet and Soreghan, 2010; Fig. 8). gravity anomalies (Fig. 9) show that this mafi c MODELING THE EFFECTS OF A The age and provenance of the continental mass extends northwestward, with slight CRUSTAL LOAD strata onlapping the ARM faults and burying the (50 km) offset, for ~500 km, well into north- hinterland paleorelief provide approximate con- eastern New Mexico, before being disrupted by Today the Wichita highland is supported by straints on the timing of subsidence. As illus- features associated with the Rio Grande Rift. a strong lithosphere that continues to support trated in Figures 4–7, faulting ceased in the late Farther northwest, the San Juan volcanic fi eld the load associated with an ~120 mGal Bouguer Pennsylvanian and post-tectonic onlap began by dominates the gravity fi eld, but Larson et al. gravity anomaly high. Concentrating on geo- Early Permian time. Furthermore, structural res- (1985) presented evidence for Cambrian rifting physical manifestations of the SOA in southern toration of the Paradox basin fi ll indicates that, extending to the Uncompahgre highland, and Oklahoma (to avoid the Laramide complica- for the Uncompahgre front, faulting had largely geophysical studies of this highland indicate tions of the Uncompahgre highlands), we verti- ceased and onlap initiated in earliest Permian that it also is underlain by a high-density, high- cally integrate the density structure derived from time (between 293 and 284 Ma according to velocity body (Snelson et al., 1998; Casillas, gravity observations and seismic velocity inter- Trudgill and Paz, 2009). More accurate con- 2004). The amplitude of the anomaly across pretations along the profi le highlighted in Fig- straints on timing await higher resolution dating the Uncompahgre highland is ~50 mGal, but ure 9. We then quantify the lithospheric load and of these continental units. fl anking strata are of lower density than those thus the fl exure forced by the Cambrian mafi c fl anking the Wichita highland, reducing the den- and ultramafi c bodies underneath (Fig. 10). The GEOPHYSICAL EVIDENCE FOR sity anomaly needed within the Uncompahgre nomenclature used here closely follows that of A CRUSTAL LOAD BENEATH THE highland uppermost crust. Moreover, new fi eld Turcotte and Schubert (2002). CORE ARM studies along this trend, in the Wet Mountains of southern Colorado (Pardo, 2009), reveal a 40 Flexural Calculations: No Horizontal Stress Gravity data provide a clear picture of the mGal gravity high associated with Cambrian extent of the SOA and the impressive mafi c mafi c and ultramafi c complexes. These results, First we defi ne the effective elastic thickness

magmatism associated with the Cambrian rift- when merged with regional gravity and mag- (Teff) of the lithosphere as

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110°W an unbroken elastic plate will behave as

− x α ⎡ x x ( ) =+⎢ ⎥⎤ wx we0 ⎣cosα sin α⎦, (5) 40 40°N

UU MCR and

V α3 w = 0 , (6) WM 0 8D

36 36 with V0 = qa from Equation 3. SU In Figure 10 we used seismic, gravity, and RGR density data to calculate the load placed on the WU AU AB SOA area by the excess mass of the maﬁ c Cam- OIZ brian rift structure rocks. We can now calculate the amount of lithospheric ﬂ exure that should

32 CBP AGM 32 result from application of Equations 5 and 6, varying the elastic thickness from 10 to 30 km. OIZ Although the mass excess may seem large, the 110°W 90°W 100°W narrowness of the mass anomaly results in very 0 100 400 modest amounts of Moho defl ection. The high km –21 –7 –3 0 3 6 12 peak amplitude of the load (consistent with the mGal ~100 mGal gravity anomaly) combined with its Figure 9. Residual gravity anomaly map calculated by subtracting a regional gravity fi eld narrowness produces a peak Moho defl ection of from the complete Bouguer anomaly values. The regional fi eld was calculated by con- ~1.4 and 0.6 km at effective elastic thicknesses tinuing upward (20 km) the complete (terrain corrected) Bouguer anomaly values. The of 10 and 30 km, respectively. In practice, this yellow line is the location of the integrated geophysical model shown in Figure 10, and amount of defl ection is barely detectable by the central portion of this line is the location of the seismic section shown in Figure 5. seismic means. A similar gravity anomaly is Abbreviations for tectonic elements: UU—Uncompahgre uplift, WU—Wichita-Amarillo observed in the Uncompahgre uplift region, but uplift, AU—Arbuckle uplift, WM—Wet Mountains, CBP—Central basin platform, AB— the amount of subsidence is smaller than that in Arkoma basin, MCR—Midcontinent Rift, RGR—Rio Grande Rift, SU—Sierra Grande the Wichita uplift region; this is consistent with uplift, AGM—Abilene gravity minimum, OIZ—Ouachita orogenic belt interior zone that the smaller modeled mafi c core of the Uncom- marks the Cambrian margin of Laurentia. pahgre region, and its more distal location relative to the southeastern part of the SOA.

Flexural Calculations: Horizontal ⎡12D( 1−ν2 )⎥⎤ 13 establish. Large-scale patterns are sometimes ==⎢ ⎥ Stress Included Theff , (1) quite clear, but second- and third-order patterns ⎣⎦E may depend on local geologic histories and density heterogeneities (Coblentz and Richardson, If the in-plane horizontal stress (P) is non- where h is elastic thickness (in m), E is Young’s zero, then the fl exural solution becomes more modulus (in Pa), ν is Poisson’s ratio, and D is 1995; Heidbach et al., 2007). In the region of the SOA, the World Stress Map database release complicated, and the fl exural parameter splits fl exural rigidity (in Nm), β γ 2008 (Heidbach et al., 2008) shows a variety of into two, labeled here as and , 3 Eh directions of the horizontal maximum stress Sh, ⎡ ⎤ D = . (2) V0 −β x −ν2 but they are dominantly perpendicular to the wx( ) = ⎢ ⎥excos(γ ), (7) 12( 1 ) 4Dβγ( 22 +β) main structure in the vicinity of the Anadarko ⎣ ⎦ basin. A smoothed stress fi eld (Heidbach et al., With these defi nitions, the governing equation with for two-dimensional cylindrical fl exure is 2010) shows a similar northward trend near ⎧ ⎫12 34°N, 95°W, but more eastward trends north ⎪⎡g(ρ−ρ)⎤12 ⎪ 4 2 γ= ⎢ min⎥ + P d w = ( ) −ρ −ρ ( )− d w and south of that location. The implication of ⎨ ⎬ (8) D qa x ( m in )gw x P , (3) ⎪⎣ 44D ⎦ D⎪ dx4 dx2 these observations is that the present-day stress ⎩ ⎭ fi eld in the central SOA is roughly isotropic and where w is vertical displacement (fl exure, in and only weakly affected by the Cambrian and 12 ⎧ 12 ⎫ 2 ⎪⎡ (ρ−ρ)⎤ m), g is gravitational acceleration (in m/s ), x is Pennsylvanian structures underneath, and an in- g min P ⎪ β=⎨⎢ ⎥ − ⎬ . (9) horizontal ordinate (in m), q is applied external plane stress value P ~ 0 is appropriate for the ⎣ 44D ⎦ D⎪ a ⎩⎪ ⎭ ρ ρ load (in N/m), m and in are mantle and infi ll present-day situation. density in (kg/m3), and P is horizontal (in plane) With a further defi nition of the fl exural As P increases in size, β decreases until it force (in N). parameter approaches zero, at which time the square root Initially, we set P = 0 and later examined in β becomes undefi ned on the real axis. This ⎡ 4D ⎤ 14 the effect of varying values of P. Appropriate α= , (4) corresponds to buckling or the elastic limit (Fig. ⎢(ρ−ρ) ⎥ values of mid-continental stress are diffi cult to ⎣ ming⎦ 10). The limit is

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3500 ⎡Eh3 g(ρ−ρ)⎤12 = ⎢ min⎥ β→ A Fc ⎢⎥2 as 0, (10) ⎣ 31( −ν ) ⎦ 3000 0.5 Distance (km) 2 50 100 150 200 250 300 350 400 λ 2500 0 which occurs at a wavelength c of Excess Mass 14 –0.5 ⎡ Eh3 ⎤ 2000 Over the Wichita Uplift λ=⎢ ⎥ . (11) c ( −ν2 ) ( ρ −ρ ) –1 30 km ⎣12 1 g min⎦ 1500 –1.5 20 km 10 km

Thus, the result of changing in-plane stresses (km) displacement Vertical –2 is that the amplitude of ﬂ exurally supported 1000 features is modulated in a nonlinear fashion, leading to buckling and thrust formation with 500 sufﬁ cient compressive stress, and subsidence

Excess Mass Relative to Mean (1000 kg/m ) 0 of topography formed by buckling upon relax- 50 100 150 200 250 300 350 400 450 ation of the high compressional stresses. The 0 –500 relaxation need not proceed as far as deviatoric Distance (km) tension, because most of the ﬂ exural relaxation occurs within the compressional ﬁ eld (Fig. 11). –1000

DISCUSSION: WHY DID THE ANCESTRAL ROCKY MOUNTAINS Wichita Uplift Anadarko Basin STOP RISING? Granite Seismic Line (Fig. 5) SW MVF NE B 0 4.3 (2.5) 6.3 4.55 (2.5) The disappearance of highland elevation in 5.3 Upper Paleozoic Post the core ARM, as recorded by preservation of 6.55 (2.63) Paleozoic 5.85 (2.6) 6.85 5.3 Permian landscapes on top of the paleohigh- (2.95) 5.65 Lower lands, records cessation of uplift, followed Proterozoic Basin? 7.15 (2.75) (2.72) 10 Paleozoic by signiﬁ cant subsidence over a broad region. 5.75

Mechanisms possibly capable of inducing such Depth (km) 5.65 Volcanics (2.63) vertical motion include orogenic collapse, nega- Mafic Igneous (Carlton Rhyolite?) tive dynamic topography, or the infl uence of 6.2 (2.7) Complex Rift Fill 6.2 (2.7) 6.2 (2.7) horizontal stresses. Any mechanism invoked to 20 explain such motion must account for the signif- 100150 200 250 300 icant vertical and areal extent of the subsidence, Distance (km) and the geologically abrupt onset following the ′ apogee of ARM tectonism. Orogenic collapse Figure 10. (A) Mass excess and fl exure computation across profi le A–A of Figure 3. The ′ is well recognized as an important process in upper plot shows the vertically integrated mass along profi le A–A using the density struc- the evolution of mountain belts (Dewey, 1988; ture derived from the model below. Note also the location of the seismic refl ection line shown Menard and Molnar, 1988; Rey et al., 2001; in Figure 5. The zero point on the vertical axis of this profi le was set to the mean value of 2 Dilek, 2006). Orogenic collapse, however, lithospheric load in units of 1000 kg/m . The excess mass anomaly is no wider than 80 km, 2 implies a large orogen with thick crust, usually and has maximum amplitude of 3800 kg/m . The inset plot shows the fl exure caused by the accompanied by partial melting to weaken the modeled lithospheric load. The curves here are labeled with effective elastic thicknesses thick crust, and typically postdates thicken- (h) of 10, 20, and 30 km. In these fl exural isostatic models, Moho defl ection would not be ing by many millions of years. Such collapse detectable by seismic refraction unless h is <10 km. Other parameters in the inset models involves gravity-driven fl ow that counteracts were held constant at Young’s modulus 70 GPa, Poisson’s ratio of 0.25, and infi ll density 3 ′ crustal thickening, reducing lateral contrasts in 2500 kg/m . (B) Velocity and density model of central portion of profi le A–A from Figure 3. 3 –3 gravitational potential energy, and is commonly Numbers in parentheses are densities (in kg/m × 10 ); other numbers are seismic P-wave associated with extensional structures in the velocities (in km/s). thickened crust and shortening in the foreland (see preceding references). These attributes do not occur in the ARM system, calling into ques- dynamic topography relate to mantle fl ow linked that any potential infl uence of dynamic topogra- tion any role of orogenic collapse as tradition- to the initiation and cessation of subduction. The phy should have affected the Gondwanan, rather ally defi ned. potential appeal of dynamic topography for the than the Laurentian, plate. Dynamic topography refers to elevation dif- ARM system derives from the presumed impor- In-plane stress acting on an inhomogeneous ferences caused by mantle fl ow, and has been tance of the Ouachita-Marathon subduction sys- crust provides an additional mechanism to invoked to explain large-scale continental tem in ARM orogenesis; however, the system explain large areas of vertical motion of the fl ooding and exposure (Gurnis, 1993; Lithgow- records southward-dipping subduction (Viele crust, including modulation of sedimentary Bertelloni and Gurnis, 1997). Uplift and sub- and Thomas, 1989; Loomis et al., 1994; Dickin- basin formation (Cloetingh, 1988; Cloetingh and sidence of large continental areas resulting from son and Lawton, 2003; Poole et al., 2005), such Kooi, 1992; Heine et al., 2008). The mechanism

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although landforms dating from the Mesozoic of the Ouachita orogenic belt. We hypothesize Tension Compression and even Paleozoic are increasingly well rec- that this shift in the regional stress fi eld and Elastic ognized (Twidale, 1998), especially from the associated cessation of northeast-oriented com- dw = 0 failure Gondwanan continents. We hypothesize that pressive stresses (Cloetingh and Kooi, 1992; limit Early Permian landscapes from the upland Cloetingh , 1988) precipitated the geologically Uncompahgre and Wichita systems were pre- abrupt reversal from orogenic uplift to epeiro- P = 0 served as a result of active regional subsidence genic subsidence tied to the existence of an Figure 11. Amplification of flexure in in Early Permian time that affected both the upper crustal mafi c load, as documented in our response to in-plane stress. Increasing hori- uplifts and surrounding regions. Exhumation data set and modeling. These results underscore zontal compressive stresses relative to a plate of these landscapes occurred in the Cenozoic, the roles of inheritance and crustal inhomogeneity with no horizontal stress (P = 0) on an elastic associated with Laramide and more recent in erecting and ultimately eradicating a classi- lithosphere with applied vertical loads will orogenesis and auxiliary drainage evolution cally enigmatic intraplate orogenic system. increase fl exure amplitudes until the elastic in Colorado, and the distal effects of the Rio failure limit is reached and buckling occurs. Grande Rift that extended to Oklahoma (Eaton, ACKNOWLEDGMENTS Conversely, placing the lithosphere under 2008). Our hypothesis predicts that Lower Research contributing to the ideas presented here relative tension will decrease the amplitude Permian postorogenic strata should thicken was funded in part by grants from the National Sci- of fl exure. This mechanism modulates load- toward the core ARM highlands, along a trend ence Foundation (EAR-0230332, EAR-0404500) and induced fl exures. Dashed line (dw; see text) perpendicular to that of the gravity anomaly and the U.S. Geological Survey National Cooperative Geo- logic Mapping Program (awards G10AC00329 and represents position of loaded and fl exed but inferred mass load. G11AC20217). The views and conclusions contained uncompressed plate. in this document are ours and should not be interpreted CONCLUSIONS as representing the offi cial policies, either expressed or implied, of the U.S. Government. We thank reviewers of horizontal stresses acting upon a mark- Geologic data from the core ARM system, C. Kluth, R. Langford, Geosphere editor D. Harry, and the Geosphere Associate Editor for constructive edly inhomogeneous crust is most consistent both long known and newly documented, indi- comments on an earlier version of this manuscript. We with both the data and modeling results from cate preservation of Early Permian landscapes are grateful to Petroleum Geo-Services for generously the ARM system. The mafi c keel of the SOA- that exhibit paleorelief of as much as 1000 m, providing access to images from their three-dimen- ARM system originated during the Cambrian and record subsidence extending over a length sional Wichita Mountain Front refl ection surveys. We thank T. Eccles, A. Shock, and D. Ambuehl for map- rifting that produced the SOA, and underpins scale of nearly 1500 km. In addition, geophysi- ping assistance, and K. Johnson and R. Burkhalter for the core uplifts of the ARM discussed here. As cal data buttressed by geological data reveal a valua ble discussions on the Permian of Oklahoma. refl ected in the subsidence history of the earlier regional-scale mafi c load underpinning these Oklahoma basin (Johnson et al., 1988; Gilbert, same regions. 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