Geochronologic and isotopic evidence for Triassic-Jurassic emplacement of the eugeoclinal allochthon in the region,

Jonathan S. Miller Department of Geology, University of North Carolina at Chapel Hill, Chapel Hill, Allen F. Glazner } North Carolina 27599-3315 J. Douglas Walker Isotope Geochemistry Laboratory, Department of Geology, University of Kansas, Mark W. Martin* } Lawrence, Kansas 66045

ABSTRACT northern Mojave Desert, which are dated INTRODUCTION at 260 to 240 Ma by U-Pb zircon in this

The geologic history of the outer conti- report. Initial Sr and ␧Nd(t) isotopic val- Lower to upper Paleozoic outer continen- nental margin (eugeoclinal) rocks in the El ues from these plutons are distinct tal-margin (eugeoclinal) rocks in the Mo- Paso Mountains and northern Mojave (<0.704 and >؉2, respectively) from con- jave Desert region have been the subject of Desert has long been important in models tinental lithosphere isotopic signatures several studies and much speculation in re- -for the development of the active continen- (>0.705 and <؊2) of both Middle Juras- gard to the timing and kinematics of conti tal margin in the western Cordillera. Cur- sic plutons in the same area and Triassic nental margin development in the western rent interpretations call for either strike- plutons in the southern and eastern Mo- United States (Fig. 1). There are essentially slip or thrust juxtaposition of eugeoclinal jave Desert. Feldspar common lead data two models for how these rocks attained rocks against miogeoclinal/cratonal (plat- for the Permian and Triassic plutons their present position in the Mojave Desert. formal) rocks, or some combination of within the eugeoclinal outcrop belt also The first model holds that they are not far- both strike-slip faulting and thrusting. Two indicate limited crustal involvement and traveled with respect to the Mojave Desert broad and interrelated aspects of the his- do not overlap previously reported values but can be traced continuously along an ir- tory of the eugeoclinal rocks are at issue: (1) for common lead data from Mesozoic regular continental margin to the Antler How much primary displacement is neces- plutons in the eastern Mojave Desert orogenic belt (Fig. 1) in western Nevada sary to account for the present position of region where Proterozoic basement is (Poole, 1974; Poole and Sandberg, 1977; the eugeoclinal rocks? and (2) When were widespread. Dickinson, 1977; Snow, 1992). The second the eugeoclinal rocks thrust against platfor- model holds that, sometime in the late The observations and data reported here mal rocks? This study primarily addresses Paleozoic (Mississippian to Late Permian), indicate that Late Permian–Early Triassic the second issue. the continental margin of North America plutons in the northern Mojave Desert and Lithologic correlation indicates that the changed from a passive to an active margin El Paso Mountains were generated within outcrop belt of eugeoclinal rocks is bounded by sinistral strike-slip truncation of the pas- or passed primarily through oceanic litho- by platformal rocks to the east and south. sive margin, carrying eugeoclinal rocks sev- sphere, but later Jurassic plutons were de- Platformal rocks are also present to the eral hundred kilometers southward from rived from and/or interacted extensively west, but many of these exposures restore to the Antler orogenic belt in western Nevada a position structurally beneath the eugeo- with continental lithosphere. We hypothe- (Burchfiel and Davis, 1972, 1975, 1981; clinal rocks when Tertiary extension is re- size that the eugeoclinal rocks were depos- Miller and Sutter, 1982; Stone and Stevens, stored, implying stacking by a thrust. New ited on oceanic crust that was thrust east- 1988; Walker, 1988; Stevens et al., 1992; U-Pb zircon geochronology and whole rock ward over Precambrian cratonal basement Martin and Walker, 1995), making them con- geochemistry and Sr, Nd, and Pb isotopic and overlying strata between approximately siderably allochthonous with respect to the data for plutons in the El Paso Mountains 240 Ma and 175 Ma, a time of little doc- Mojave Desert region (Fig. 1). and northern Mojave Desert lend insight umented tectonic activity in the Mojave Recent work by Glazner et al. (1989) and into the timing of this thrusting. The out- Desert region. Postulated Permian or late Walker et al. (1990a) has shown that regard- crop belt of eugeoclinal rocks coincides Middle Jurassic east-directed thrusting is less of which of the above scenarios is cor- with the only known Permian and Triassic incompatible with our data and observa- rect, Mojave eugeoclinal rocks lie structur- plutons in the El Paso Mountains and tions. The data also fill an important gap ally above Paleozoic cratonal/miogeoclinal in palinspastic reconstructions of the (platformal) rocks (present, respectively, in early Mesozoic arc and indicate that this the hanging wall and the footwall of the Wa- *Present address: Servicio Nacional de Geo- arc was northwest-trending and oblique to logı´a y Minerı´a-Chile, Grupo de Geologı´a Re- terman Hills detachment fault) after Mio- gional, Avda. Santa Marı´a 0104, Casilla 1347, Paleozoic facies trends in the Mojave cene extension is restored (Fig. 2). Sixty to Santiago, Chile. Desert. seventy kilometers of Miocene slip across

GSA Bulletin; December 1995; v. 107; no. 12; p. 1441–1457; 7 figures; 3 tables.

1441

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/107/12/1441/3382179/i0016-7606-107-12-1441.pdf by guest on 02 October 2021 MILLER ET AL.

posed structures that juxtapose the eugeo- clinal and platformal facies belts. Much of the relevant Paleozoic stratigra- phy in the region has also been obliterated by Mesozoic intrusions; remnants of these strata are preserved as roof pendants in the younger plutons. The geochemical and iso- topic characteristics of these plutons have been used to evaluate tectonic models, most notably by Kistler and Peterman (1978) who demonstrated that the El Paso Mountains and Lane Mountain areas (Fig. 2) have plu- 87 86 tons with Sr/ Srinitial (Sri) ratios less than 0.706. These ratios are anomalous with re- spect to ratios considered typical of cratonic

North America, where Sri of plutons is gen- erally greater than 0.706 (Kistler and Peter- man, 1978), and they interpreted this zone (roughly the eugeoclinal corridor as shown in Fig. 2) as a failed rift of probable Pre- cambrian age. In contrast, Stevens et al. (1992) interpreted this zone to be an in-

board ‘‘oceanic terrane’’ (east of the Sri ϭ 0.706 isopleth), bounded on the east by al- lochthonous eugeoclinal rocks. In this paper we use a combination of field, zircon geochronologic, geochemical, and Rb-Sr, Sm-Nd, and Pb-Pb isotopic data to address the mode and timing of thrust juxtaposition of the eugeoclinal rocks with the platformal rocks in the Mojave Desert region. Although the data presented here primarily address thrusting of the eugeocli- nal rocks over the platform, they are impor- tant in evaluating possible strike-slip trun- cation of the margin because the timing of Figure 1. Generalized geologic elements that form the framework for understanding the thrusting bears indirectly on the viability of geologic history of the eugeoclinal rocks in the Mojave Desert. The eugeoclinal rocks in the primary strike-slip displacement of the eu- Mojave Desert are apparently displaced approximately 300 km southward from correlative geoclinal rocks. Antler Belt rocks in western Nevada and eastern California. The Mesozoic batholith com- pletely surrounds and intrudes the outcrop belt of eugeoclinal rocks. The position of the FIELD RELATIONS isopleth is from Kistler and Peterman (1978). This line is poorly defined in 0.706 ؍ (Sr(i the Mojave Desert. Figure modified from Walker (1988). The outcrop belt of Paleozoic eugeoclinal rocks in the Mojave Desert is bordered on the east, south, and west by Late Proterozoic