Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
1 1 1 1 1 TECTONOPHYSICS . \, ELSEVIER 1 1 Tectonophysics 305 (1999) 141-152 ~ 1 \ '0/41.1.632 6027; FAX: 1 ,Park, PA 16802, USA. , 16802, USA. Phone: Karlsruhe, Germany. Neogene basins of the northern Rio Grande rift: partitioning and asymmetry inherited from Laramide and older uplifts \ Karl S. Kellogg * u.s. 1 Geological Survey, Mail Stop 913, Box 25046, Federal Center. Denver, CO 80225, USA Jlhampton Je 1 Received 3 March 1998; accepted 141uly 1998 1 :A IL 'Jpon-Thames Abstract 1 ,. 1t. I 1 Arbor, MI lOr, MI m 11 cs and the 13t with its uest from lit except 1 Three asymmetric Neogene basins in the northern Rio Grande rift of New Mexico and Colorado - the San Luis basin. the upper Arkansas River graben, and the Blue River graben - are tilted toward large flanking normal faults and lie astride the similarly asymmetric Late Cretaceous-early Tertiary (Laramide) San luan-San Luis. Sawatch. and Front Range-Gore Range uplifts, respectively. The steep, thrust-faulted side of each uplift is on the same side as the down-rotated side of each of the Neogene basins. In addition. the direction of stratal tilt changes northward across the Villa Grove accommodation zone from east in the San Luis basin to west in the upper Arkansas River graben. This accommodation zone coincides approximately with the northward change from the east-directed San Juan-San Luis uplift to the west-directed Sawatch uplift. These observations. supported by seismic-reflection studies across the San Luis basin and studies of several other superimposed pairs of rift basins and Laramide uplifts, suggest that the basin-bounding normal faults are listric and merge at depth with the older thrusts. which are also listric and root into the crust at about 15-16 km. The Blue River graben is complicated by lack of basin fill and a thrust history along the west side of the Gore Range that is at least as old as late Paleozoic. Nonetheless. the Neogene valley is demonstrably tilted west and lies astride an overall west-directed thrust system. similar to other thrust-and-basin relationships in the northern Rio Grande rift. © 1999 Elsevier Science B.Y. All rights reserved. 1 1 1 1 1 1 1 Keywords: Rio Grande rift; Colorado; Laramide orogeny; Neogene extension; inherited structure 1 ·Aalaysia, ~ates are , 1. Introduction nerican 1-mail: 5047; ?230; 509 , the ler} The Rio Grande rift extends northward from New Mexico into the heart of the southern Rocky Mountains, where the deep basin fill that defines major rifting terminates at the north end of the upper Arkansas River Valley near Leadville. Colorado (Tweto. 1979; Fig. I), However. Neogene normal • Tel.: + 1 303 236 1305; Fax: [email protected] + 1 303 2360214; E-mail: faults and grabens that are approximately coeval with formation of the Rio Grande rift can be traced along trend at least as far north as the Wyoming border; the Blue River graben is the largest of these northernmost rift structures. In Colorado and northern New Mexico, the Rio Grande rift developed along the axial zone of the Late Cretaceous-early Tertiary (Laramide) San Juan-San Luis uplift (Tweto, 1975, 1979; Chapin and Seager, 1975; Cordell, 1978; Eaton, 1979; Chapin and Cather, 1994; Russell and Snelson, 1994), which, in turn, developed along 0040-1951/99/5 - see front matter © 1999 Elsevier Science B. V. All rights reserved. PI!: S0040-1951(99)00013-X ,. .. ...... "\-,. " 1 , 1 1 1 1 1 142 K.s. Ke/logg/Tt'ctO/lOphysics 3U5 (/YYYj N/-/52 ~MAP I .I AREA COLORADO f--;}~-.i :j BASIN-FILL DEPOSITS Including Santa Fe. Dry Union. and . Troublesome Formations ' ~ '. I . '1 GHM i . ~. ~ ,ROUG\'\ NEOGENE VOLCANIC ROCKS Basalt and subordinate rhyolite f'I'[l ~ OLIGOCENE VOLCANIC AND SEDIMENTARY ROCKS Mostly volcanic rocks of intermediate composition .. TERTIARY AND UPPER CRETACEOUS INTRUSIVE ROCKS CJ , " LOWER TERTIARY TO CAMBRIAN SEDIMENTARY ROCKS D PROTEROZOIC ROCKS - . . •. . . Normal fault with Neogene movement-Bar and ball on downthrown side; dotted where concealed I ; . \',' ':~ "~.. .~'. \ .~.), • '::~.;, j + . ~ ,;. . ",. -,..- ...... Thrust or reverse fault with inferred Laramide movement-Teeth on upthrown side; major late Paleozoic movement on west side of Gore Range ", ~ .\ " Monocline axis, showing direction of maximum dip °CI======S2.5.......50KM Fig.!. Map showing major tectonic features of the Colorado portion of the Rio Grande rift. simplified and modified from Tweto (1979). Cross-sections A-A', B-B', and C-C are shown in Fig. 3. AG = upper Arkansas River graben: APF = Antelope Pass normal fault: BRF = Blue River normal fault: BRG = Blue River graben: CF = Castle Creek fault zone: 'ET = Elk Range thrust zone: GF = Gore fault: GHM = Grand Hogback monocline: K = Kremmling: KT = Kerber Creek thrust: L = Leadville: MR = Mosquito Range: PP = Poncha Pass: S = Salida: VGAZ = Villa Grove accommodation zone; WMP = western Middle Park basin: WRT = Williams Range thrust. . , K.S. Kellogg/Tectonophysics 305 (1999) 1-11-152 •. ::~:~~~~~ " • . J. :·~~~:~·"1 ~ ," ••.·..-1 ~ .~ r~ ~. .~ ~~ .1,1. and '~1 ~l. ;.1 ,., 'CKS ':~')/ite .' structures that were as old as Proterozoic (Tweto, 1979). Stratal tilt within basins of the Rio Grande rift alternates from east to west, segmenting the rift system (Chapin and Cather. 1994: Russell and Snelson, 1994) similar to the East African rift (e.g .• Bosworth et al.. 1986). Each segment of the Rio Grande rift is a complex north-striking half graben that is separated from adjacent segments by an accommodation or transfer zone, across which stratal tilts reverse. The accommodation zones are crustal flaws (fault zones or poorly defined structural lineaments) which in places exploited Laramide and older zones of weakness and locally acted as a conduit for magma (e.g .• Russell and Snelson. 1994). This study explores the relationships between the direction of stratal tilt within each major normalfault-bounded segment of the northern Rio Grande rift and the steep, thrust-faulted side of the Laramide uplift within which the segment lies. The evidence strongly suggests that the geometry of the Tertiary· basins may be genetically linked to the symmetry of the older Laramide uplift. Three grabens of the northern Rio Grande rift, underlying the San Luis basin, the upper Arkansas River Valley, and the Blue River Valley (Fig. 1), support a model, based, in part, on ideas presented by Wise (1963): Scholten (1967); Royse et al. (1975); Schmidt et al. (1984); Russell and Snelson (1994), and Kellogg et al. (1995), which explains how Laramide faults and uplifts controlled the placement and orientation of Neogene faulting and basin formation. This paper tests the applicability of the model, described in the next section, to the major structural features along and adjacent to various parts of the northern Rio Grande rift. 2. Basement uplifts and Neogene grabens: a possible genetic link , '" Deep seismic reflection profiles across several Laramide uplifts, such as the Wind River Mountains in Wyoming and the Manzano uplift in New Mexico, indicate that on at least one side listric thrusts, flattening at depth into the middle crust at about 15-16 km, bound the uplifts (Sharry et ai., 1986; de Voogd et aI., 1988; Russell and Snelson, 1994). I·B The bounding thrusts commonly control the position of Tertiary normal faults by offering zones of weakness that can be exploited during crustal extension (Royse et at.. 1975; Smith and Bruhn, 1984; Arabasz et al.. 1992; Kellogg et aI.. 1995). Tertiary basin-bounding normal faults also appear to root into older thrusts in the southern Albuquerque basin (Russell and Snelson, 1994). Reiter et al. (1992) presented a quantitative, isostatic model for the reactivation of Laramide thrusts and the formation of half grabens by rotation of domino-style crustal blocks. Southwestern Montana contains many excellent examples of Neogene extensional features that reactivate Laramide structures. At least four Neogene extensional valleys as wide as 20 km formed along the axial zones of large (as wide as 30 km) Laramide uplifts (Scholten, 1967). The thrusts define the east side of each uplifts and produce large basement overhangs above a footwall syncline in Paleozoic and Mesozoic rocks (e.g. Kellogg et aI., 1995) (Fig. 2A). According to a model for structural inversion (Schmidt and Dresser, 1984; Schmidt et aI.. 1984; Kellogg et ai., 1995), the unsupported eastern side of a basement arch in Montana underwent rotational sagging into the underlying or adjacent basin sediments, thereby producing extension. fracturing, and possibly normal faulting along the crestal zone of the arched basement block. Similar crestal normal faulting ('keystone faulting') was recognized long ago in the basement uplift of the Owl Creek Mountains. Wyoming (Wise. 1963). The fractures were later exploited during Tertiary crustal-wide extension. resulting in large-scale collapse and development' of a basin along the axial part of the Laramide arch (Fig. 2B). 'Perched basement wedges' (Lageson. 1989), bounded on one side by Laramide-age thrusts and on the other side by Neogene normal faults, typically result from this process (Fig. 2C). In most cases. strata in the basins adjacent to the basement wedge dip toward both the major valleybounding normal faults and older uplift-bounding thrusts. The idea that normal faults commonly exploit older thrust is well established. For example. seismic studies across the Idaho-Wyoming thrust belt show that Neogene normal faults commonly sole into underlying thrust ramps (Royse et al.. 1975). Field re- .' .J ,\ ~ ,\ 144 K.S. KellogglTectonophvsics 305 (J999) J.JJ-J52 Mzpz Laramide sedimentary rocks ""- . . ~--I-~-·~.;:c;'·~--··-------:-~-······\T7·--·-·~.A .-.-- ......---'\ ~ • I • ;"" A . • " . ... . . l.. --M~2p'~'~ pCu LATE CRETACEOUS· EARLY EOCENE KM 5 SEA LEVEL -5 -10 -15-t=:=~===::::=-_~.:...:.........2P~R~E~S~E~N~T..2..~..J Fig. 2. Schematic diagrams showing major tectonic elements during evolution of an asymmetrical Laramide uplift and collapse durin Neogene extension: no vertical exaggeration. PC" = Precambrian rocks: M~~ = Mesozoic and Paleozoic rocks. Adapted, in part, fror Schmidt et al. (1984) and Kellogg et al. (1995) . . . ~" " , " .. ~ ... ' lations hips also demonstrate that some nonnal faults adjacent to basins in southwestern Montana sole into older thrust surfaces (Kellogg et a!., 1995); similar relationships have been mapped in southeastern Idaho (Kellogg, 1992). The mechanisms by which Laramide (and earlier) crustal shortening controlled Neogene extension are complicated and still poorly understood, and the model outlined here for listric thrusts and nonnal faults, although favored by the author, is one of several that have been proposed. Seismic evidence for listric Laramide thrusts and Neogene nonnal faults commonly is equivocal and some researchers suggest that basin-bounding nonnal faults may be planer to at least 15 Ian depth (Dozer and Smith, 1985) or steepen with depth and root into thrusts that ,also steepen downward (e.g" Sales. 1983). A new school of thought propose that north-striking Laramide faults in New Mexico and southern Colorado have a large component of dextral slip and a relatively small component of reverse slip (e.g Bauer and Raiser, 1995; Wawrzyniec and Geissman 1995). For example, movement along one of tht major faults bounding the east side of the southern· most San Juan-San Luis uplift (the Picuris-Peco~ fault in the southernmost Sangre de Cristo Mountains near Santa Fe. New Mexico) has an ancestry that is as old as Proterozoic and an inferred 37 Ian of mostly Laramide dextral strike-slip motion (Bauer and RaIser. 1995); the fault is viewed as a positive flower structure that steepens downward to an essentially vertical attitude. If one accepts that the Laramide orogeny was marked by significant crustal shortening, however, then it is difficult to reconcile . such shortening along downward-steepening faults . In addition, the inferred. predominantly strike-slip, non-listric geometry inferred for the Picuris-Pecos fault precludes exploitation of the fault by Neogene nonnal faults along the east side of the adjacent rift basin (Espanola Basin) . 3. Gcologic setting of the northern Rio Grande rift ~'during ,;.. from " . ,~:/ .,,. ./.~ ,'\ ' ," :.1. ;; ~;(e.g. 'man . .;: the lem.~cos iDun.;stry .'km ·'auer ;usi,1 an rthe :Mal ::,cile . ·fP.ts. \ . \~' ~~P. I.: Os hne .• Soft Laramide crustal contraction and resultant uplift along the San Juan-San Luis arch began about 70 Ma (late Campanian) and created a highland that shed clastic detritus into bounding basins until the Late Eocene (Tweto. 1975), at which time a widespread erosion surface extended across most of Colorado (Epis and Chapin. 1975). Rio Grande rifting began shortly after 29 Ma (Tweto, 1979; McIntosh et aI.. 1992) and was marked by a change in volcanism from the intermediate compositions that characterized the widespread Oligocene volcanic fields (Steven. 1975) to bimodal basalt-rhyolite (mostly basalt) volcanism that characterizes the Rio Grande rift (Lipman and Mehnert, 1975). This fundamental change in volcanic style coincides with a change throughout the western United States from subduction-driven to extension-driven tectonic regimes in the Late Oligocene to Early Miocene (Christiansen and Lipman, 1972). A SUbsequent period of rapid faulting and extension in New Mexico and the San Luis basin of Colorado culminated in the Middle to Late Miocene (Chapin and Cather, 1994) and was accompanied or closely followed by voluminous basaltic volcanism between 3 and 8 Ma (Seager and Morgan. 1979) . Despite paleobotanical evidence that suggests that the Rocky Mountains have been topographically high since the Eocene (e.g., Gregory and Chase, 1994), other studies indicate that the rise of the modem Rocky Mountains was accompanied by a period of rapid faulting and extension in the Middle to Late Miocene (Chapin and Cather, 1994; Steven et aI.. 1997). Such uplift is consistent with apatite fission-track ages which indicate that the Sawatch and Sangre de Cristo ranges underwent significant uplift since about 20 Ma (Bryant and Naeser, 1980; Lindsey et aI., 1986; Kelley et aI., 1992). 3.1. The San Luis basin and adjacent uplifts The San Luis basin extends from about 10 km south of Salida, Colorado, to about 80 km south of the Colorado-New Mexico border; only the Colorado portion of the basin is shown in Fig. 1. The basin is bordered on the west by a gently east-dip- ping surface underlain mostly hv Proterozoic crvstalline rocks and Oligocene volcanic rocks and' is bounded on the east by the Sangre de Cristo normal fault system. which also defines the western marO'in of the rugged Sangre de Cristo Mountains (Figs~ 1 and 3A). The northern San Luis basin unde~ent 8-12% extension (Kluth and Schaftenaar. 1994). increasing southward along the Rio Grande rift to greater than 28% extension in central New Mexico (Chapin and Cather, 1994). Seismic reflection surveys and deep exploratory drilling indicate that the San Luis basin consists of two east-tilted half grabens. separated by the north-trending Alamosa horst (Brister and Gries. 1994). The eastern part of the eastern half graben (Baca graben) contains as much as 5.6 km of the Upper Oligocene to middle Pleistocene Santa Fe Group above a basal sequence as old as Eocene. Other researchers report that the basin is as much as 6.4 km deep (Kluth and Schaftenaar, 1994). The near absence of pre-Tertiary sedimentary rocks beneath the San Luis basin is a relict of erosion over the crest of the former Laramide uplift. The 'perched basement wedge' (terminology of Lageson, 1989) of Proterozoic rock along the west side of the present Sangre de Cristo Mountains (Fig. 3A) is a remnant of the eastern flank of the San Juan-San Luis uplift. The crystalline rocks of the basement wedge were thrust eastward during the Laramide orogeny above mostly Pennsylvanian and Permian sandstone and conglomerate; Neogene normal faults subsequently exploited several of the thrust surfaces (Lindsey et aI., 1983). Structural relief across the Sangre de Cristo normal fault system, which bounds the west margin of the basement wedge, is at least 9200 m (Kluth and Schaftenaar, 1994). The system of Laramide thrusts and Neogene normal faults extends along or near the east side of the entire San Luis basin. a style of deformation that is very similar to that interpreted along the east side of the southern Albuquerque basin in New Mexico (Russell and Snelson. 1994). The northern end of the Laramide thrust system along the west side of the Sangre de Cristo Mountains disappears under the San Luis basin and reappears on the west side of the valley as the northdirected Kerber Creek thrust (Lindsey et aI., 1983) (Fig. 1). This geometry suggests overall northeast 146 K.S. Kellogg/Tectonophysics 305 (1999) 141-152 ~I.~ -,. .... ~:~:.~.'.: : 5 . .... ...;. .. ,..., 0 KM -5 -10 C GORE RANGE BLUE RIVER WILLIAMS BLUE RIVER VALLEY RANGE \ FAULT ~ _\ THRUST, GORE \ FRONT RANGE C' 5=:~i!\~~L:~'OC' o -'--~.-j~/;''t KM5 - ~~t~:~() \ '.) "'~ \\, " "~\.\~~~~. \\ ,'t., , (P~~MIAt:I:~ENNSYL~~NIAN) ._............ " , . , I., TROUGH ~. ...... ',', " " . -10~~----~~~--~~~~~~~~~ Fig. 3. Cross-sections indicated in Fig. I; no vertical exaggeration. Symbols for rock units are the same as for Fig. I; dashed lines suggest stratal relationships at depth. Scale is 2 times that of Fig. I. , .. .... ~ '.' ' Laramide movement, with a right-lateral component along the north-striking part of the thrust system and a left-lateral component on the Kerber Creek thrust. Sales (1983) suggested that the Tertiary normal faults along the Sangre de Cristo fault system merge at depth with the older thrusts, which generally steepen downward, a geometry based, in part, on laboratory modeling experiments. Deep seismic reflection studies resolve neither the orientation nor the curvature of the Sangre de Cristo fault system, although modeling suggests that the most likely orientation is about 60°. to a depth of about 6 km (Kluth and Schaftenaar, 1994). Using this angle, a graphical method reported by Kluth and Schaftenaar (1994) indicates that the depth of detachment (flattening) is at about 16 km, the depth of the brittle-ductile transition zone estimated from heat-flow studies. 3.2. Villa Grove accommodation zone VanAlstine (1968) suggested that the upper Arkansas River graben is structurally continuous with the San Luis basin, but westward stratal dip of Miocene rocks and the down-to-the-east normal fault along the west side of the upper Arkansas River graben dies out just north of Poncha Pass (Fig. 1), the topographic gap between the San Luis basin and the upper Arkansas River graben. The area surrounding Poncha Pass is complexly faulted and contains Proterozoic rocks that are exposed across virtually .... • ... ,.j', ~. . .~~ " ~ '. .. '. " jl. ':.) ~~;;':~,;!;~'~;;;};i:.·:,: ~. . . . . ' . , " "~' .' • " ..... ,J, ( , • ..;:~,~. "', :,- f~:' I~~~:: thJl':~ .~~ ~.'. . K.S. Kellogg I Tectonophysics 305 (19<)<) 1-11-152 :·~~~.~;.~·'fr~h:,+~~;'!I"'·~~the entire rift. South of Poncha Pass, stratal dip is . . "."~ ·?~;';-l':~#~'~.-··~;!east and major normal faulting is on the east side -:''''':';~':''<~';'~'~~''';!'''. ',,',,;/-Uof the San Luis basin ' : alona the Sanare de Cristo " . ~ ;' e .' ::: .' . -:fault zone (compare Fig. 3A and 38), The reversal in .. ", ' ~~;stratal dip across Poncha Pass is about 20 krn north ~':of the Villa Grove accommodation zone (VGAZ in ,1:Fig. 1) of Chapin and Cather (1994), a proposed . ?":northeast-trending crustal-scale stI1lctural lineament \ :or zone of weakness across which stratal tilt suppos:~:: edly reverses, Despite the 20-krn relocation in the ,~~ present study, the name 'Villa Grove accommodation :": zone' is retained, .~~ A Laramide transition zone between the strongly ,~!; west-directed Sawatch uplift and the east-directed . ~~ San Luis uplift is approximately coincident with ~~ the Villa Grove accommodation zone. The northern ' .. '.: .;' :"):;\' .~.~ limit of east-directed thrusts adjacent to the San Luis .' ,"':>'.;:" :'!\.},';/ ..., basin is about ~~ krn south of the accommodation '. ~. :" ·'2.(.j:' .:' , zone, at the position of the Kerber Creek thrust. The ....:',: )1: ·,:·/~f·::~:'·~tl s?uthern limit of west-di:ec~ed thrusting on the west r .. ·•• ! \~\ot~~;i~'::'1:~':'.'~ Side of the Sawatch uplIft IS not clear, but appears . ,:,,~.:.~':,:..;: .. I:-/.~Aj to be at about the same latitude as the Villa Grove l~ accommodation zone. .. ' ~ ',,' ".\'" ~ ;>",1 , ", ~),; " .·~'1Ii , • I ;:( 3.3. The Sawatch lIplift , ,. I.,. , • ,," " . '. I . " ,- "~\"~1'~: .. ··Ai :<i The Laramide Sawatch uplift (Sawatch anticline of Tweto, 1975) is a large north-plunging arch of . ~:l Proterozoic rock, the east flank of which, from the ~)t crest of the Mosquito Range eastward into South 'r' \,J Park, is overlain by relatively undeformed, mostly J,- ~ flat-lying late Paleozoic and Mesozoic rocks (Tweto, 1979). In contrast, most of the west side of the uplift contains a complex of steep, discontinuous faults which, near Aspen, is called the Castle Creek fault zone (Bryant, 1979). Total displacement across the fault zone is about 3600-4300 m, and beds adjacent to the zone dip steeply west or are overturned. Faults dip both east and west and are characteristically discontinuous along strike, although many of the faults are clearly east-dipping thrust or reverse faults (Spurr, 1898; Bryant, 1979). Tweto et al. (1978) interpreted most of the faults along the west side of the Sawatch uplift to be normal faults, including those of the Castle Creek fault zone. However, intrusive relationships with the fault zone indicate that the faults formed during the Laramide orogeny (Bryant, 1979), suggesting that the zone is contrac- ;~ 1/) ~, .', 1~7 tional rather than extensional. Recent mapping along the west side of the Sawatch uplift. north of the Castle Creek fault zone, has demonstrated the contractional character of the west margin of the uplift (Wallace and Blaskowski, 1989). In addition, the steeply dipping to overturned beds adjoining the uplift on the west characterize a large footwall syncline structure, similar to those commonly mapped adjacent to and beneath Laramide basement-involved thrusts (e.g., Kellogg et aI., 1995). Inferred northward migration of the Colorado Plateau may have added a component of dextral strike slip along the fault zone (Wawrzyniec and Geissman, 1995), although supportive data are inconclusive . Numerous Laramide and younger, mostly felsic plutons that lie along the northeast-trending Colorado mineral belt intrude the Sawatch uplift. A large gravity low coincides with the belt and indicates that the plutons form the apophoses of a large, mostly hidden plutonic complex (Tweto, 1975). The northwest-striking, southwest-directed Elk Range thrust crops out west of the Castle Creek fault zone (Fig. 1). The thrust, largely a beddingplane structure in Permian and Pennsylvanian rocks, cuts rocks as young as Cretaceous (Bryant, 1979); total transport distance is at least 5-6 krn. Bryant (1979) suggested that the Late Cretaceous or Early Paleocene Elk Range thrust formed by gravity sliding off the Sawatch uplift. Alternatively, although the Elk Range thrust is oblique to the west margin of the Sawatch uplift, it is geometrically permissible that the thrust roots beneath the large west-directed basement block that forms the Sawatch uplift (Fig. 3B). To the northwest, contraction along the Elk Range is transferred to a west- to southwest-directed blind thrust system that underlies the Grand Hogback monocline. The blind thrust is likely an out-of-basin thrust involving a basement wedge (Perry and Grout, 1988). 3.4. The IIpper Arkansas River graben The north-striking upper Arkansas River Valley occupies the northernmost major graben in the Rio Grande rift system and splits the east-central portion of the Sawatch uplift. Although complicated in detail, the graben contains a west-dipping basin-fill sequence (Upper Miocene and Lower Pliocene Dry -----------~ 148 K.S. Kellogg / Tectonophysics 305 (1999) 141-152 Union Formation) that is truncated on the west side by a large 'master' normal fault with at least 3000 m vertical offset (Knepper, 1976) (Fig. 3B). Resistivity and gravity data (Tweto and Case. 1972) also indicate that the bedrock surface beneath the Dry Union Formation near Buena Vista slopes steeply westward and is truncated by the large east-dipping normal fault (Tweto. 1978) (Fig. 3B). The east side of the graben contains a number of down-to-the-west normal faults that step up almost to the crest of the Mosquito Range. Many of these faults have a Laramide ancestry, as shown by their relationship to dated Laramide intrusive rocks (Tweto, 1979). "These west-dipping faults originated as antithetic faults in the flank of the Laramide Sawatch anticline, and they were reactivated in Neogene time to serve as elements of the upper Arkansas Valley graben structure." (Tweto, 1979, p. 44.) 3.5. The Gore and Front Range uplifts The Gore Range is a narrow, rugged, uplifted block of Proterozoic rock bounded on the west by the Gore fault and on the east by the Blue River normal fault (Fig. 1). The Blue River Valley follows a 5 to 9 km wide graben that is floored mostly by Upper Cretaceous shale (Tweto et aI., 1978; Kellogg, 1997) and separates the Gore Range from the western side of the Front Range. The Williams Range thrust bounds the graben on the east and is a low-angle to nearly horizontal structure that probably steepens farther to the east under the Front Range block, where it presumably overlies basement rock (Fig. 3C). The Williams Range thrust defines the western structural boundary of the Front Range, which, on the east, is bounded by high-angle Laramide contractional faults (Fig. 1). Erslev (1993) suggested that the Williams Range thrust is a back thrust in an overall eastdirected Laramide thrust system, which implies that that Front Range is completely detached. The Gore fault has had a long geologic history, the details of which are somewhat controversial. It was active during Proterozoic, late Paleozoic, and Late Cretaceous-early Tertiary time (Tweto, 1979) and approximately defines the eastern structural margin of the late Paleozoic central Colorado trough (Fig. 3C). This trough collected a thick, eastwardthinning clastic assemblage, banked against the east- em margin of the trough and shed westward from the Ancestral Front Range uplift (DeVoto. 1980). The uplift began during early Pennsylvanian time and created a region that remained topographically high until Late Jurassic time; sedimentary rocks older than Jurassic are missing from an area that includes most of the present Front Range. Gore Range. Park Range. Middle Park and the northern Blue River graben (Tweto. 1975). Tweto et al. (1970) interpreted the Gore fault. on the west side of the Gore Range. as a normal fault. but a more recent interpretation suggests that it represents major Pennsylvanian and Early Permian west-directed contractional faulting that accompanied uplift of the Ancestral Front Range (Ye et al.. 1996). Evidence is lacking for major Laramide movement along the Gore fault (Tweto and Lovering, 1977), although Snyder (1980) documented major west-directed Laramide thrusting along the western margin of the Park Range (northern continuation of the Gore Range). The complex pattern of faulting and the steep to overturned eastern margin of the central Colorado trough mirrors that on the west side of the Sawatch uplift, all of which supports a model for both late Paleozoic and Laramide contractional movement along the Gore fault. 3.6. The Blue River graben and western Middle Park Although deep sediment-filled basins of the Rio Grande rift end at the northern end of the upper Arkansas River graben, major Neogene faults related to Rio Grande rifting extend for a considerable distance farther north. The Neogene Blue River normal fault, along the east side of the Gore Range (Fig. 3C). is a major element of the Rio Grande rift system that underwent several thousand meters of down-to-theeast movement, probably no later than the Pliocene (West, 1978). In addition, numerous north-striking normal faults in the Blue River graben are almost entirely east-dipping (Kellogg. 1997), indicating that the Blue River graben is a west-tilted structure. The valley, however, contains little Neogene basin-fill deposits, indicating that the drainage basin formed by the graben was neither structurally deep nor closed to the north, where the Blue River empties into the Colorado River near Kremmling (Fig. 1). The western Middle Park basin (Fig. I) also forms a west-tilted Neogene half graben, filled by K.S. KelluggI Tectonuphvsics 305 (1999) 1·J/-152 ...... " - l ~I . ,", . .', : ~ ~ I. : ~~. ,.. '. ., - ~ ... , ".' .. ,'-' , ";,1iocene basin-fill deposits of the Troublesome Fora sequence of tuffaceous siltstones, sand~tones, and conglomerates (lzett, 1975). On the west, o1he down-to-the-east Antelope Pass nonnal fault :)ounds the Middle Park basin and approximately ';'ollows the trace of the Williams Range thrust. ~pation, •. Discussion •. ", ~" 'l ,. ":'./ OJ ~. At several localities along the northern Rio 'Grande rift there appeoars to be an association between Neogene nonnal faults and Laramide thrust .'faults, strongly suggesting that the two types of faults are genetically linked. The model presented . here for the development of Tertiary basins along the , 'crests of earlier thrust-bounded uplifts explains this :.'association; the San Luis basin and its relationship _H:.~with the San Juan-San Luis uplift is an excellent ex...'iample (Fig. 3A). In addition, the near coincidence of :,;the Villa Grove accommodation zone, where stratal ::1 tilt reverses direction, and the boundary between the .i)0" Sawatch uplift and the San Juan-San Luis uplift, ,~J where thrust-transport direction also switches direc.,:, tion, does not seem merely fortuitous. The Villa : Grove accommodation zone appears to exploit a pre; existing zone of crustal weakness that is at least as ;' old as Laramide. It is worth noting, although not ,i discussed in this paper, that similar structural precursors may have also existed for other accommodation ': zones (the Embudo, Santa Ana, Tijeras, and Socorro :< accommodation zones of Chapin and Cather, 1994) ., farther south along the Rio Grande rift (Russell " and Snelson, 1994; S.M. Cather, written commun., 1998). As applied to the Sawatch uplift, the model 0' r does not as clearly define the relationship between Laramide and Neogene structures as it does for the San Juan-San Luis uplift. About 30 km separates the west margin of the uplift and the upper Arkansas River graben, a distance considerably wider than other recognized perched basement wedges. The western margin of the Sawatch uplift is not universally interpreted as a contractional margin (e.g. Tweto et aI., 1978), although evidence presented here strongly suggests that it is contractional. If true, faults along the west margin of the Sawatch uplift, possibly including the Elk Range thrust, represent .Y 0 . .- ~ ,.' ... ' ',:' ,,' O· . 1.+9 the surface traces of a listric west-directed fault system that underlies the Sawatch uplift and accommodated Laramide crustal shortening, eastward tilting. and uplift. The Dry Union Fonnation in the upper Arkansas River graben dips toward both the master nonnal fault along the west side of the valley and the steep, thrust-faulted side of the Sawatch uplift (Fig.3B). The upper Arkansas River graben lies astride the central part of the Sawatch uplift. The relatively large distance between the valley and the west side of the uplift, where the model would predict that the basement overhang would sag into the adjacent sedimentary basin, is difficult to reconcile. Possibly, arching and gravitational spreading (extension) concurrent with Laramide intrusive activity under the Sawatch uplift may have contributed to extensional strain as well as facilitated movement along thrusts, thereby pennitting the wide 'perched basement wedge: A complicated relationship also exists between the Gore Range-Front Range uplift and the Blue River graben, but nonetheless indicates that the graben inherited its geometry from older contractional structures. The Gore fault, along the west side of the Gore Range, is not entirely a Laramide structure; indeed, most of the movement along the fault may be late Paleozoic in age. Nonetheless, the Blue River graben tilts strongly west and sits above a west-directed thrust system, suggesting that the nonnal faults sole just as easily into thrusts that are partly Paleozoic in age as they do into thrusts that are exclusively Laramide. Irrespective of the age of the Gore fault, westward tilting of the southern Blue River graben and western Middle Park basin was accommodated along nonnal faults, interpreted as listric, that sole into the thrust (Fig. 3C). The Gore Range represents a 'perched basement wedge' bounded on the west by the Gore thrust or reverse fault and on the east by the Blue River nonnal fault. Similarly, westward rotation of the western Middle Park basin resulted by movement along the Antelope Pass nonnal fault, which may merge at depth with the Gore thrust. East-directed, commonly blind Laramide thrusts that probably also have a late Paleozoic ancestry also bound the east margin of the Front Range uplift, although a Neogene rift valley associated with this margin did not fonn. Apparently, crustal extension 150 K.S. Kellogg/Tectonophysics 305 (1999) 141-152 coeval with Rio Grande rifting did not extend to the e:lst margin of the Front Range uplift. 5. Conclusions .' ; ., ., This study documents a style of deformation that is characteristic, not only of the northern Rio Grande rift. but also of large parts of the Rocky Mountain region of North America. Future studies may judge whether there are analogues throughout the world. The major conclusions of this study are summarized as follows. (1) An empirical association between the sense of asymmetry of several basins of the northern Rio Grande rift and the sense of asymmetry of the Laramide uplift on which the rift basin sits suggests a genetic link. The down-rotated side of each basin appears on the same side as the thrust-faulted side of the uplift. The thrust faults that bound the uplifts are interpreted to be listric and flatten in the middle crust at about 15-16 km. (2) The Villa Grove accommodation zone bounds the down-to-the-east San Luis basin portion of the Rio Grande rift and the down-to-the-west upper Arkansas River graben, and also appears to define a zone that separates the east-vergent San Juan-San Luis arch from the west-vergent Sawatch uplift. (3) Fault relationships on the west side of the Sawatch uplift suggest that a west-directed Laramide thrust, defined, in part, by the trace of the Castle Creek fault zone, underlies the uplift. The upper Arkansas River graben lies astride the Sawatch uplift and tilts west into a large 'master' normal fault. (4) The Blue River graben and western Middle Park basin lie astride the Front Range-Gore Range uplift and tilt to the west against large east-facing normal faults. Major late Paleozoic movement along the thrust on the west side of the Gore Range, associated with uplift of the ancestral Rocky Mountains, reactivated during the Laramide. (5) The above relationships suggest a model, similar to one first summarized by Schmidt et a\. (1984) and expanded upon by Kellogg et al. (1995), in which rotational sag of a thrust-bounded basement overhang into an adjacent sedimentary basin produced zones of extension in the arched basement block. During subsequent crustal extension, the al- ready-extended zones in the arched central part of the fault blocks underwent gravitational collapse, producing an asymmetrical graben along the axial part of the arch. The master normal fault controlling the down thrown side of the graben roots into the older thrust. The resultant 'perched basement wedge' is bounded on one side by a thrust and on the other side by a younger normal fault. Acknowledgements I am indebted to Bruce Bryant, S.M. Cather, E.A. Erslev, S. Haldeway, D.H. Knepper, D.R. Lageson, S.A Minor, R.B. Scott, and AR. Wallace for very helpful reviews. Most of these reviewers also provided invaluable discussions, both in the field and office. References Arabasz. W.1 .• Pechmann. J.e.. Brown. E.D .. 1992. Observational seismology and the evaluation of earthquake hazards and risk in the Wasatch Front area. Utah. In: Gori, P.L., Hays, W.w. (Eds.), Assessment of Regional Earthquakes Hazards and Risk along the Wasatch Front, Utah. U.S. Geol. Surv. Prof. Pap. 1500-D, DI-036. Bauer, P.W., Raiser, S., 1995. The Picuris-Pecos fault-repeatedly reactivated~ from Proterozoic(?) to Neogene. New Mexico Geol. Soc. Guidebook, 46th Field Conference, Geology of the Santa Fe Region, pp. 111-115. Bosworth, W., Lambiase, J .. Keisler, D., 1986. A new look at Gregory's Rift - the structural style of continental rifting. Eos 67 (29), 577 and 582. Brister, B.S., Gries, R.R., 1994. Tertiary stratigraphy and tectonic development of the Alamosa basin (northern San Luis basin), Rio Grande rift, south-central Colorado. In: Keller, G.R., Cather, S.M. (Eds.), Basins of the Rio Grande Rift Structure, Stratigraphy, and Tectonic Setting. Geol. Soc. Am . Spec. Pap. 291, 39-58. Bryant, B., 1979. Geology of the Aspen 15-minute quadrangle, Pitkin and Gunnison Counties, Colorado. U.S. Geol. Surv. Prof. Pap. 1073, 146 pp. Bryant. B., Naeser, C.w., 1980. The significance of fission-track ages of apatite in relation to the tectonic history of the Front and Sawatch Ranges, Colorado. Geol. Soc. Am. Bull. 91. 156-164. Chapin, e.E., Cather, S.M., 1994. Tectonic setting of the axial basins of the northern and central Rio Grande rift. In: Keller, G.R., Cather, S.M. (Eds.). Basins of the Rio Grande Rift Structure, Stratigraphy, and Tectonic Setting. Geol. Soc. Am. Spec. Pap. 291, 5-25. '--. ' ...... K.S. Kellogg I Tectollophvsics 305 I f')<JY) /.1/-/52 , ',1 :::ltl~u~ /..' :::: <' ;~)..:.f::~ ... ~.;~;~,. ,.~ ~t" t 'j ..:.; I' j "• '.1 ,I h',' " .. ~.. . Chapin. e.E .• Seager, W.R .. 1975. Evolution of the Rio Grande rift in the Socorro and Las Cruces areas. New Mexico Geol. Soc. Guidebook of Las Cruces Country, 26th Field Conference, pp. 297-322. Christiansen, R.L.. Lipman. P.W.. 1972. Cenozoic volcanism and plate-tectonic evolution of the Western United States, II. Late Cenozoic. Philos. Trans. R. Soc. London A 271, 249-284. Cordell. L., 1978. Regional geophysical setting of the Rio Grande rift. Geol. Soc. Am. Bull. 89. 1073-1090. de Voogd. B .• Serpa. L.. Brown. L.D .. 1988. Crustal extension and magmatic processes. COCORP profiles from Death Valley and the Rio Grande rift. Geol. Soc. Am. Bull. 100. 15501567. DeVoto. R.H .• 1980. Pennsylvanian stratigraphy and history of Colorado. In: Kent. H.C .. Porter. K.W. (Eds.). Colorado Geology. Rocky Men. Association Geologists. Denver. pp. 71101. Dozer. D.L. Smith. R.B .. 1985. Source parameters of the October 18. 1983 Borah Peak. Idaho. earthquake from body wave analysis parameters. Bull. Seismol. Soc. Am. 89.1041-1051. Eaton. G.P.• 1979. A plate-tectonic model for late Cenozoic crustal spreading in the western United States. In: Riecker. R.E. (Ed.). Rio Grande Rift - Structures and Magmatism. American Geophysical Union. Washington. D.e.. pp. 7-32. Epis. R.C .• Chapin. e.E .. 1975. Geomorphic and tectonic implications of the post-Laramide. late Eocene erosion surface in the Southern Rod..-y Mountains. In: Curtis. B.F. (Ed.). Cenozoic History of the Southern Rocky Mountains. Geol. Soc. Am. Mem. 144.45-74. Erslev. E.A.. 1993. Thrusts. back-thrusts. and detachment of Rocky Mountain foreland arches. In: Schmidt. C.1 .• Chase. R.B .• Erslev. E.A. (Eds.). Laramide Basement Deformation in the Rocky Mountain Foreland of the Western United States. Geol. Soc. Am. Spec. Pap. 280. 339-358. Gregory, K.M .• Chase. e.G .. 1994. Tectonic and climatic significance of a late Eocene low-relief. high-level geomorphic surface. Colorado. J. Geophys. Res. 99 (BIO). 20141-20160. Izett. G.A.. 1975. Late Cenozoic sedimentation and deformation in northern Colorado and adjoining areas. In: Curtis. B.F. (Ed.), Cenozoic History of the Southern Rocky Mountains. Geol. Soc. Am. Mem. 144. 179-210. Kelley. S.A.. Chapin. C.E .• Corrigan. J .• 1992. Late Mesozoic to Cenozoic cooling histories of the flanks of the northern and central Rio Grande rift. Colorado and New Mexico. New Mexico Bur. Mines and Min. Res. Bull. 145. 38 pp. Kellogg. K.S .. 1992. Cretaceous thrusting and Neogene block rotation in the northern Portneuf Range region. southeastern Idaho. In: Link. P.K .. Kuntz. M.A.. Platt. L.B. (Eds.). Regional Geology of Eastern Idaho and Western Wyoming. Geol. Soc. Am. Mem. 179.95-113. Kellogg. K.S .. 1997. Geologic map of the Dillon 7.5' quadrangle. Summit and Grand Counties. Colorado. U.S. Geol. Surv. Open-File Rep. OF97-738. scale 1:24.000. Kellogg. K.S .. Schmidt. C.1 .. Young. S.W.• 1995. Basement and cover-rock deformation during Laramide contraction in the northern Madison Range (Montana) and its influence on 151 Cenozoic basin formation. Am. Assoc Pet. Geol. 79 (8). 1117-1137. Kluth. e.F.. Schaftenaar. C.H .. 199-1. Depth and geometry of the northern Rio Grande rift in the Sun Luis basin. south-central Colorado. In: Keller. G.R .. Cather. S."!'. (Eds.). Basins of the Rio Grande Rift - Structure. Stratigraphy. and Tectonic Setting. Geol. Soc. Am. Spec. Pap. 291. 27-38 . Knepper. D.H .. Jr.. 1976. Late Cenozoic structure of the Rio Grande rift lone. central Colorado. In: Epis. R.C .. Weimer. RJ. (Eds.). Studies in Colorado Field Geology. Prof. Contrib. CO. School Min. 8.421-..00 . Lageson. O.R.. 1989. Reactivation of a Proterozoic continental margin. Bridger Range. southwestern Montana. In: French. D.E .• Grabb. R.F. (Eds.). Geologic Resources of Montana. I. Montana Geol. Soc .. Billings. MT. pp. 279-298. Lindsey. D.A .. Lohnson. B.R .. Andriessen. P.A.M.. 1983. Laramide and Neogene structure of the northern Sangre de Cristo Range. south-central Colorado. In: Lowell. J.D. (Ed.). Rocky Mountain Foreland Basins and Uplifts. Rocky Mountain Assoc. Geol.. Denver. CO. pp. 219-228 Lindsey. D.A .• Andriessen. P.A.M .. Wardlaw. B.R .. 1986. Heating. cooling. and uplift during Tertiary lime. nonhern Sangre de Cristo Range. Colorado. Geol. Soc. Am. Bull. 97. 11331143. Lipman. P.w.. Mehnert. H.W.. 1975. Late Cenozoic basaltic volcanism and development of the Rio Grande depression in the southern Rocky Mountains. In: Cunis. B.F. (Ed.). Cenozoic history of the Southern Rocky Mountains. Geol. Soc. Am. Mem. 144. 119-154. McIntosh. w.e.. Chapin. C.E.. Ratte. J.e.. Sutter. 1.F.. 1992. Time-stratigraphic framework for the Eocene-Oligocene Mogollon-Oatil volcanic field. southwest New Mexico. Geot. Soc. Am. Bull. 104.851-871. Perry Jr.• W.1 .• Grout. M.A .. 1988. Wedge model for late Laramide basement-involved thrusting. Grand Hogback monocline and White River uplift. western Colorado. Geol. Soc. Am. Abstr. Prog. 20 (7). 384-385. Reiter. M .• Barrol!. M.W.. Cather. S.M .. 1992. Rotation buoyancy tectonics and models of simple half graben formation. 1. Geophys. Res. 97 (B6). 8917-8926. Royse. F.. Jr.. Warner. M.A .. Reese. D.L.. 1975. Thrust belt structural geometry and related stratigraphic problems. WyomingIdaho-northern Utah. In: Bolyard. D.W. (Ed.). Deep Drilling Frontiers in Central Rocky Mountains. Rocky Mountain Assoc. Geologists. Denver. CO. pp. 41-54. Russell. L.R.. Snelson. S .. 1994. Structure and tectonics of the Albuquerque Basin segment of the Rio Grande rift - insights from reflection seismic data. In: Keller. Keller. G.R .. Cather. S.M. (Eds.). Basins of the Rio Grande Rift - Structure. Stratigraphy. and Tectonic Setting. Geot. Soc. Am. Spec. Pap. 29i, 83-112. Sales. J.K .. 1983. Collapse of Rocky Mountain basement uplifts. In: Lowell. J.D .• Gries. R. (Eds.). Rocky Mountain Foreland Basins and Uplifts. Rocky Mountain Assoc. Geol.. Denver. CO. pp. 79-98. Schmidt. C.1 .. Dresser. H .. 1984. Late Cenozoic structural in- ( ,i j ~ .. ~ . " • 152 K.S. Kellogg / Tectonophysics 305 ( /999) / oJ / - /52 ! ". • . ., version in a region of small extensional strain. SW Montana. Geo1. Soc. Am. Abstr. Prog. 16 (6). 646-647. Schmidt. C.1 .. Sheedlo. M .. Werkema. M.• 1984. Control of range-boundary normal faults by earlier structures. southwestern Montana. Geo1. Soc. Am. Abstr. Prog. 16 (4). 253. Scholten. R.. 1967. Structural framework and oil potential of extreme southwestern Montana. In: Henderson. L.B. (Ed.). Montana Geological Society 18th Annual Field Conference Guidebook. Montana Geo1. Soc .. Billings. MT. pp. 7-19. Seager. W.R .. Morgan. P.. 1979. Rio Grande rift in southern New Mexico. west Texas. and northern Chihuahua. In: Riecker. R.E. (Ed.). Rio Grande Rift - Structures and Magmatism. American Geophysical Union. Washington. D.e.. pp. 87-106. Sharry. J.. Langen, R.T.. Jovanovich. D.B .. Jones. G.M .• Hill. N.R .• Guidish. T.M .. 1986. Enhanced imaging of the COCORP seismic line. Wind River Mountains. In: Barazingi. M .• Brown. L. (Eds.), Reflection Seismology - A Global Perspective. Am. Geophys. Union. Geodyn. Ser. 13. 223-236. Smith. R.B .. Bruhn. R.L.. 1984. Intraplate extensional tectonics of the eastern Basin-Range - inferences on structural style from seismic reflection data. regional tectonics. and thermalmechanical models of brittle-ductile deformation. J. Geophys. Res. 89 (B7). 5733-5762. Snyder. G.L.. 1980. Geologic map of the northernmost Gore Range and southernmost northern Park Range, Grand. Jackson. and Routt Counties. Colorado. U.S. Geol. Surv. Misc. Invest. Map 1-1114. scale 1:48.000. Spurr. J.E .. 1898. Geology of the Aspen mining district. Colorado. U.S. Geol. Surv. Monogr. 31,260 pp. Steven, T.A., 1975. Middle Tertiary volcanic field in the southern Rocky Mountains. In: Curtis. B.F. (Ed.). Cenozoic History of the Southern Rocky Mountains. Geol. Soc. Am. Mem. 144. 75-94. Steven. T.A.. Evanoff. Emmett and Yuhas. R.H.. 1997. Middle and late Cenozoic tectonic and geomorphic development of the Front Range of Colorado. In: Bolyard. D.W., Sonnenberg. S.A. (Eds.), Geologic History of the Colorado Front Range. Rocky Mountain Assoc. Geol.. Denver. CO. pp. 115-124. Tweto. 0 .. 1975. Laramide (Late Cretaceous-Early Tertiary) orogeny in the southern Rocky Mountains. In: Curtis. B.F. (Ed.). Cenozoic History of the Southern Rocky Mountains. Geol. Soc. Am. Mem. 144, 1-44. Tweto, 0 .. 1978. Northern rift guide I, Denver-Alamosa. Col- orado. Guidebook to Rio Grande rift in New Mexico and Colorado. New Mexico Bur. Min. Miner. Resour. Circ. 163. 13-27. Tweto. 0.. 1979. The Rio Grande rift system in Colorado. In: Riecker. R.E. (Ed.). Rio Grande Rift - Tectonics and Magmatism. American Geophysical Union. Washington. D.C .. pp.33-56. Tweto. 0 .. Case. J.E .. 1972. Gravity and magnetic features as related to geology in the Leadville 30-minute quadrangle. Colorado. U.S. Geol. Surv. Prof. Pap. 726-C. 31 pp. Tweto. 0 .. Lovering. T.S .• 1977. Geology of the Minturn 15minute quadrangle. Eagle and Summit Counties. Colorado. U.S. Geo1. Surv. Prof. Pap. 956. 96 pp. Tweto, 0 .. Bryant, Bruce. and Williams. F.E .. 1970. Mineral resources of the Gore Range-Eagles Nest Primitive Area and vicinity. Summit and Eagle Counties, Colorado. U.S. Geol. Surv. Bull. 1319-C. 127 pp. Tweto, 0 .. Moench. R.H .• Reed. I.e.. Jr.. 1978. Geologic map of the Leadville 1° x 2° quadrangle. northwestern Colorado. U.S. Geol. Surv. Misc. Invest. Ser. Map 1-999, scale 1:250.000. VanAlstine. R.E .• 1968. Tertiary trough between the Arkansas and San Luis valleys. Colorado. U.S. Geol. Surv. Prof. Pap. 600-C. CI58-CI60. Wallace, A.R .• Blaskowski. M.1 .• 1989. Geologic map of the Mount Jackson quadrangle and the eastern part of the Crooked Creek Pass quadrangle. Eagle County. Colorado. U.S. Geol. Surv. Misc. Invest. Ser. Map 1-1909, scale 1:24.000. Wawrzyniec. T.F.. Geissman. J.W., 1995. Transpressional kinematics of the Elk Range thrust and the Castle Creek structural zone - evidence for Laramide northward translation of the Colorado Plateau. Geol. Soc. Am. Abstr. Prog. 27 (6). 123124. West. M.W.. 1978. Quaternary geology and reported surface faulting along east flank of Gore Range. Summit county, Colorado. Q. CO. School Min. 73 (2). 66 pp. Wise, D.U .• 1963. Keystone faulting and gravity sliding driven by basement uplift of Owl Creek Mountains, Wyoming. Am. Assoc. Pet. Geol. 47 (4). 586-598. Yeo H.. Royden. L .. Burchfiel Schue. C.• Schuepbach. M.. 1996. Late Paleozoic deformation of interior North America - the greater Ancestral Rocky Mountains. Am. Assoc. Pet. Geol. 80. 1397-1432.