Structural Discordance Between Neogene Detachments and Frontal Sevier Thrusts, Central Mormon Mountains, Southern Nevada

Home , Autochthon (geology), Meadow Valley Mountains, Mormon Mountains

TECTONICS, VOL. 4, NO. 2, PAGES 213-246, FEBRUARY 1985

STRUCTURAL DISCORDANCE BETWEEN NEOGENE DETACHMENTS AND FRONTAL SEVIER THRUSTS, CENTRAL MORMON MOUNTAINS, SOUTHERN NEVADA

Brian Wernicke

Department of Geological Sciences, Harvard University, Cambridge, Massachusetts

J. Douglas Walker and Mark S. Beaufait

Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts

Abstract. Detailed geologic mapping in The geometry and kinematics of normal the Mormon Mountains of southern Nevada faulting in the Mormon Mountains suggest provides significant insight into that pre-existing thrust planes are not p•ocesses of extensional tectonics required for the initiation of low-angle developed within older compressional normal faults, and even where closely orogens. A newly discovered, WSW-directed overlapped by extensional tectonism, need low-angle normal fault, the Mormon Peak not function as a primary control of detachment, juxtaposes the highest levels detachment geometry. Caution must thus be of the frontal most part of the exercised in interpreting low-angle normal east-vergent, Mesozoic Sevier thrust belt faults of uncertain tectonic heritage such with autochthono•s crystalline basement. as those seen in the COCORP west-central Palinspastic analysis suggests that the Utah and BIRP's MOIST deep-reflection detachmentinitially dipped 20-25ø to the profiles. Although thrust fault west and cut discordantly across thrust reactivation has reasonably been shown to faults. Nearly complete lateral removal be the origin of a very few low-angle of the hanging wall from the area has normal faults, our results indicate that exposed a 5 km thick longitudinal it may not be as fundamental a component cross-section through the thrust belt in of orogenic architecture as it is now the footwall, while highly attenuated widely perceived to be. We conclude that remnants of the hanging wall (nowhere more while in many instances thrust fault than a few hundred meters thick) reactivation may be both a plausible and structurally veneer the range. The attractive hypothesis, it may never be present arched configuration of the assumed. detacbnnent resulted in part from progressive "domino-style" rotation of a INTRODUCTION few degrees while it was active, but is largely due to rotation on younger, In Bally et al.'s [1966] classic structurally lower, basement-penetrating account of the Cordilleran foreland fold normal faults that initiated at and thrust belt in Canada, evidence was high-angle. presented suggesting that a period of extension, characterized by west-dipping, Copyright 1985 lis'tric normal faults, followed eastward by the American Geophysical Union. 'thrusting and apparently reactivated the older thrust faults. "Backslippage" on Paper number 4T0792. thrust faults has been documented by 0278-7407/85/004T-0792510. O0 seismic reflection profiling throughout 214 Wernicke et al.: Central Mormon Mountains

114• 30' W

:57' N

parts of the thrust belt not severely an earlier compressional phase of movement overprinted by extensional tectonics [e.g., McDonald, 1976; Keith, 1982; [e.g., Royseet al., 1975; Royse, 1983]. Drewes, 1967, 1981]. The normal faults commonly have up to The Mormon Mountains, located along the several kilometers of displacement, and eastern margin of the Basin and Range form basins in which older strata are physiographic province (Figure la), is a antithetically rotated into the fault well-exposed example of low-angle normal surface, with older strata dipping more faults superimposed upon thin-skinned steeply than younger. In some parts of compressional structures. Here, the Cordilleran thrust belt (particularly structures associated with the Sevier the Nevada-Utah sector or Sevier orogenic orogeny are distinguishable from the belt, and its hinterland to the west), a extensional faults because of their group of enigmatic terrains are classical fold-thrust belt geometry and characterized by large-scale younger- their detailed characterization in over-older faulting much more complex than relatively undisturbed ranges along strike the indisputably "backslipped" portions of of the Sevier belt to the south [Burchfiel the thrust belt in Canada and Wyoming. In at al., 1974, 1982; Davis, 1973; Bohannon, these terrains the concept of thrust fault 1981, 1983a, b; Cart, 1983; Axen, 1984]. reactivation has been widely applied to Thus, this terrain offers an unusual faults which lack prima facie evidence of opportunity to examine the influence of Wernicke et al.: Central Mormon Mountains 215

,,, ...•...... •;i..-..-::..:-.- ::?-:.:.:-• 2,•.,

' / ,-.<__-tLv/, '_ 2,.) ,-' a'," '--':?' k /• • /\ I.•,I • -4•:•,• ,,--- , l,•' ,, x /,:•'•--"I I.'--; ( '7' /' 7C)e3 ,- F •,• • ..•:"'--"•:•..,....?__•2•:•6.3,.65,•" ,'•,- ¾.WIREGRASS.=.% HACKB•Ri•{-'x"., • '•. "JIL.•r• - •' •."• •/•..:•••iii'"•'•2•:•;.::.-.'•,,"", •SPRING • %•. %,•SPRING•) •--•\ ß /. -, ..,,'

RoxNEe'>.-'- •;r• •PETROGLYPHSDavidson Peak Z5' < •Moupa Peuk NW Z5' L-+ ?.5' .... deep Trail 0 • 2 l , i krn Fig. lb. Localities referred to in text, superimposed on contacts from Figure 3. Mormon Peak allochthon is shaded.

preexisting compressional structures on should have no trouble forming in the large-magnitude extensional tectonics, as absence of any preexisting anistropy. well as an opportunity to gain new insights into the extensional st•ctures Previous Studies themselves. This report summarizes the results of Apart from the brief mentions of the detailed mapping from the 1979, 1980 and MormonMountains by Spurr [1903] and 1981 field seasons, which covered the Longwell [1926], C. M. Tschanz [1959; central part of the Mormon Mountains Tschanz and Pampeyan, 1970] was the first (Figures la, lb). The results of this to describe the geology of the map area study have major implications for the (Figure la). He mappedthe area during interpretation of the many orogenic belts the late 1950's and early 1960's in which have undergone a period of reconnaissance as part of a report on the compression followed by extension. In geology and mineral deposits of Lincoln particular, the process of reactivation of County, in which he outlined the basic thin-skinned th•st faults appears to be distribution of rock types. However, due of lesser importance than the formation of to a lack of detailed stratigraphic new low-angle normal faults in information, limited field time, and accommodating crustal extension. Our exceptional structural complexity, he was documentation of this process bears not able to identify the major structural heavily on the interpretation of deep features of the area discussed in this reflection profiling in "collapsed" report. 01more [1971] described and thin-skinned thrust belts. It appears mapped the structure of the East Mormon that low-angle normal faults mapped or Mountains and a portion of the Tule detected seismically within older thrust Springs Hills. He interpreted the geology belts may not be tacitly assumed to be of the East Mormon Mountains in terms of reactivated thrusts. Further, since large-scale, east-directed thrust faulting If If low-angle normal faults can break on new locally modified by backslipping ; planes discordant to thrusts, then they however, most of the faults he mapped as 216 Wernicke et al.' Central Mormon Mountains

severely modified by Tertiary extensional BIRD SPRING FORMATION(top not exposed) Limestone,gray, aphanlhcta coarse-grained,cherry, sandy toter- deformation. Olmore' s interpretations, beds, highly fossiliferous similar to those of many workers who tried MONTE CRISTO LIMESTONE ( 279 m) YELLOWPINELIMESTONE (53m see Bullionfor htholo•y) to view low-angle normal faults in terms l ARROWHEAD LIMESTONE(3m) Limestore,hcjht olive-cjra)' t aphanmc, thru-bedded of compressional thrusting, predated major BULLION LIMESTONE (110m) Limestone,hght olive-gray, thru-to thick-bedded, very fine to coarse- breakthroughs in the understanding of grained, locallycherry, crino•lal

ANCHOR LIMESTONE (•,5 m) extensional tectonics in the Basin and -'X L•mestone,dark gray, fine-grained, very cherty DAWN LIMESTONE (78m) Range made during the 1970's [for example, Limestone,light ohve-gray, very fine to coerse-groned, thin-to rneclum- bedded•chert), toward top• fossaliferous, locally cross-laminated Anderson, 1971; Wright and Troxel, 1973; SULTAN LIMESTONE (216 m) Proffett, 1977] ß Nonetheless, his mapping CRYSTAL PASS LIMESTONE (69m) L•rnestone,hght c,lroy, ophonit•, laminated sandstone marker bedneer top has provided important information for VALENTINE LIMESTONE (79m) Lm-•slone,hght gray, fine-grated, very thin-bedded, brecclated, sd•.eous interpreting the area considered in this Dolomite,light gray, fine to coo'se-gramed,thin-bedded, brecc•ed report. Longwell mapped the southern IRONSIDE DOLOMITE (68 m) Dolon'•te,oave-Uack, fine to coarse-cJraned,kx•nated to lhm4)edded, vuggy portion of the Mormon Mountains in Sdtydalom•te, grayish orange, laminated tothin-bedded; dolaren•lc reconnaissance for the 1:250,000 geologic UNDIFFERENTIATED ORDOVICIAN (109 m) Dolomite,medium gray, Ol:honiticto medium-grained,t•n--bedded, mapof Clark County [Longwell et al., vuggy,brecc•oted _/EUREKA QUARTZITE (5m) 1965], and published several reports on POGONIP GROUP (152 m) the geology of the Muddy Mountains and Silty dalomde,brown•h gray, medium-grained, laminated to thln-bed- North MuddyMountains [Longwell, 1921, L•mestoneand shale, rned,dm and dive-gray, fine- to ooorse-gralned, ded•foss•h o•_ohtic,ferou• fossiliferous 1926, 1949, 1962]. Ekren et al., [1977] SIIty dolomite, pale orange,free to coarse-grained,laminated to thin-bedded,chertyt brecclated mapped part of the northernmost Mormon NOPAH FORMATION (102 m) •te, hghtdrve-gray, fine to (:oame-<•d, sdty,cherry, Iooally bmcc•ated Mountains as part of a reconnaissance

SdtyDUNDERBURGdolorrute, orang•:,hSHALE to(12m)green•:,h gray, r•p-up clasts, glaucond•c sand study of Tertiary rocks in Lincoln County. BONANZA KING FORMATION (769m) BANDED MOUNTAIN MEMBER (569 m) Und 5 - Dolomite,light ohve to darkgray, fine-to coarse-graned, STRATIGRAPHY laminatedto th•n-bedded,cherry w•th columnarstrornatolites near top _/ Unit4- Dolomde,light to darkgray, fine-to ccx3rse-cjra•ned, lam•nated to very thm-beddecL,•3nt strcmatolltesnear base The stratigraphy of the central part of J toolomitic tt'un-beddeds•ltstone, grayish orange, fine tomed•um-graned, laminated the Mormon Mountains consists of an -/ Dolomite,light ohve-gray, medium tocoarse-grained, tt'un-bedded, locally sllty, oohhc,black bond at base approximately 2000 m thick Cambrian Unit 3- Dolornlte,I•Jht to darkgray, fine to coome-grained,lamlr•3- through Pennsylvanian sequence of shallow ted to thin-bedded,burrow mottled, algol Iominites ol bc•, sdty bed at top marine rocks transitional between the thick miogeoclinal sediments to the west Umt 2 - Dolomite,light and dark gray,fine to medium-grained,lorn- and the thin veneer of cratonal sediments mated to thin-bedded,burrow mottled, locallysllty characteristic of the Grand Canyon region UnitI - Dolomite,light to darkgray, fine to medium-grained,lamin- to the east. This sequence nonconformably ate(I, locallysilty overlies Proterozoic crystalline rocks and J Siltydolomite, lightbrown, laminated, locallysandy, referred toinform- ally as the '•ilty umt" is in turn overlain unconformably by PAPOOSE LAKE MEMBER (20Ore) Dolorn•te,light to dark gray,fine to coarse-grained,laminated to thick- Quaternary alluvial and landslide deposits bedded,locally sllty, burrowmottled (Figure 2 and Plate 1). The Paleozoic strata can be divided into three major

Dok•itlc hmestoneand dolomite,gray, fine-grained, very th•n-bedded, sequences: a 284 m thick lower terri- oncolitic genous sequence, a 1205 m thick middle BRIGHT ANGEL SHALE (139 rn ) Slitstone,sandstone. and shale, yellowishtO greenish brown, thin wavy dolomite sequence, and 511-m-thick upper bedded,strongly b[oturbated Limestone,dark to mediumgray, fine to medlurn-cjronecksllty,orcohtc limestone sequence. The lower terrigenous sequence consists of the Tapeats Sandstone TAPEATS SANDSTONE (145m) and Bright Angel Shale (Lower to Middle Arkoslcsandstone, grayish red, medium to coarse-grained,thin to thick-bedded,brown•h gray sandstones and sliMonesin upperpart, locally conglomerahc Cambrian), the middle dolomite sequence is composed of the Bonanza King Formation,

METAMORPHIC AND IGNEOUS ROCKS Nopah Formation, Pogonip Group, Ely Springs Dolomite, and the Ironside Dolomite Member of the Sultan Limestone (Middle Cambrian through Late Devonian), Fig. 2. Stratigraphic column, central and the upper limestone sequence consists Mormon Mountains. of the Valentine and Crystal Pass Limestone Members of the Sultan Limestone, the Monte Cristo Limestone, and the lower thrusts are unrelated to the Sevier part of the Bird Spring Formation (Late orogeny. Structures in the Tule Springs Devonian through Pennsylvanian). The Hills not thought to have experienced lower terrigenous sequence contains "backslippage" by Olmore have been several thin carbonate units within the Wernicke et al.: Central Mormon Mountains 2]7

Bright Angel Shale, and the dolomite and Monte Cristo Limestone. The geometry of limestone sequences are intercalated with the Mormon thrust system iadicates that it several sandstone, siltstone and shale was formed by east-directed, units. ramp-decollement style thrusting. The Thicknesses of stratigraphic units thrust faults are either bedding-parallel given above and in Figure 2 were or cut up-section to the east (Plates 1 determined by Jacob's staff measurement in and 2). Figure 4 depicts the geometry of the easternmost part of the map area and the thrust system by showing, in map view, in the East Mormon Mountains. Pronounced the lithologies immediately below the sole thickening of the lower part of the of the thrust system. This map indicates miogeoclinal sectioa occurs within that the base of the thrust system autochthonous rocks (below the basal overrides Mississippian rocks in western Sevier allochthon or Mormon thrust plate, areas of exposure, and ramps up across Figure 3) across the area. Using cross Pennsylvanian rocks to the east. section thicknesses, it is estimated that Enigmatically, the decollement localized Cambrian strata are about 50% thicker in in the most isotropic units in that part the western Mormon Mountains than in the of the section: the footwall decollement East Mormon Mountains, giving a total over much of the Mormon Mountains occurs east-to-west thickening of roughly 400 m. in the Bullion and Yellowpine Limestones, both massive, coarse grained, poorly STRUCTURAL GEOLOGY bedded units, rather than in the well bedded, finer-grained limestones Four major structural levels produced immediately above and below (Sultan by compressional and extensional faulting Limestone and Bird Spring Formation, of the miogeoclinal package are separated Figure 2). Even more enigmatic is that by, from bottom to top, floor faults the hanging wall of the Mormon thrust of duplexes below the Mormon thrust, the detached everywhere in strong dolomites of Mormon thr•st, and the Mormon Peak the Banded Mountain Member of the Bonanza detachment. These levels will be referred King Formation, largely in units 2 and 3, to here, from bottom to top, as the rather than in weaker shales and autochthon, parautochthon, Mormon thrust limestones in the Bright Angel Shale only plate, and the Mormon Peak allochthon a few hundred meters below. A similar (Figure 3). The parautochthon and Mormon geometry of decollement thrusting exists thrust plate were emplaced over the to the south in the Spring Mountains autochthon by east-directed thrusting [Burchfiel et al., 1982; Willemin et al., during the Mesozoic Sevier orogeny. In 1981], where the major thrusts are Miocene time, both the autochthon and detached just a few hundred meters above thrust stack were dismembered by a system the Bright Angel Shale in strong Bonanza of west-dipping, low-angle normal faults, King dolomites. These observations, the largest of which is the Mormon Peak combinedwith Bohannon's [1983 a, b] detachment. Normal faulting caused mapping in the Muddy Mountains, define a varying amounts of eastward tilting of 200 km-long segment of the Cordilleran rock units in the map area; in the foreland fold and thrust belt in which the footwall of the Mormon Peak detachment, major frontal decollement ignored structurally lower rocks (autochthon and presumably weak layers in the sedimentary Precambrian basement) lie to the west of wedge and broke the "hard way" (Figure 5). structurally higher rocks (parautochthon The parautochthonous duplexes beneath and Mormonthrust plate). the Mormon thrust dip northeastward and are folded by northwest-trending, Mormon Thrust System northeast-vergeat overturned folds, suggesting transport to the northeast The Mormon thrust system, exposed in (Fig. 6). Parautochthonous slices were the eastern half of the map area (Plate 1 probably derived from a ramp between the and Figure 3), is composedof the Mormon Cambrian and Mississippian decollement thust plate, containing the Banded portions of the thrust system, because Mountain Member of the Bonanza King bedding in the slices generally intersects Formation and the Nopah Formation, and the bounding faults at high angle. The structurally lower duplexes which contain general northwest strike of strata in the every formation from the Nopah Formatioa slices suggests that the ramp from which to the Yellowpine Limestone Member of the they were derived may also have had a 218 Wernicke et al.. Central Mormon Mountains Wernicke et al.: Central Mormon Mountains 219

•Westernlimit,exposures ß ofautochthonous œbb• c• ••. ,•..•-r • •ern limit,ex•res ] I ofaut•h• •b II MPb

Mmb ß II and ß

• • • w,..•.•ss

I , I km Fig. 4. Map showing footwall geology of the Mormonthrust system. Barbed line shows map trace of the base of the thrust system from Plate 1.

northwesterly strike. In addition, the Here, recumbent folds directly beneath the ramp east of the Mississippian decollement Mormon thrust are overturned to the portion of the thrust system may also southeast (Plate 1), suggesting that the strike northwest because the footwall Mormon thrust system may have had a period pinchout of the Bird Spring Formation of southeast transport. A thrust fault strikes northwest (Fig. 4). Ramps between two parautochthonous slices certainly need not strike perpendicular to (location A, Figure 4; Plate 1) has been transport direction, but the consistency folded and is overturned to the north. of the northwest orientation with the This indicates that folding of slices, and northeast vergent folds in the therefore at least part of the movement on parautochthon and northeast dip of the Mormon thrust, followed imbrication of parautochthonous slices suggests a rather these two parautochthonous slices. simple initial geometry of breakage, with The location of the ramp between the duplexes derived from a Cambrian- Cambrian and Mississippian decollement Mississippian ramp subsequently emplaced portions of the Mormon thrust is unknown. against the next ramp to the east. The source terrain (original Although these geometries are most simply underpinnings) of the Mormonthrust plate interpreted as the result of must lie west of the westernmost northeast-directed thrusting, they are not autochthonous outcrops of its oldest restrictive evidence for such a transport stratigraphic units (Banded Mountain direction. Any transport direction dolomites, Figure 4), but the upper deduced for the Mormon thrust system segment of the ramp may have overlapped does not take into account any possible these exposures. The easternmost possible rotations of the entire system about a position of the top of the ramp vertical axis during Tertiary deformation. corresponds to the western limit of No systematic pattern of folding was exposures of autochthonous Monte Cristo observed in the Mormon thrust plate. Both Limestone, which corresponds to the southeast- and northeast-overturned folds westernmost outcrops of the Mormon thrust are present, along with both northeast- itself (Figure 4). Perhaps the ramp lay and northwest-striking, open to tight over the western part of the mapped area, upright folds, with no apparent preferred because older-over-younger imbrications orientation of axes. within the Bonanza King Formation there Folds in the autochthon were observed are common, particularly in the area west at one locality (Figure 4, location B). of Davies Spring (Figures lb, Plate 1). 220 Wernicke et al.. Central MormonMountains

W E

s Fig. 6. Axesof macroscopic,flexural-slip folds in parautochthonous slices suggesting northeastward transport.

The presence of these slices may indicate proximity to the ramp. However, these imbrications are also intimately associated with younger-over-older, I_as Vegas west-directed normal faults beneath the Valley Shear MormonPeak detachment. Thus, the Zone older-over-younger relationships may simply be a consequenceof juxtaposing (Restored} pieces of higher thrust allochthons onto the autochthon by normal faulting and be unrelated to the original position of the Cambrian-Mis sis sippian ramp.

9 , 20• No data within the map area yield km information as to the precise timing of thrust faulting. Regional constraints suggest a mid-Jurassic to Late Cretaceous GassPeak- Wheeler age for compressional deformation Pass plate [Armstrong,1968; Carr, 1980; Burchfiel and Davis, 1971, 19811. The total amount of shortening represented by the Mormon Keystone-Muddy Mtn- thrust system is at least 30 km, based on Glendale plate reconnaissance outside the area of Plate 1 Contact,Red Spring, [Wernicke,1982; Wernickeet al., 1984]. Summit, and Eastern Imbricate Normal Fault Belt

Mormon plates A group of west-dipping, rotated low-angle normal faults and associated • Autochthon high angle faults that disrupt the thrust terrane is present in the eastern part of Fig. 5. Major thrust plates of the Sevier the map area (Plate 1, Figure 3). The orogenic belt in southern Nevada. The two largest of these, the Horse Spring fault lowest thrusts consistently detach in (Plates 1 and 2; Figure 3) has dolomites from the CambrianBonanza King approximately 3 km of offset near Whitmore Formation. Between the Keystone-Muddy Mine (Plate 2, D-D'), but almost Mountain and Gass Peak-Wheeler Pass completely dies out only 6 km to the thrusts is a broad, regional synclinorium north. Along section A-A' (Plate 2), the consisting of miogeoclinal rocks which Horse Spring fault may be represented by a have been folded and faulted on small series of normal faults which cut thrusts. autochthonousDevonian and Mississippian Wernicke et al.: Central •ormon Mountains 221 rocks and basal slivers of the Mormon Bonanza King Formation contact, along a thrust system, but cumulative extension segment between Horse Spring and Hackberry along these faults is only about 200 m. Spring. Here the footwall rocks are An interesting kinematic problem of the folded over to the west. Numerous bedding Horse Spring fault is the difference in plane faults within the limestone unit of tilt between the hanging wall and footwall the Papoose Lake Member are apparently in its central area of exposure, as seen folded over, and slices of basal Papoose in section D-D' (Plate 2). As noted in Lake are discordantly faulted across the Wernicke and Burchfiel (1982) planar Bright Angel Shale (sections F-F' and normal faults cannot cause differential B-B', Plate 2). tilt between hanging wall and footwall The Horse Spring fault dips west to unless the blocks deform internally in northwest to the north of Wiregrass some way. Concave upward listric faults, Spring, but abruptly chan•es to a because they are curved, force southwest dip of about 30 v in its southern differential rotation and tilt the hanging exposures. In its southeasternmost wall more steeply than the footwall. exposures, the fault becomes difficult to Since footwall strata are inclined 20-30 define, and is represented on Plate 1 as a degrees more steeply than hanging wall vastly oversimplified zone of faults near strata, and no evidence was found for hill 4322. Chaos structure [Noble, 1941] significant internal deformation of the is spectacularly developed along the fault blocks (except within ca. 100 m of southeastern segments of the fault, the fault plane), neither the planar nor particularly in a canyon 0.5 km due north concave upward listric models are of the petroglyphs (Figure lb), where adequate. Palinspastic restoration of several hundred meters of section are section D-D' elucidates the basic problem omitted across a complex fault zone (see (Figure 7): the fault cuts through the Wernicke and Burchfiel [1982], and Wright footwall section much more quickly than and Troxel [1973] , for an explanation of the hanging wall section, suggesting that the genesis of chaos). the two were not initially in direct In general, the southern portion of the contact. Apparently a wedge of material eastern imbricate belt is much more once lay between the two. The complex than the relatively simple reconstruction in Figure 7 was done so as tilted-block geometry of the northern to minimize the volume of this wedge by portions. Near bedding-parallel faults restoring the hanging wall to its are far more abundant, and brecciation is easternmost possible position. The so intense that in most instances it took simplest kinematic scheme to account for considerable effort to find an outcrop ths missing wedge is to have it squeezed suitable for taking an attitude. As out to the west from between the hanging complicated as Plate 1 is in this area, it wall and footwall. The geometry of the barely begins to document the complexity faults bounding the wedge derived from of deformation. this reconstruction indicates that the Faults structurally beneath the Horse upper one was an east-directed thrust Spring fault generally do not display fault emplacing older rocks over younger. differential tilt between hanging wall and The lower, more steeply dipping fault, in footwall, and are thus believed to be of conjunction with the Horse Spring fault, planar rotational-type as described in forms a downward steepening normal fault Wernicke and Burchfiel [1982]. The total zone, which serves to accommodate extension of the eastern imbricate belt differential tilt between the hanging wall along section D-D' (Plate 2) is and footwall. Undeformed regions are approximately 3 km. The total extenslon separated from imbricate normal fault along F-F' (Plate 2) is a similar amount, systems by concave upward listric fault since a reconstruction of the section zones if the blocks dip toward them should not allow overlap of the Mormon [Wernicke and Burchfiel, 1982], and it is thrust. It is noteworthy that such a suggested here that concave downward fault small amount of extension can cause such systems may serve as boundaries if the an extreme amount of deformation. blocks dip away from the undeformed The direction of extension in the region, as appears to be the case here. eastern imbricate belt cannot be Drag folding directly beneath the Horse determined with precision, but two lines Spring fault is well developed where the of evidence suggest that it is fault crosses the Bright Angel Shale- approximately east-northeast. The first 222 Wernicke et al.' Central Mormon Mountains

ß, I ,.• o ,.• ,-4 •

•:•0 .• o

ß• • o

• o

o $ •

• o .•

.m o •• o o

o • • • o o o • o •

• • o

ß• • o

$ o '•

• • o • .• o • Wernicke et al.- Central Mormon Mountains 223

• 40

• 20

o 0 I0 20 30 40 50 60 70 80 90 80 70 60 50 40 ;50 20 I0 0 degrees e(•st of south degrees e(]st of north DIP DIRECTION Fig. 8. Tilt direction histogram of eastern imbricate belt suggesting an extension direction of N80W+20ø.

is the sense of rotation of autochthonous their formation was not influenced by the Paleozoic rocks. In areas where Mormon thrust, despite their close slickenside striae are abundant within structural proximity. These facts are an simple imbricate normal fault terranes, exception to the notion that low-angle their trend tends to parallel the dip normal faults developed within older direction of rotated strata [e.g., compressional thrust belts are reactivated Anderson, 1971; Davis et al., 1980; Davis thrust faults because the thrusts are and Hardy, 1981] . Thus, assuming that the presumably planes of weakness. The autochthonous Paleozoic section was eastern imbricate belt and its basal roughly horizontal or gently inclined to detachment developed discordantly across the west prior to imbricate normal the older structural grain, suggesting faulting (see below), the extension that (1) preexisting planes of weakness direction may be inferred from their sense are not required for the initiation of of tilt. Tilt directions of autochthonous thin-skinned normal fault systems, and sedimentary rocks (Figure 8) suggest an (2) preexisting structural features such extension direction of approximately as thrust faults do not always influence N80øE. The second, in accord with the their localization. The relationships first result, is derived from two observed here suggest that the fracture of intersecting fault planes which do not crystalline basement and its autochthonous offset each other, located 1.7 km ENE of cover along relatively high-angle planes peak 7083 (just north of section D-D', was easier than reactivating an old, very Plate 2). Regardless of the chronology of low-angle decollement thrust. This is not movement on these two faults, this surprising in view of the likelihood that geometry is only possible if the slip-line the least principal stress was nearly of at least one of the faults is parallel parallel to the geoid during extensional to their line of intersection. The tectonism. orientation of both fault planes is As will be discussed in more detail well constrained, and their line of below, two faults in the eastern imbricate intersection trends S82øWand plunges 25 ø belt cut the Mormon Peak detachment and (Figure 9), in crude agreement with the rotate it eastward (Plate 1). extension direction based on the average dip direction of rotated strata. Central High-Angle Fault System As discussed in Wernicke et al. [1984], an east-directed subhorizontal normal In the central, structurally simplest fault exposed in the Tule Springs Hills part of the map area, a system of and East Mormon Mountains may underlie the high-angle faults are present in the Mormon Mountains at shallow depth and may autochthon st ructurally above the Horse function as a basal detachment for the Spring fault but below the Mormon Peak eastern imbricate belt. The eastern detachment (Plate 1). The faults are of imbricate belt and inferred basal small normal displacement (10-100 m), and detachment involve autochthonous generally dip between50 ø and 70ø . Some crystalline basement (Plate 1), and thus of the faults change orientation along 224 Wernicke et al.: Central Mormon Mountains strike. Collectively, they account for very little extensional strain. The faults may be divided into two sets: a north-northeast-striking set and a northwest-striking set (Figure 10). The apparent sense of throw on the north-northeast striking set is consistently down to the west-northwest, and on the northwest striking set, generally down to the southwest. Assumingaverage strikes of N49øWand N23øE(Figure 11) and a dip of 60ø for the two fault sets in the more organized part of the system (most of the scatter, particularly in the northwest striking $ set, are from faults in the northeastern Fig. 9. Intersection of nonoffsetting part of the area adjacent to the Horse faults, eastern imbricate belt. Spring fault), the faults plot on a stereogram as a near-perfect conjugate shearsystem, with a least •rincipal allochthon, marked at its base by a stress axis oriented at N76E35 ø, trending conspicuous topographic ledge formed by nearly parallel to the principal the Mormon Peak detachment. The elongation direction inferred for the detachment surface forms a smoothly eastern imbricate normal fault system. contoured dome (Figure 12) which mantles Unfortunately, the actual slip direction the domal topography of the range. Shown on these faults is unknown due to lack of on Fig. 12 is a set of topographic offset linear features and slickenside contours which serve to constrain the striae, so it is not possible to projection of the detachment east of peak rigorously support this interpretation. 6365 (it must project above ridges Another, albeit more complex consisting of lower plate strata). The interpretation, would be to consider slip structural relief on the dome is about 1 on the faults as purely downdip and to km over the width of the map area. call attention to the fact that the two Beneath the detachment, a similar broad sets occur largely in separate spatial domal structure is expressed by the domains: a northwest-striking set to the orientation of autochthonous (relative to north and the north-northeast-striking set the Mormonthrust system) Paleozoic to the south. In this case, two separate strata. Figure 13a shows the strike and principal elongation (and presumably, dip direction of attitudes taken in the least principal stress) directions could domal structure, omitting the central be inferred, oriented perpendicular to the high-angle fault system and formational strike of each fault set, and perhaps a contacts. Figure 13b shows a structure third corresponding to the domain of contour map on the base of the Tapeats north-northwest striking faults adjacent Sandstone in the same area drawn by to the northern part of the Horse Spring extrapolating to its stratigraphic depth fault (Plate 1). at each of the points shown. Contouring The fact that one of the faults cuts was done without attempt to extrapolate the Mormon Peak detachment at one end and faults of the central high angle fault is truncated by it at the other strongly system to depth. Although Figures 15a and suggests that the high-angle faults were 15b have approximately the same form, they active during the final stages of differ in that the contour on the Tapeats displacement on the Mormon Peak detachment has an eastward elongate domal crest while (Plate 1; fault A, Figure 10). None of the structural form indicated from the high-angle faults can be shown to attitudes shows a localized crest. The postdate final movement on the Horse localized crest in Figure 13a is Spring fault. coincident with the western part of the elongate crest in the Tapeats, and the Mormon Peak Detachment easten part of the elongate crest is the locus of maximum relative uplift due to The structurally highest tectonic unit northeast- and northwest-striking faults in the map area is the Mormon Peak of the central high-angle fault system. Wernicke et al.. Central Mormon Mountains 225

FAULT A

'-..

Fig. 10. Summary diagram, central high-angle fault system. Bar and ball on downthrown block. Reverse faults marked with an R. Mormon Peak allochthon is stippled.

The difference in form of the two diagrams warping of the autochthonous Paleozoic may thus be attributed to the high-angle would remain. Although there is no a faulting. Since the locus of uplift of priori reason to assume that the the central high-angle fault system does detachment was initially planar, the crude not coincide with the center of the coincidence (on the scale of the range) of well defined dome in bedding attitudes it folding of the lower plate about an seems unlikely that the two structures east-west axis with the detachment fault formed synchronously. arch suggests that the two may be an An important problem is the expression of the same process. It is relationship between the domal structure clear that a substantial degree of folding expressed in the autochtonous Paleozoic of the Paleozoic section predated at least section and the domal form of the late movement on the detachment. If the detachment surface. Inspection of Figures folding of the lower plate accompanied the 12 and 13 suggests that the two structural early history of movement on the forms are certainly not perfectly detachment there must have been enough coincident, the detachment appearing to be movement thereafter to smooth out the form much smoother and having less total of the detachment by truncation and relief. Figure 14 shows two removal of the early-domed surface. cross sections comparing the form of the Alternatively, folding of lower plate lower plate strata and the detachment, and strata may be related to Mesozoic it is clear, at least in cross section deformational events, in which case its A-A', that if the curvature of the spatial association with the domed detachment were removed, a pronounced detachment is merely coincidental. We 226 Wernicke et al.- Central Mormon Mountains

O, = S IOøE 2 ø N although lower plate structures are in •: s8oow 55 ø • detail clearly discordant to the faults [e.g., Davis et al., 1980; Rehrig and Reynolds, 1980]. Since it has been shown that (1) the central high-angle fault system was active during the final stage of emplacement of w E the MormonPeak allochthon; (2) the doming of lower plate strata significantly predates these stages; and inferred that (3) domingof lower plate strata and the development of the central high-angle fault system are diachronous, it follows that doming largely preceded development S of the central high-angle fault system. Flg. 11, Posslble orientations of Both early doming and later high-angle principal stress axes during development faulting are here interpreted to be of the central high-angle fault system. synchronous with movement on the Mormon Peak detachment. If the doming of miogeoclinal strata in the lower plate favor the former hypothesis, on the basis accompanied early movement on the of similar relations in a number of detachment, it seems likely that the Tertiary detachment terrains in the present domed configuration of the southern Basin and Range province. In detachment is a consequence of continued these terrains, axes of folds in lower folding, rather than an initially plate mylonitic foliation are remarkably curviplanar surface of breakage. coincident with domes in the detachments, The detachment cuts downsection toward

o o o o o oO0•,•\ \•\ /\

/ Wernicke et al.' Central MormonMountains 227

0 I":"2 •000

krrl b Fig 13. a) Attitude symbols delineating a domal structure in the autochthon. b) Structure contour on the Tapeats Sandstone datum (feet above sea level). Points show locations where elevation of datum was calculated. Mormon Peak allochthon is stippled.

the west in the footwall. The easternmost the Mormon Peak detachment, it must be klippe in the map area rests upon the assumed that the preextension structural Mormon thrust plate only a few hundred configuration of the Mormon Mountains was meters above the thrust, and the next typical of that of the frontalmost klippe to the west upon autochthonous segments of undisrupted thin-skinned Devonian and Mississippian rocks. The fold-thrust belts. The absence of any remainder of the klippen to the west cut significant structures in the autochthon gradually downward through the autochthon, related to compressional deformation, and until the westernmost exposed klippen rest the simple ramp-decollement geometry of upon Precambrianbasement (Plates 1 and 2, the Mormon thrust system suggest that this Figure 3). is the case. In addition, where An important aspect of the thrusted postdetachment domino-style faulting has terrain is that it provides a reasonably affected the autochthonous sedimentary accurate means by which to measure the package and Mississippian decollement amount of tilting due to the Miocene segment of the Mormon thrust in the extensional event. Cross-sections and eastern imbricate belt, faults now dipping reflection seismic profiles of thrust 20ø-45ø eastwardand makinga high-angle belts which have not been disrupted by (near 90ø) with beddingpalinspastically later events typically show that the restore such that either bedding was autochthonous sedimentary package and initially horizontal (e.g., Figure 17) or bedding-parallel portions of the basal dipping to the east. A large westward thrust fault are very gently inclined preextension dip of these strata would (typically between3 and 8 degrees) away require the imbricate normal faults to from the foreland [e.g., Bally et al., have rotated from an initial west dip 1966; Royse et al., 1975; Cook et al., through the vertical to their current dip, 1979; Price, 1981; Jordan et al., 1983]. an extremely unlikely kinematic scenario. To place constraints on the rotation due Thus, assuming a horizontal or shallow to extension, and hence the initial dip of west dip of the autochthonous strata gives 228 Wernicke et al.: Central Mormon Mountains

B l st A - A' east 8000 I .... t'øøø 6000 me•l• • 6O00

4000 __o• pc,teac'ha•e•'• Oe 14000 2000 • I2000

sea level sea level

A B-B' A'

south I away __ north 6000 toward• 6000

4000 4O00

2000 '2000

sea level sea level

-2000 -2000 no vertical exaggeration Fig. 14. Cross sections of contour maps in Figures 13b and 12, comparing the structural form of the Mormon Peak detachment and the lower plate dome.

a maximum estimate for the initial dip of Basin and Range province who have noted the MormonPeak detachment (Figure 15). the same difference in degree of (Figure 15a) shows cross section B-B' fracturing between compressional and (Plate 2) with all the major blocks extensional faults [e.g., Guth, 1981; extrapolated down to the base of the Seaget, 1970]. In cases where it is Tapeats sandstone. The reconstruction ambiguous whether a low-angle fault is a shown in Figure 15b was done so as to thrust or a low-angle normal fault, the restore the Tapeats to a common, planar presence of large amounts of clast-rotated datum. In doing so, the eastern and breccia favors the latter. western traces of the Mormon Peak In addition to pervasive brecciation on detachment restore into remarkable outcrop scale, the Mormon Peak allochthon alignment, making an angle with the is highly internally deformed at map Tapeats of about 17ø. Addinga scale, and could be characterized as a o o preextension westward dip of 3 -8 for the tectonic megabreccia. Consistently, near Tapeats across the entire area suggests bedding-parallel faults omit stratigraphic the initial dip of the Mormon Peak section, high-angle faults have normal detachmentwas about 20o-25ø . geometry, and both either tangentially As is common with faults in the eastern merge with or are truncated by the imbricate normal fault system, intense detachment. Prior to extension, the brecciation occurs adjacent to the allochthon was composed of an intact detachment, and in both the upper and Paleozoic section ranging from Banded lower plates persists for several tens of Mountain dolomites at the base up to the meters away from it. The breccias are Mississippian and Pennsylvanian Bird characterized by considerable clast Spring Formation at the top. It has been rotation and highly variable grain size broken into a series of tilted normal and sorting. In contrast, the thrust fault blocks. Although the Paleozoic faults of the Mormon thrust system sections within the blocks were probably generally do not have clast-rotated somewhat deformed during the Sevier breccias associated with them. Where orogeny, no evidence was found within any present, they are volumetrically minor, of the blocks to suggest major deformation and usually occur within a few meters in the map area before block faulting, (rather than tens of meters) of the although mapping by Tschanz and Pampeyan faults. These observations are consistent [1970] in the MeadowValley Mountains to with those of a number of workers in the the west suggests that substantial Wernicke et al.. Central Mormon Mountains 229

z o o • m

o z

• z uJ 0

i! •o•o

z < z 0 0 -- o •' •

z • - o 230 Wernicke et al.- Central Mormon Mountains

115ø45'W I .! .! $6ø$O'N /

$6Ol5'N

'6O

o IO Tertioryvolconics a sediments Glendoleplate- Mormon Pk. ollochfhon

Mormon plate end perautochthon • Autochthon

• = horizontal bedding Figure 16. Tectonic map showing the structural relationship between the Mormon Mountains and Meadow Valley Mountains. Geology in part from Tschanz and Pampeyan[1970] and Ekren [1977] ß

Mesozoic compressional deformation and separating the tilted blocks generally dip tilting is present within the Mormon Peak in the opposite direction to bedding in allochthon there (Figure 16). In the the blocks, as would be expected from Mormon Mountains, the normal faults imbricate normal faulting of a Wernicke et al.: Central Mormon Mountains 231

• 0-2 i•-• 2- 4

¾• 6- 8 • 8-10 E I I0- 12 contour intervol: % per % area

277 poles

$ Fig. 17. Equal-area plot showing tilt directions in the Mormon Peak allochthon. Most of the tilts are less than 30ø , with over 90%less than 45ø .

near-horizontal package of rock where "antiformal" type, since the blocks dip faults rotate to a moderate or low dip away from the boundary [Stewart, 1980]. from an initial steep dip. In addition, The basic pattern of tilt-parallel and at several localities in the north part of tilt-perpendicular boundaries observed in the Mormon Mountains (about 5-10 km north the Mormon Peak allochthon is highly of the northern boundary of the map area), analogous to regional tilt patterns tilted Tertiary ignimbrites rest observed in the Basin and Range province disconformably or with very slight angular [Rehrig et al., 1980; Stewart, 1980; unconformity on the Bird Spring Formation Wernicke and Walker, 1982]. in the Mormon Peak allochthon (work in The east-northeast extension direction progress). For these reasons, it appears inferred from the tilt direction of upper that the magnitude and direction of plate rocks is consistent with that tilting of Paleozoic rocks in the Mormon inferred from the eastern imbricate belt Peak allochthon is largely the result of and central high-angle fault system. The Tertiary imbricate normal faulting. slightly scattered pattern of tilts The direction of tilt of the blocks observed in Figure 17 might be that (Figure 17) is separable into east- and expected from imbricate normal faulting west-tilted domains. The west-tilted acting on a package of randomly (but not domains show a preferential tilt toward steeply) tilted strata. the southwest, while east-tilted domains Since the tilted blocks in the show no preference between southeast and allochthon form a coherent package not northeast (most of the southeast tilts affected by thm•sting, they must have been occur in blocks near peak 6365). derived from a single thrust plate. This The boundaries of the tilt domains requires that Cambrian rocks in the within the allochthon in the western part westernmost-exposed klippen of the of the map area trend roughly allochthon have moved at least 8 kin, the east-northeast, parallel to the tilt distance from these klippen to the direction of the blocks. The change in footwall cutoff of the Mormon thrust. tilt direction across the boundaries does Matching Banded Mountain strata in the not take place along discrete fault zones, Mormon Peak extensional allochthon with but seems to be accommodated along diffuse Banded Mountain in the Mormon thrust plate zones (Plate l). Two domain boundaries in connects the Paleozoic section in the the central and eastern part of the map allochthon with the Mormon thrust plate. area trend perpendicular, or at least at a However, regional considerations suggest high-angle, to tilt direction, and are of that these rocks were derived from a 232 Wernicke et al.' Central •ormon Mountains higher tectonic element of the Sevier plate are Upper Cambrian and the oldest orogen, the Glendale-Muddy Mountain thrust strata in the Mormon Peak allochthon are plate. Middle Cambrian, the preextension Glendale In co,ramonwith the Muddy Mountains and thrust in the study area need only have Spring Mountains IBohannon, 1983a, b; duplicated a small amount (less than Burchfiel et al., 1974, 1982; Axen, 19841, several hundred meters) of section. These the Mormon Mountains contain two major considerations indicate likely paleodepths thrust famlts: the Mormon thrust and the of autochthonous basement exposed in the higher Glendale thrust. South of the map western Mormon Mountains of about 5-8 kin. area, the Mormon thrust plate dips gently The age of extensional faulting in the southward. Abomt 10 km south of the map area is mid- to late-Miocene. Mapping somthernmost klippe of the Mormon Peak by B. Wernicke et al. (unpublished map, al!ochthon in the map area, Banded 1984) and by Ekren et al., I19771 in the Mountain dolomites of the Glendale thrust northern Mormon Mountains indicates that plate are thrust over the Jurassic Aztec imbricately normal-faulted mid- to late Sandstone and the Cretaceous (?) Overton Miocene volcanic rocks are truncated by Conglomerate. These Mesozoic rocks form a the Mormon Peak detachment. The highest contiguous (but structurally disrupted) unit in the volcanic section, which rests stratigraphic seque•ce with Cambrian disconformably on mid-Miocene ash-flow strata in the Mormon thrust plate to the tuffs, is a basaltic andesite that yielded north IG. J. Axen, personal communication, a whole-rock K-At age of 8.5+0.3 Ma 19831. The Mormonthrust is therefore not IArmstrong, 1970, sample 20311ß Assuming the highest thrust fault in the frontal this to be its true age, it follows that part of Sevier orogen at the latitude of the Mormon Peak detachment must have been the Mormon Mountains. active at this time, since upper plate Reconnaissance mapping by us and by normal •aults either merge with or are Tschanz and PampeyanI19701 outside the truncated by the detachment. Although the map area indicates that the Mormon Peak volcanic sections are conformable, lack of a!!ochthon is structurally continuous with angular unconformities does not folded late Paleozoic and early Mesozoic necessarily indicate that extensional strata exposed in the Meadow Valley tectonism had not begun prior to eruption Mountains (Figure 16). These folded rocks of the basaltic andesite. In fact, the are part of a regionally continuous Miocene section on the Mormon Peak synclinorium of folds and small thrust allochthon in the Meadow •alley Mountains faults that occurs in the Keystone-Muddy is still flat lying (Figure 16), even Mountain-Glendale thrust plate. though it has been transported at least Therefore, rocks of the Mormon Peak 8 km to the west-southwest on the Mormon extensional allochthon are part of the Peak detachment. Thus, no data in the Glendale-Muddy Mountain thrust plate and immediate area directly bear on the not the Mormon thrust plate. precise time of onset of extensional If two thrust plates are cut out along tectonism. It is probable that extension the detachment, then its structural in the Mormon Mountains was partly coeval omission in western areas of exposure with west-southwest directed extension in comld approach 10 km, the thickness of two the Lake Mead Area, over 60 km to the full miogeoclinal sections thrust above an south of the mapped area, described by autochthon comprised of Cambrian through Anderson I1971, 19731, Anderson et al., Mississippian rocks. A minimum omission I19721, and BohannonI1979, 1983a, bl. of 4 km is suggested by the reconstruction They have shown on the basis of structural in Figure 15b, assuming very little and sedimentologic analysis of synorogenic post-thrusting westward dip of the Tapeats clastic and volcanic deposits, that the sandstone and a topographic slope toward most intense west-southwest directed the foreland. Both the initial westward extension occurred between about 14 and 10 dip of the autochthon and eastward dipping Ma ago. topographic slope conspire to increase the Unconformably overlying the extended estimate of omission, as does any terrane are flat-lying clastic rocks of overburden that lay above the easternmost, the Muddy Creek Formation, which fill the structurally highest rocks in the footwall modern valleys. The maximum age of these of the detachment (Fig. 15b). In deposits in the vicinity of the Mormon addition, since in the map area the Mountains is unknown, and it is therefore youngest strata within the Mormon thrust possible that low-angle normal faulting in Wernicke et al.: Central MormonMountains 233

the Mormon Mountains continued well past lineations were shown to predate upper 8.5 Ma ago, perhaps into the Pliocene. plate imbricate normal faulting by more than 40 Ma [Davis et al., 1980]. In DISCUSSION others, the tectonites formed only 5-10 Ma prior to imbricate normal faulting [Rehrig Both the eastern imbricate fault system and Reynolds, 1980; Keith et al., 1980]. with its inferred basal detachment and the In either case, the data seem to be Mormon Peak detachment do not appear to be incompatible with models which view lower spatially coincident with older Mesozoic plate ductile strain simply as a thrust faults. synchronous, in situ accommodation of Since the hanging wall detached within upper plate imbricate normal faulting, a Bonanza King dolomites, the characteristic mechanism of crustal extension suggested detachment plane for thrust faults in the by many workers prior to the recognition region, it is possible that the •ormon of the significance of metamorphic core Peak detachment followed an old thrust complex terrains [e.g., Wright and Troxel, plane (Glendale thrust?) along part of its 1973; Proffett, 19771. There has been trajectory. However, its penetration into considerable debate among workers in the crystalline basement in the westen Mormon lineated terrains as to whether or not the Mountains is extremely difficult to tectonites are genetically related to reconcile with a reactivated thrust fault upper plate extensional deformation and geometry. Again, thrust fault geometries the basal detachments themselves. both in the Mormon Mountains and Geologists who regard the two events as throughout southern Nevada demonstrate separated by several tens of millions of that basement was not involved in the years do not consider the two to be frontalmost part of the thrust belt. In necessarily genetically related, while the region shown in Figure 5, the those who interpret them as being decollement planes of the two frontal separated by only a few million of year thrusts carry Cambrian dolomites without favor models in which they are part of the exception; nowhere are they observed to same deformational system. The data carry crystalline basement. This presented here indicate that all of the observation holds for the frontalmost attributes of Mormon Mountains geology in parts of the vast majority of thin-skinned common with metamorphic core complexes fold-thrust belts developed within described above need not form in close continental margin sedimentary prisms. proximity to mylonite tectonites, and The Mormon Mountains have many of the provides further documentation for the characteristics of a widespread group of similar conclusion of Davis et al. [1980] terrains which have been termed and Shackelford [1980] . "metamorphic core complexes" [Davis and The occurrence of an unmetamorphosed Coney, 19791, including a domal range Paleozoic section beneath the Mormon Peak form, a domal, structurally higher-over- detachment, deformed only by minor normal lower, low-angle dislocation surface faulting (central high-angle fault system) structurally veneering the range, and during translation of the detachment, imbricate normal faulting both above and conclusively demonstrates that the lower below the dislocation surface [e.g., plate was extended at most only a few Shackelford, 1980]. Unlike the hundred meters during emplacement of the metamorphic core complexes, rocks beneath Mormon Peak allochthon. Thus, lower plate the dislocation surface in the Mormon extension cannot be invoked as a driving Mountains show no indication of ductile mechanism for upper plate translation and deformation or mylonitization. As extension, since the Mormon Peak originally hypothesized by Davis and Coney allochthon has been translated more than [1979] and Rehrig and Reynolds [1980], the 8 km relative to the lower plate. A deformation in the lower plates of the megalandslide or gravity-slide mechanism metamorphic core complexes, charac- for the emplacement of the Mormon Peak teristically consisting of a shallowly allochthon is equally untenable, because dipping mylonitic foliation containing a there clearly are no "toe" relationships mineral lineation which trends parallel to indicative of shortening in ranges to the the upper plate extension direction, west. In the Meadow Valley Mountains, serves as an accommodating mechanism for there is no indication of any complex mid- upper plate extension. However, in some to late-Miocene deformation. As mentioned of the core complexes, the mylonitic above, throughout the Meadow Valley 23• Wernicke et al.' Central Mormon Mountains

GEOLOGIC MAP OF THE CENTRAL PART OF THE MORMON MOUNTAINS LINCOLN COUNTY, NEVADA Geology by Brian P. Wernicke, J. Douglas Walker, and Mark S. Beaufait

MAP UNITS EXPLANATIONOF SYMBOLS ATTITUDES Quaternary I oa Qa, Alluvial deposits Tertiarv- TQIs, Landslide deposits Strike and dip of bedding I To,'] 4%. Quaternary ANGULAR UNCONFORMITY Strike and dip of overturnedbedding 4o.• Missi•ippian-œ MPb, Bird Spring Formation Strike and dip of bedding overturned more than 180 vermian I MPb] Mm Monte Cristo Formation Strike of vertical bedding

Mmy, Yellowpine Member Horizontal bedding Horizontal bedding, upside-down Mmb, Bullion Member

Mina, Anchor Member Approximatestrike and dip of bedding Strike and dip of foilation Mmd, Dawn Member Mississippian Strike of vertical foliation Mmda, Undifferentlated Dawn and Anchor Members

Ds, Sultan Formation •o.• Dip direction and dip of fault

Dsi, Ironside Member 4o• Dip direction and dip of overturned fault

OD, Undlfferentlated Ordovician through Devonian • Approximatedipdirection anddip of fault

O, Ordoviclan rocks Ordovician o• CONTACTS(DASHEDWHEREAPPROXIMATELY LOCATED,DOTTED WHERE CONCEALED, Ou, Ely Springs Dolomite QUERIEDWHERE INFERRED, TEETH ON UPPER PLATE OF LOW-ANGLE FAULTS) Op, Pogonip Group .... ?-- Depositionai

•n, Nopah Formation High-angle fao!t

•b, Bonanza King Formation i I i , , Low-angle normalfault

ßCbb, Banded Mountain Member • Thrustfault •bbS, Unit $ •bb4, Ui•it 4 • Mormonthrust Cbb3, Unit 3 • Low-anglen•rmal faults placing older rocks over Cambrian Cbb2,Cbbl, Unit 21 younger in western part of map area • MormonPeak detachment '•bR. Papooee•.ake Member •'bpd, DOlomiteunit .•bpl, Limestone unit FOLD AXES (TRACESDASHED WHERE APPROXIMATELY LOCATED) Ca, Bright Angel Shale UPRIGHT: .•t, Tapeats Sandstone t -'10 Axialtrace and plunge of anticline NONCONFORMITY • •10 Axialtrace andplunge of syncline Precambrian pCm, Precambrian metamorphicand igneousrocks SMALL-SCALE(ARROW ON STEEP LIMB WHENASSYMETRiC): •, •10 Trend and plunge of anticline •10 Trend and plunge ofsyncline 0 0.5 1.0 I i I OVERTURNED AND RECUMBEN]': N miles • •-10 Axial trace and plunge of anticline 0.5 1.0 1.5 2.0 • •10 Axialtrace andplunge of syncline

Topography by U.S.Geoiogical Survey Assisted by S.C. Semken, D.L. Olgaard and J.M. Stock

Wernicke et al.: Central MormonMountains 237

Mountains and in Meadow Valley Wash, large-scale strain accommodation usually mid-Miocene ash-flow tuffs as old as observed in areas which have experienced 12.5+0.3 Ma (K-At, sunidine, sample 203F, large-scale compression [e.g., Milnes and Armstrong I19701 and an underlying undated Pfiffner, 1980; McClay and Coward, 1981; lacustrine limestone lie in near Price, 1981], except that the slabs are horizontal depositional contact upon bounded by normal faults instead of thrust folded and east-vergent thrust faulted faults [Wernicke, 1981] . Paleozoic and Mesozoic strata ITschanz and The most basic kinematic aspect of the Pampeyan,1970; Ekren et al., 19771, and extensional system in the Mormon Mountains thus could not have been significantly (and many other areas of large-scale deformed as a result of movement on the crustal extension) is the duality of MormonPeak detachment (Figure 16). It is detachments (which initiate at low angles thus geometrically required that the of 10ø-30ø) and rotational imbricate detachment continue into the subsurface to normal faults (which initiate high angles the west beneath the Meadow Valley of 60o-90o). In both cases blocks between Mountains. Further, the probable 5-8 km the faults rotate, but rotation of paleodepth for western exposures of the detachments and undeformed portions of the detachment seem too large for the blocksthey bound is at a maximum20o-30 ø detachment to be the sole fault of a large range, while "domino style" blocks may gravity slide. rotate as muchas 70o-80ø ß In extended As proposedin Wernicke [1981], the areas, the "division of labor" amongfault simplest means to generate the core types appears to be that detachments(here complextectonic association [Davis and defined as normal faults which init[ate at Coney, 1979] is by large movementon low-angle) are the first-order rooted low-angle normal faults. Whether accommodators of crustal pull-apart, while or not mylonitic shear zones are present "domino" style and listtic normal faults in the lower plate depends on the local serve to penetratively dilate the shallow geotherm and the magnitude of displacement portions of the blocks between them. It on the detachment, which in several is in this sense that the term extensional studies have been shown to be on the order allochthon is applied: they are rock of at least 30-60 km [Bartley and masses whose lower bounds are normal Wernicke, 1984; Allmendinger et al., 1983; faults which initiate at low angle. As Compton and Todd, 1979; Davis et al., such they are the first-order building •82]. blocks of extensional orogens. The reconstruction in Figure 15b The imbricate geometry of extensional suggests that the Mormon Peak allochthon allochthons in the Mormon Mountains, and was wedge shaped. The apparent westward the fact that normal faults of the eastern structural simplification of the imbricate belt offset the Mormon Peak allochthon is therefore probably due to detachment illustrate some important its increased ability to resist internal aspects of the kinematics of extensional fragmentation resulting from frictional systems. First, it suggests that resistance on the detachment [Wernicke detactunents are not single, regional 1981, Figure 3c]. A similar westward subhorizontal surfaces which accommodate structural simplification exists in the differing styles of crustal extension lower plate, with the eastern imbricate (e.g., brittle vs. ductile) but are belt giving way westward to the components of large-scale systems of structurally simple central high-angle imbricated extensional allochthons, whose fault terrain. Perhaps this also is the bounding detachments need not merge at any product of a west-thickening, wedge-shaped particLllar level within the crust. allochthon above the inferred detach•nent Second, these imbricate systems may at depth beneath the eastern imbricate progressively step downward. Such a belt. The overall picture of extensional chronology may also apply to west-central strain is thus envisioned as large-scale, Utah and the Snake Range areas (Bartley east-over-west simple shear of two and Wernicke [1984]; Wernicke [1982], see imbricate, wedge-shaped slabs which are below). Third, folding of the detachments penetratively extended by rotational along axes perpendicular to transport imbricate normal faulting [Wernicke and occurs as the result of reverse-drag Burchfiel, 1982] concentrated in the thin, flexing of initially planar, gently eastern portion of the slabs (Figure 18). dipping surfaces in the hanging wall of This is precisely the mechanism of rotational imbricate normal faults within 238 Wernicke et al.- Central Mormon Mountains

/ /

Fig. 18. First-order scheme for the accommodation of upper crustal extensional strain in the Mormon Mountains area. Large, imbricate slabs are sheared past one another on low-angle normal •faults while their shallow ends experience relatively penetrative extensional strain by a variety of normal fault types. Movement on structurally lower detachments causes post-displacement faulting and folding of structurally higher ones.

younger, structurally lower extensional large-scale, east-directed, decollement- allochthons (Figure 18). Folds with axes style thrust faulting disrupted the parallel to transport (flexing of the miogeoclinal wedge during the Late Mormon Peak detachment and autochthonous Jurassic (?) through Late Cretaceous Paleozoic strata, section A-A', Figure 14; Sevier orogeny. The Mormon thrust was see also Frost [1981] ) are most likely the active during this time, and moved the result of a small component of regional Mormon thrust plate to the east, shortening perpendicular to the regional dislocating slabs of Cambrian through extension direction. The mechanism of Mississippian rocks from a ramp between transport-perpendicular fold development decollement portions of the thrust, might be isostatic rebound of the crust folding them about northwest-trending beneath inhomogeneously extended axes, and accreting them against a extensional allochthons [Wernicke, 1985; structurally higher Mississippian-Triassic Spencer, 1984]. However, at least in the footwall ramp. In accord with regional case of the MormonPeak (this report), relationships IBurchfiel and Davis, 1971; northern Snake Range and House Range Carr, 1980; Axen, 1980; Bohannon, 1983a, detachments [Bartley and Wernicke, 1984] b l, the Glendale plate was subsequently and Tucki Mountain fault (K. V. Hodges et eraplaced on top of the Mormon plate. al., 1984) reverse drag on younger, Following a period of presumed tectonic structurally lower faults seems to be the quiescence from Paleocene through early mechanism of folding. Miocene (?) time, large-scale, thin- skinned extensional tectonism began to STRUCTURAL SUMMARY dismember the Sevier orogenic belt in a west-southwest direction, as inferred from Following conformable sedimentation several independent kinerotic arguments. from Early Cambrian through Early Jurassic The major exposed extensional structure, time in the Mormon Mountains area, the Mormon Peak detachment, developed Wernicke et al.: Central Mormon Mountains 239

obliquely across the major thrust plates in Scotland [Brewer and Smythe, 1984] show of the Sevier orogen rather than that both areas have experienced a period reactivating them. It has a minimum of crustal extension following compression displacement of 8 kin, and juxtaposes the (Figures 19 and 20). Combinedwith highest structural levels of the Sevier surface geology and drilling results, we orogenic belt with Preca:nbrian crystalline believe these reflection profiles are basement of the autochthon. Subsequently, perhaps illustrative of concepts deduced an inferred, structurally lower detachment from our mapping. of perhaps equal importance formed beneath The reflector geometry of a portion of the eastern imbricate belt within the COCORP west-central Utah survey is Precambrian crystalline basement, and thus shown in Figure 19a. Although we do not also did cot reactivate a preexisting wish to give a detailed interpretation of thrust. The Mormon Peak detachment formed this reflector geometry here, some aspects with a dip of approximately20o-25 ø to the of •t are remarkably compatible with west, and the initial stages of movement geometries we have observed. The geometry on the fault are interpreted to have been of the Sevier Desert detachment (lower accompanied by folding of the area about detachment on Figure 19b) at its shallow ENE-trending axes. Late in its history of end seems to be highly analogous to the movement, as this broad warping continued, Mormon Peak detachment. At point A, it the central high-angle fault system became crosses the Payant thrust at an angle of active, further deforming the system, and about 15ø-20ø , similar to that betweenthe was locally active following final Mormon Peak detachment and Mormon thrust emplacement of the Mormon Peak allochthon. (Figure 15). The arching at B is Based on cross-cutting relationships, the interpreted here to be related to drag or eastern imbricate belt was active after sympathetic shear beneath the detachment, final emplacement of the easternmost as observed beneath western portions of klippe of the Mormon Peak allochthon and the Mormon Peak detachment. The the last movements on the easternmost interpretation presented here is not fa,•lts of the central high-angle fault unique. For example, the detachment at system. These relationships do not point A may reactivate a ramp portion of preclude partially synchronous movement on the thrust, then sole along the old thrust westermuost segments of the Mormon Peak parallel to autochthonous stratigraphy at detachment and faults in the eastern depth, thereby reactivating a thrust along imbricate belt, but do suggest that the its entire trajectory. Relationships emplacement of the Mormon Peak allochthon mapped in the Mormon Mountains simply largely predated their development. The demonstrate that a nonreactivation o current eastward dip of about 20 of the interpretation is perfectly viable, and detachment in the easternmost klippe may not be ruled out based on the (Plate l) is the result of rotation by erroneous assumption that all low-angle faults in the eastern imbricate belt. normal faults in extended thrust belts All major elements of the modern follow old thrust planes. topography between the Meadow Valley Similar to the Mormon Peak and lower Mountains and the East Mormon Mountains extensional allochthons, the Sevier Desert appear to be linked to the final extensional allochthon exhibits westward structural configuration of the structural simplification. East of point extensional allochthons. No evidence was C, the reflector geometry and well data found for a younger episode of widely [Mitchell, 1979] suggest fragmentation of spaced high-angle normal faults ("Basin the upper plate by west-dipping imbricate and Range" faulting). The total amount of normal faults, whereas west of C there is crustal extension accommodated within the no evidence of any major internal mapped area is a minimum of 8-11 km extension. (varying with latitude), hilt is The normal faults at and west of C cut permissably as great as 20 to 25 km. a 4.2 Ma-old basalt flow (or its equivalent stratigraphic horizon) found in IMPLICATIONS FOR DEEP REFLECP[ON PROFILES the Gulf no. 1 Gronning well (TD 2458 m; OF EXTENDED THRUST BELTS Lindsey et al. [1981]), yet farther west, at point D, we interpret the same horizon Deep seismic reflection profiling north (which may not contain the basalt at D) to of the Mor.mon Mountains along strike of overlap a growth-fault basin formed in the the Sevier orogenic belt [Allmendinger et hanging wall of the House Range al., 1983] and along the Caledonian front detachment. The relationships at C and D 240 Wernicke et al.: Central Mormon Mountains

HOUSE RANGE SEVIER DESERT CANYON RANGE

• a 600 1000 1400

E b Hou•• •tac• Co.•/o.?b•Gulfno.1Gronning Well ß .?.•:)?::::L:::-'""...... •.'.:u.:.:•j:::.:.:.?:?.:.'.'. • 10 10 Km

x 20 20 • •E-EXTE•N HOCK• OF: EXTEN•NAL •OGENIC HOCK• fi ff TertiaryDetachments

< . ':'4'•H.... Rangeextens•nal a•chhn •;• Post- 4.2 Ma • • P.... t RangeThust • SevierDesert extensional allochthon • Pre- 4.2Ma PrecambrianBasement [• Autochthon

Fig. 19. Interpretation of reflector geometry from COCORPwest-central Utah survey. a) Reflector geometry, after Allmendinger et al. I1983]; b) Structural interpretation discussedin text. See Figure 1 for location of profile. thus suggest that a substantial amount of boundary fault with the detachment at movemeat on the $evier Desert detachment depth (point G) suggest that its trace postdates movement on the eastern segment west of G may have continued moving into of the House Range detachment. This is the Quaternary, and could even still be extremely significant because the large active. The two detachments may thus have arch at E developed in the House Range moved simultaneously. detachment appears to be the result of None of the available data constrain reverse-drag flexiag above the younger, the time of initiation of the two structurally lower detachment (cf. Figures detachments: either one or the other may 16 and 20; see also Bartley and Wernicke have formed first, or they may have I !9841 , Figure 3) ß Although no attempt initiated synchronously. At any rate, was made to depth-correct the seismic similar to the Mormon Mountaias area, data, Allmendinger et al. I19831 have deformation induced by movements of shown that the segment of the House Range structurally lower detachments seems to detachment along E retains its current rotate and deactivate at least parts of easterly dip (though of only a few higher ones. We suspect this may be the degrees) once the effect of velocity predominant mechaaism of along-strike pull-up in the House Range is taken into folding of detachments throughout the acco•nt. Restoring the basalt horizon to Basin and Range. the horizontal (unflexing the hanging wall The large-scale geometry of the of the Sevier Desert detachment) rotates extending orogen in Utah may be the segmeat of the House Range detachment interpreted as the development of along E to the horizontal. Thus, at about low-angle normal faults (detachments) 4 Ma before present, movementon the boundiag extensional allochthons which structurally deeper Sevier Desert fail by imbricate, rotational normal detachment rotated the House Range faulting (both listtic aad planar) at detachment through the horizoatal, thereby their shallow ends. Although it is deactivating it. However, to the west, difficult to rule out the possibility that the linear topographic scarp along the the detachments there in all cases west side of the House Range (at point F) reactivate thrusts, processes observed in and the coalescence of the western the Mormon Mountains make plausible Wernicke et al.: Central Mormon Mountains 241

• •ew A 3400 2600 1800 1000 200E c RIFTBASINS •e - _ .•'-- --- •'•• =•.-•-I

MOINEALLOCHTHON •o 15 '••--•.•. ' 20'km '•15

MANTLE LITHOSPHERE FlannanNormal8hearZone Outer I$1•$ Normal8hear Zone 2•0• Figure 20. Interpretation of MOIST profile. a) Reflector geometry from Brewer and Smythe [19841; b) Structural interpretat ion discussed in text; c) Location map of HOIST profile relative to major on-land structures.

interpretations which include substantial been made to depth-correct our discordance between the two, including the interpretation, and as such Figure 20 involvement of autochthonous crystalline cannot be construed as a true basement as far east as the Gulf no. 1 cross-section. Gronning well. Brewer and Smythe [1984] interpret the The detachments are interpreted here reflector geometry primarily in terms of not to merge at any particular decoupling Caledonian thrusts which later experienced horizon in the crust. Although the Utah back-slippage as normal faults during reflection data do not preclude the Pertoo-Triassic (?) rifting. Based on this possibility of a decoupling horizon in the interpretation, they conclude that the lower crust between differing styles of Caledonian thrusts are more akin to deeply extension, we have difficulty in seeing penetrating, Wind River-type structures how the orogenic stockwerk geometries of [Soper and Barber, 1982] than they are to crustal extension as envisioned by a thin-skinned thrusting [McClay and Coward, number of workers [e.g. Hamilton, 1982; 1981] apparent beneath the Blue Ridge and Miller et al., 1983] might apply. We view Inner Piedmont of the southern the "brittle-ductile transition" as simply Appalachians [Cook et al., 1979] ß the gradual change from brittle shear to Although clearly plausible and in many ductile shear along crustally penetrating respects attractive, their interpretation normal faults and shear zones, rather than hinges quite heavily upon the assumption a subhorizontal boundary which exerts that normal faults in compression zones profound influence on extensional always reactivate thrusts. If this is not geometry. At this time, we see no assumed, there is little need for any of compelling mechanical reasons which the shallowly inclined reflectors to be preclude our hypothesis. old thrust planes, particularly the Outer In fact, the MOIST seismic reflect ion Isles and Flannan structures. No surface profile across the Caledonian front in information is available for the Flannan, Scotland (Figure 20) [Brewer and Smythe, and the surface exposures of the Outer 1984] provides the most substantive Isles "thrust" [Sibson, 1977] yield little evidence to date that single, low-angle firm basis for determining its age and its normal shear zones may penetrate not only sense and amount of slip. Our into the lower crust, but well into the interpretation, which is merely one of mantle. many that are possible, views the Outer We have reproduced Brewer and Smythe's Isles and Flannan structures as deeply line-drawing interpretation of the MOIST rooted normal faults and shear zones which data and present below it an inter- extend the Caledonian foreland pretation emphasizing the importance of lithosphere. We depict the base of the the Permo-Triassic (?) extensional orogen Caledonian orogen to be the Moine thrust superimposedupon the Caledonian (Figure zone, discordantly overprinted by deeply 20). It is stressed that no attempt has rooted normal faults. If the Flannan and 242 Wernicke et al.: Central •.,•ormon Mountains

Outer Isles reflectors are indeed virgin large strains without creating a great normal faults, then the Moine thrust deal of structural relief. It should be exposed on the surface is the northwest noted that the dip of deeper segments of limit of Caledonian orogenesis. This in the Outer Isles structure may be steeper turn raises the possibility that the than shown here because of the velocity on-land exposure of the Outer Isles pull-up effect of deeper crustal rocks. "thrust" is a late Paleozoic normal shear Similar to the Flannan reflectors, then, •one, having no relation whatsoever to the shear zone may actually steepen as it early Paleozoic mountain building. In crosses the Moho [cf. Wernicke, 1984] , view of the interpretative nature of perhaps a result of stress refraction near arguments for a thrust-fault origin of the the mechanical interface between weak Outer Isles structure, this hypothesis may lower crust and strong mantle. As is the be worth considering. case with the Sevier Desert profile, we Our interpretation suggests that the see little evidence for an extensional crt•st along the MOIST profile has nearly orogenic stockwerk on MOIST, although this doubled in width. We note that the normal possibility cannot be ruled o•t on the faults, without depth correction, dip basis of reflector geometry alone. The about 15ø-20ø to the southeast and the data suggests to us that single, low-angle steepest observed bedding in the normal faults and shear zones penetrate half-grabensis dippingabout 15ø-20ø. not only deeply into the continental Applying a "megadomino" kinematic model to crust, but also into at least the upper 15 these values suggests 80-100 percent km of mantle lithosphere. Indeed, one increase in original width of the line, could scarcely imagine a reflector using the relevant equations for geometry less indicative of rheological tilted-block geometry [see Wernicke and i•fluences on the geometry of fault zones Burchfiel, 1982, Figures 2 and 3]. This at depth. estimate is minimum, because possible We present our interpretations of these velocity pull-down from the basinal rocks profiles neither as the "final word" nor maximizes both the near surface fault dip even as necessarily more attractive than and the dip of bedding in the grabens. others. Our main purpose is to illustrate These pull-downs toward the graben center how co•cepts of orogenic architecture are conspire to minimize the extension most clearly documented by hands-on estimate. A preextenslon cmlstal observation of relationships in the field. thickness of 40 to 50 km is indicated. Without this kind of information there is Fault-bed angles of 30o-40ø suggesta little basis for i•terpreting the deep similar initial dip for the faults, reflection profiles. In particular, we somewhat steeper than the probable initial suspect that the concept of thrust fault dips for the Sevier Desert, House Range reactivation may be far less important a and Mormon Peak detachments. The overall process than the blocking out of pattern of extension bears strong extensional allochthons discordantly similarity to Davis' [1983] concept of across the grain of the older continental rifting, except the initial compressional ones. dips are perhaps somewhat lower than his suggested45 ø. Althoughthe Moho Acknowledgements. The authors reflector is not sharply offset by any of acknowledge support from NSF grants EAR the structures except the Flannan, viewing 7713637 awarded to B.C. Burchfiel, EAR its reflector geometry (Figure 20) along a 7926346 awarded to B.C. Burchfiel and P. low-incidence-angle line of sight reveals Molnar, and EAR 8319767 awarded to B. two broad upwarps aligned with downdip Wernicke. projections of the Outer Isles and next-higher subparallel structures. We REFERENCES interpret this in terms of broad shearing of the Moho, similar to that accomplished Allmendinger, R. W., J. W. Sharp, D. Von by the more obvious Flannan structure. Tish, L. Serpa, L. Brown, S. Kaufman, J. The shear zone geometry for the Moho Oliver, and R. B. Smith, Cenozoic and proposed here suggests it could be offset Mesozoic stucture of the eastern Basin as much as 20 km or more along the Outer and Range province, Utah, from COCORP Isles structure, an amount commensurate seismic-reflection data, Geology, 11, with o,•r interpretation of its surface 532-536, 1983. offset. The shear zone model permits Anderson, R. E., Thin-skin distension in Wernicke et al.' Central Mormon Mountains 243

Tertiary rocks of southeastern Nevada, geometry along the Caledonian- Geol. Soc. Am. Bull., 82, 43-58, 1971. Appalachian orogen, J. Geol. Soc. Anderson, R. E., Large-magnitude late London, 141, 105-120, 1984. Tertiary strike-slip faulting north of Burchfiel, B.C., and G. A. Davis, Clark Lake Mead, Nevada, U.S. Geol. Surv. Mountain thr•st complex in the Prof. Pap., 794, 18 pp., 1973. Cordillera of southeastern California, Anderson, R. E., C. R. Longwell, R.L. Geologic S•mmary and Field Trip Guide, Armstrong, and R. F. Marvin, Univ. Calif. Riverside Mus. Contrib. 1, Significance of K-Ar ages of Tertiary 1-28, 1971. rocks from the Lake Mead region, Burchfiel, B.C., R. J. Fleck, D. T. Nevada-Arizona, Geol. Soc. Am. Bull., Secor, R. R. Vincelette, and G. A. 83, 273-287, 1972. Davis, Geology of the Spring Mountains, Armstrong, R. L., Sevier orogenic belt in Nevada, Geol. Soc. Am. Bull., 85, Nevada and Utah, Geol. Soc. Am. Bull., 1013-1022, 1974. 79, 429-458, 1968. Burchfiel, B.C., and G. A. Davis, Mojave Armstrong, R. L., Geochronology of Desert and environs, in edited by W. G. Tertiary igneous rocks, eastern Basin Ernst, The Geotectonic Development of and Range Province, western Utah, California, Prentice Hall, Englewood eastern Nevada, and vicinity, U.S.A., Cliffs, N. J., 217-252, 1981. Geochim. Cosmochim. Acta, 34 203-232, Burchfiel, B.C., B. Wernicke, J. H. 1970. Willemin, G. J. Axen, and S.C. Cameron, Axen, G. J., Geological relations and A new type of decollement thrust, mechanical implications for thrust Nature, 300, 513-515, 1982. faulting at La Madre Mountain, Carr, M.D., Upper Jurassic to Lower northeastern Spring Mountains, Nevada, Cretaceous (?) synorgenic sedimentary Geol. Soc. Am. Abstr. Programs, 12, 3, rocks in the southern Spring Mountains, 95, 1980. Nevada, Geolog•v, 8, 385-389, 1980. Axen, G. J., Thrusts in the eastern Spring Cart, M.D., Geometry and structural Mountains, Nevada- Geometry and history of the Mesozoic thrust belt in mechanical implications, Geol. Soc. Am. the Goodsprings district, southern Bull., in press, 1984. Spring Mountains, Nevada, Geol. Soc. Bally, A. W., P. L. Gordy, and G.A. Am. Bull., 94, 1185-1198, 1983. Stewart, Structure, seismic data and Compton, R. R., and V. R. Todd, 01igocene orogenic evolution of southern Canadian and Miocene metamorphism, folding and Rock Mountains, Bull. Can. Pet. Geol., low-angle faulting in northwestern Utah- 14, 337-381, 1966. Reply, Geol. Soc. Am. Bull., part I, 90, Bartley, J. M., and B. Wernicke, The Snake 307-309, 1979. Range decollement interpreted as a major Cook, F. A., D. S. Albaugh, L. D. Brown, extensional shear zone, Tectonics, 3, S. Kaufman, J. E. Oliver, and R. D. 64 7-657, 1984. Hatcher, Jr., Thin-skinned tectonics in Bohannon, R. G., Strike-slip faults of the the crystalline southern Appalachians; Lake Mead region of southern Nevada, COCORP seismic reflection profiling of edited by J. M. Armentrout et al., the Blue Ridge and Piedmont, Geology, 7_, Cenozoic Paleogeography of the Western 563-567, 1979. United States, Pacific Section, Society Davis, G. A., Relations between the Economic Paleotologists and Keystone and Red Springs thrust faults, Mineralogists, 129-139, Los Angeles, eastern Spring Mountains, Nevada, Geol. 1979. Soc. Am. Bull., 84, 3709-3716, 1973. Bohannon, R. G., Mesozoic and Cenozoic Davis, G. H., and P. J. Coney, Geological tectonic development of the Muddy, North development of the Cordilleran Muddy, and northern Black Mountains, metamorphic core complexes, Geology, 7, Clark County, Nevada, Geol. Soc. 120-124, 1979. Am. Mere. 157, 125-148, 1983a. Davis, G. A., J. L. Anderson, E. G. Frost, Bohannon, R. G., Geologic map, tectonic and T. J. Shackelford, Mylonitization map, and structure sections of the Muddy and detachment faulting in the and northern Black Mountains, Clark Whipple-Buckskin-Rawhide Mountains County, Nevada, U.S. Geol. Surv. Map terrane, southeastern California and 1-1406, Washington, D.C., 1983b. western Arizona, Mem. Geol. Soc. Am. Brewer, J. A., and D. K. Smythe, MOIST and 153, 79-130, 1980. the continuity of crustal reflector Davis, G. H., and J. J. Hardy, The Eagle 244 Wernicke et al.- Central Mormon Mountains

Pass detachment, southeastern Arizona: Geol. Soc. Am. Bull., 94, 341-361, 1983. Product of mid-Miocene listtic (?) Keith, S. B., Evidence for late Laramide normal faulting in the southern Basin southwest vetgent underthrusting in and Range, Geol. Soc. Am. Bull. 92, southeast California, southern Arizona, 749-762, 1981. and northeast Sonora, Geol. Soc. Am. Davis, G. A., J. L. Anderson, D. L. Abstr. Programs, 14 (4) 177, 1982. Martin, D. Krummenacher, E.G. Frost, R. Keith, S. B., S. J. Reynolds, P. E. Damon, L. Armstrong, Geologic and M. Shafiqullah, D. E. Livingston, and geochronologic relations in the lower P. D. Pushkar, Evidence for multiple plate of the Whipple detachment fault, intrusion and deformation within the Whipple Mountains, southeastern Santa Catalina-Rincon-Tortolita California- a progress report, in crystalline complex, southeastern Mesozoic-Cenozoic Evolution of the Arizona, Geol. Soc. Am. Mem. 153, Colorado River Trough Region, 217-268, 1980. California, Arizona and Nevada, edited Lindsey, D. A., R. K. Glanzman, C. W. by E.G. Frost and D. L. Martin, 1-27, Naeser, and D. J. Nichols, Upper Cordilleran, San Diego, 1982. Oligocene evaporites in basin fill of Davis, G. H., Shear-zone model for the Sevier Desert region, western Utah: Am. origin of metamorphic core complexes, Assoc. Pet. Geol. Bull., 62, 251-260, Geology, 11, 342-347, 1983. 1981. Drewes, H. D., Geology of the Connors Pass Longwell, C. R., Geology of he Muddy Quadrangle, Schell Creek Range, Mountains, Nevada, with a section to the east-central Nevada, U.S. Geol. Surv. Grand Wash Cliffs in western Arizona, Prof. Pap., 557, 93 pp., 1967. Am. J. Sci., 5th ser., _1, 39-62, 1921. Drewes, H. D., Tectonics of southeastern Longwell, C. R., 1926, Structural studies Arizona, U.S. Geol. Surv. Prof. Pap., in southern Nevada and western Arizona, 1144, 96 pp., 1981. Geol. Soc. Am. Bull., 37, 551-584, Ekren, E. B., P. P. Orkild, K. A. Sargent, 1926. and G. L. Dixon, Geologic map of Longwell, C. R., Structure of the northern Tertiary rocks, Lincoln County, Nevada, Muddy Mountains area, Nevada, Geol. Map 1-1041, U.S. Geol. Surv., Soc. Am. Bull., 60, 923-967, 1949. Washington, D.C. Longwell, C. R., Restudy of the Arrowhead Frost, E.G., Structural style of fault, Muddy Mountains, Nevada, U.S. detachment faulting in the Whipple Geol. Surv. Prof. Pap. 450-D, D82-D85, Mountains, California, and Buckskin 1962. Mountains, Arizona, Ariz. Geol. Soc. Longwell, C. R., E. H. Pampeyan, B. Dig., 1•3, 25-29, 1981. Bowyer, and R. J. Roberts, Geology and Guth, P. L., Tertiary extension north of mineral deposits of Clark County, the Las Vegas Valley shear zone, Sheep Nevada, Bull. Nev. Bur. Mines Geol., 62, and Desert Ranges, Clark County, Nevada- 218 pp., 1965. Geol. Soc. Am. Bull., 92, 763-771, 1981. McClay, K. R., and M.P. Coward, The Moine Hamilton, W. B., Structural evolution of thrust zone- an overview, in Thrust and the Big Maria Mountains, northeastern Nappe Tectonics, edited by K. R. McClay Riverside County, southeastern and N.J. Price, Spec. Publ. Geol. Soc. California, in edited by E.G. Frost and London• 9, 241-260, 1981. D. L. Martin, Mesozoic-Cenozoic McDonald, R. E., Tertiary tectonics and Evolution of the Colorado River Trough sedimentary rocks along the transition, re$ion• California, Arizona• and Nevada, Basin and Range province to plateau and edited by E.G. Frost and D. L. Martin, thrust belt province, Utah, in edited by location, Cordilleran pp. 1-27, 1982. J. G. Hill, Symposium on Geology of the Hodges, K. V., J.'D. Walker, and Wernicke, Cordilleran Hinseline, Rocky Mtn. Assoc. B. P., Tertiary folding and extension, of Geol., Denver, Colo., 281-317, 1976. Tucki Mountain area, Death Valley Miller, E. L., P. B. Gans, and J. Garing, region, CA, Geol. Soc. Am. Abstr. The Snake Range decollement: An exhumed Programs,16 (6), 540, 19•84. mid-Tertiary ductile-brittle transition, Jordan, T. E., B. L. Isack, R. W. Tectonics, 2, 239-263, 1983. Allmendinger, J. A. Brewer, V. A. Ramos, Milnes, A. G., and A. O. Pfiffner, and C. V. Ando, Andean tectonics related Tectonic evolution of the central Alps to geometry of subducted Nazca plate, in the cross-section St. Gallen-Como, Wernickeet al.' Central MormonMountains 245

Eclogae Geolß Helv., 73, 619-633, 1980. Soper, N. J., and A. J. Barber, A model Mitchell, G. C., Stratigraphy and regional for the deep structure of the Moine implications of the Argonaut Energy No. Thrust Zone, J. Geolß Soc. London, 1_39, 1 Federal, Millard County, Utah, Basin 127-138, 1982ß and Range Symposium, Rocky Mtn. Assoc. Spencer, J. E., The role of tectonic of Geol. Symposium,503-514, Denver, denudation in the warping and uplift of Colo., 1979. low-angle normal faults, Geology, 12, Noble, L. F., Structural features of the 95-98, 1984. Virgin Spring area, Death Valley, Spurt, J. E., Descriptive geology of California, Geol. Soc. Am. Bull., 52, Nevada south of the fortieth parallel 941-1000, 1941. and adjacent portions of California- Olmore, S. D., Style and evolution of U.S. Geol. Surv. Bull., 208, 229 pp., thrusts in the region of the Mormon 1903. Mountains, Nevada, Ph.D. thesis, Univ. Stewart, J. H., Regional tilt patterns of of Utah, Salt Lake City, 1971. late Cenozoic basin-range fault blocks, Price, R. A., The Cordilleran foreland western United States, Geol. Soc. Am. thrust and fold belt in the southern Bull., 9__1,460-464, 1980. Canadian Rocky Mountains, in Thrust and Tschanz, C. M., Thrust faults in Nappe Tectonics, edited by K. R. McClay, southeastern Lincoln County, Nevada and N. J. Price, Spec. Publ. Geol. Soc. (abstr.) Geol. Soc. Am. Bull., 70, London9_, 427-447, 1981. 1753-1754, 1959.• Proffett, J. M., Jr., Cenozoic geology of Tschanz, C. M., and E. H. Pampeyan, the Yerington district, Nevada, and Geology and mineral deposits of Lincoln implications for the nature and origin County, Nevada, Nev. Bur. Mines Bull., of Basin and Range faulting, Geol. Soc. 73, 188 pp., 1970. _A_m.Bull., 88, 247-266, 1977. Wernicke, B., Low-angle normal faults in Rehrig, W. A. and S. J. Reynolds, Geologic the Basin and Range province- Nappe and geochronologic reconnaissance of a tectonicas in an extending orogen, northwest-trending zone of metamorphic Nature, 291 (5817), 645-648, 1981. core complexes in southern Arizona, Mem. Wernicke, B. P., Processes of extensional Geol. Soc. Am. 153, 131-158, 1980. tectonics, Ph.D. thesis, 171 pp., Mass. Rehrig, W. A., M. Shafiqullah, and P. E. Inst. of Technol., Cambridge, 1982. Damon, Geochronology, geology, and Wernicke, B., Uniform-sense simple shear listtic normal faulting of the Vulture of the continental lithosphere, Can. J. Mountains, Maricopa County, Arizona, Earth Sci., in press, 1985, Ariz. Geol. Soc. Dig. 12, 89-110, 1980. Wernicke, B., and B.C. Burchfiel, Modes Royse, F., M. A. Warner, and D. L. Reese, of extensional tectonics, J. Struct. Thrust belt structural geometry and Geol., 4, 105-115, 1982. related stratigraphic problems, Wernicke, B., and J. D. Walker, Some Wyoming-Idaho-northern Utah, in Deep geometrical aspects of extensional Drillin.$ Frontiers in the Central Rocky allochthons, Geol. Soc. America Abstr. Mountains, edited by D. W. Bolyard, Programs., 14,(7), 645, 1982. Rocky Mtn. Assoc. of Geol., Denver, Wernicke, B., P. L. Guth, and G. J. Axen, Colo., 1975. Tertiary extension in the Sevier thrust Royse, F., Extensional faults and folds in belt of southern Nevada, in Western the foreland thrust belt, Utah, Wyoming, Geological Excursions (guidebook), Idaho, Geol. Soc. Am. Abstr. Programs, edited by J.P. Lintz, Mackay School of 15 (5), 295, 1983. Mines, 4, 473-510 Reno 1984 Seaget, W. R., Low-angle gravity glide Willemñn, J. H., B.C. B•rcbœiel, M.D. structures in the northern Virgin Cart, G. J. Axen, C. S. C•meron, and Mountains, Nevada and Arizona, Geol. G. A. Davis, Detailed geom•bry of a Soc. Am. Bull., 8_•1,1517-1538, 1970. foreland thrust- the •eystone thr•t, Shackelford, T. J., Tertiary tectonic so•thern Nevada and •outhea•tern denudation of a Mesozoic-early Tertiary California, Geol. So½. •m. ^bstr. (?) gneiss complex, Rawhide Mountains, Programs,13, (7), 581, 1981. western Arizona, Geology,8,190-194,1980. Wright, L. A., and B. W. Troxel, S ibson, R. H., Fault rocks and fault Shallow-fault interpretation of Basin mechanisms, J. Geol. Soc. London, 133, and Range structure, southwestern Great 191-213, 1977. Basin, in Gravity and Tectonics, edited 246 Wernicke et al.: Central Mormon Mountains

by K. A. de Jong and R. Scholten, John B. Wernicke, Department of Geological Wiley, New York, 397-407, 1973. Sciences, Harvard University, Cambridge, MA 02138. M. S. Beaufait and J. D. Walker, Department of Earth, Atmospheric and ?lanetary Sciences, Massachusetts Institute of Technology, Cambridge, (Received May 10, 1984; 02139. accepted May 22, 1984.)