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Downloaded from geosphere.gsapubs.org on May 31, 2012 Geosphere Load-induced subsidence of the Ancestral Rocky Mountains recorded by preservation of Permian landscapes Gerilyn S. Soreghan, G. Randy Keller, M. Charles Gilbert, Clement G. Chase and Dustin E. Sweet Geosphere 2012;8;654-668 doi: 10.1130/GES00681.1 Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geosphere Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. 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Notes © 2012 Geological Society of America Downloaded from geosphere.gsapubs.org on May 31, 2012 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. 654 For permission to copy, contact [email protected] © 2012 Geological Society of America Downloaded from geosphere.gsapubs.org on May 31, 2012 The rise and demise of the Ancestral Rocky Mountains 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, sub sidence the transition between syntectonic and post- in within