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].