George C. Dunne

PALEOZOIC AND MESOZOIC EVOLUTION OF EAST-CENTRAL CALIFORNIA

East-central California, which encompasses an area located on the westernmost part of sialic North America, contains a well-preserved record of Paleozoic and Mesozoic tectonic events that reflect the evolving nature of the Cordilleran plate margin to the west. After the plate margin was formed by continental rifting in the Neoproterozoic, sediments comprising the Cordilleran miogeocline began to accumulate on the subsiding passive margin. In east-central California, sedimentation did not keep pace with subsidence, resulting in backstepping of a series of successive carbonate platforms throughout the early and middle Paleozoic. This phase of miogeoclinal development was brought to a close by the Late Devonian-Early Mississippian Antler orogeny, during the final phase of which oceanic rocks were emplaced onto the continental margin. Subsequent Late Mississippian-Pennsylvanian faulting and apparent reorientation of the carbonate platform margin are interpreted to have been associated with truncation of the c...

Geology of the Inyo Mountains Volcanic Complex: Implications for Jurassic paleogeography of the Sierran magmatic arc in eastern California

An ∼3.1-km-thick volcanic complex exposed in the southern Inyo Mountains, east-central California, records Jurassic subaerial depositional environments along the east flank of the Sierran arc. This complex, which we name the Inyo Mountains Volcanic Complex, is subdivided into lower, middle, and upper stratigraphic intervals. The 200–580-m-thick lower interval comprises predominantly epiclastic strata deposited on alluvial fans and adjacent river flood plains that were inclined northeast. Mafic lava flows and rare reworked tuff in this interval record the onset of Jurassic(?) volcanism in this part of the arc. The 300–700-m-thick middle interval is composed predominantly of intermediate to silicic lava flows and tuffs representing a major episode of volcanism ending at ca. 169 Ma that is contemporaneous with emplacement of numerous plutons in the region. The >2260-m-thick upper interval is composed of epiclastic strata with minor intercalations of volcanic rock. Most of this interval accumulated on low-gradient flood plains that hosted evaporative lakes and that were episodically invaded by alluvial fan complexes. Three new U-Pb age determinations constrain the lower half of the upper interval to have been deposited during the interval from ca. 169 Ma to 150 Ma. The uppermost part of the complex remains undated but probably accumulated prior to 140 Ma. The Inyo Mountains Volcanic Complex is part of a belt of volcanic complexes that are the easternmost preserved Jurassic complexes of the Sierran arc. These complexes share sufficient similarities to suggest that they represent a distinctive arc-flank depositional province significantly different from that represented by coeval volcanic complexes preserved in roof pendants farther west, closer to the magmatic axis of the arc. Similarities among arc-flank complexes include predominantly to exclusively subaerial settings, substantial (>30%) portions of epiclastic strata, and existence at times of north- to northeast-inclined paleoslopes. We infer on the basis of the varying types and amounts of volcanic rocks that whereas most complexes in the arc-flank province were rarely if ever proximal to major eruptive centers, complexes in two areas (White Mountains and eastern Mojave Desert) were at times located in or adjacent to such centers. These differences lead us to speculate that the east flank of the Jurassic arc consisted of eastward-projecting volcanic salients separated by arc recesses—typified by the Inyo Mountains area—in which epiclastic deposition was dominant.

Age of Jurassic volcanism and tectonism, southern Owens Valley region, east-central California

Remnants of the eastern fringe of the volcanic and volcanogenic sedimentary cover of the Mesozoic Sierra Nevada batholith are exposed in the southern Inyo Mountains and adjacent Alabama Hills of east-central California. Six new U-Pb dates on volcanic units and crosscutting intrusions reveal that the upper parts of both the Inyo Mountains and Alabama Hills sections accumulated during Middle and Late Jurassic time. During this same interval, both sections were steeply tilted and locally folded during one or more episodes of contractile deformation occurring in the east Sierran thrust belt. Differences between the largely undated lower parts of the Inyo Mountains and Alabama Hills sections suggest that they were once located farther apart, then later brought into proximity by thrust faulting or strike-slip faulting. The Inyo Mountains and Alabama Hills sections are similar to partly coeval strata in the White Mountains, in that both contain abundant sedimentary strata that were in part deposited in or near terrain of moderate topographic relief. Together, these areas seem to compose a distinctive arc-marginal depositional province different than that represented by partly coeval strata preserved in pendants to the west.

Structure and evolution of the East Sierran thrust system, east central California

[1] A belt of arc-parallel, northeast vergent contractional deformation, the East Sierran thrust system (ESTS), crops out for � 150 km along the east side of the Sierran continental margin arc. The ESTS is nowhere wider than � 20 km, and it accommodated an estimated minimum of � 9.3 km of horizontal shortening. Remarkably, it experienced repeated episodes of broadly coaxial and coaxial-planar contractional deformation beginning prior to 188 Ma and continuing past 140 Ma. We postulate that the ESTS resulted primarily from episodic underthrusting of the back arc lithosphere beneath the east edge of the Sierran arc, facilitated by a buttressing effect of the arc. As a result of this process, rocks along the east flank of the batholith, including the ESTS, were episodically shortened against the arc buttress. The ESTS experienced significant deformation during the Nevadan orogeny, indicating that contractional to transpressive deformation affiliated with this event affected the eastern wall rocks of the arc as well as its western wall rocks. INDEX TERMS: 8102 Tectonophysics: Continental contractional orogenic belts; 9609 Information Related to Geologic Time: Mesozoic; 8015 Structural Geology: Local crustal structure; 9350 Information Related to Geographic Region: North America; KEYWORDS: eastern California, Mesozoic, structure, Cordilleran, thrust system. Citation: Dunne, G. C., and J. D. Walker (2004), Structure and evolution of the East Sierran thrust system, east central California, Tectonics, 23, TC4012, doi:10.1029/2002TC001478.

From Jurassic shores to Cretaceous plutons: Geochemical evidence for paleoalteration environments of metavolcanic rocks, eastern California

Volcanic and plutonic rocks exposed in east-central California record a long history of metasomatism and/or metamorphism within the Mesozoic Cordilleran continental arc. We use whole-rock and mineral elemental compositions, along with standard and cathodoluminescence petrography to characterize alteration histories of Late Triassic to Middle Jurassic metavolcanic rocks in the Ritter Range, White-Inyo Mountains, and Alabama Hills. Although alkali-metasomatism is widespread and pervasive, ratios and abundances of Ce, Th, Tb, and Ta suggest that mafic protoliths from the White-Inyo Mountains were shoshonitic, whereas those from the Ritter Range were calc-alkaline. Alkali exchange apparently modified the compositions of many metavolcanic rocks. Much of this metasomatism may have occurred at low-temperature (T) conditions, and attended or shortly postdated deposition of the volcanic protoliths. High δ 18 O values for K-rich metatuffs from

Geology and structural evolution of Old Dad Mountain, Mojave Desert, California

Rock units representative of most pre-Cenozoic formations in the southern Cordilleran geosyncline, as well as major structures developed during multiple periods of Mesozoic and Neogene deformation, are exposed at Old Dad Mountain, Mojave Desert, California. Precambrian metamorphic basement is overlain by upper Precambrian through Permain miogeosynclinal strata; intruding and (or) resting upon these strata are predominantly igneous rocks, which are interpreted as parts of the eastern margin of a Mesozoic Andean-type volcanic-plutonic arc. Structures formed during three phases of deformation of probable Mesozoic age are exposed at Old Dad Mountain. An important unconformity gives evidence of Late Triassic and (or) Early Jurassic unrest, and a thrust fault of this age may underlie Old Dad Mountain. A younger northwest-trending shear zone appears to have accommodated a significant component of left slip during Mesozoic time; it may be an analog of longitudinal faults within and behind modern volcanic-plutonic arcs. Remnants of the Playground thrust plate that moved eastward or southeastward at least 2.5 km during late(?) Mesozoic time overlie this shear zone. Superimposed on these older features are steeply dipping faults, gentle folds, and landslide masses related to movements on the Old Dad normal fault, a Basin and Range fault of Neogene age. Recognition of early Mesozoic and probable late Mesozoic structures of compressional origin at Old Dad Mountain lends support to the hypothesis that the eastern Mojave Desert is the site of widespread overlap of early Mesozoic and late Mesozoic orogenic belts.

Stratigraphic record of subduction initiation in the Permian metasedimentary succession of the El Paso Mountains, California

Petrologic investigation of Permian metasedimentary rocks in the El Paso Mountains reveals a rock record interpreted to be consistent with the sedimentary pattern of the upper continental plate of a nascent subduction zone, based on geodynamic modeling and comparison with a Cenozoic example (Puysegur Ridge, New Zealand). Facies changes reveal a history of uplift (conglomerate), followed by subsidence (carbonate turbidite deposits) and deeper-water sedimentation (argillite, with portions deposited below the carbonate compensation depth [CCD]), and then gradual shallowing accompanied by the onset of nearby intermediate volcanism (volcaniclastic and bioclastic sediments) and construction of a volcanic edifice (andesitic lavas) in a shallow-marine environment. Comparison with Permian global sea-level curves indicates that initial uplift (relative sea-level fall) followed by deep subsidence (relative sea-level rise) are likely due to tectonic rather than eustatic effects. Shallowing during volcaniclastic sedimentation could have been due to both arc edifice building and global sea-level fall. Sandstone modal analysis suggests that the basin evolved from a tectonic setting involving compressive uplift to an arc basin setting. Geodynamic modeling implies the involvement of a transform/truncation fault in subduction initiation. Magmatic trends based on Permian paleogeography and timing suggest a limited nucleation of subduction in the El Paso Mountains followed by propagation southward. Furthermore, subduction initiation modeling suggests regional lithospheric flexure that may be reflected in coeval basins and uplift in the northern Mojave, Death Valley, and Inyo Mountains regions as well as in coeval facies changes on the western edge of the Colorado Plateau. Overall, the Permian section of the El Paso Mountains may be one of the few preserved Paleozoic sedimentary records of subduction inception along a continental margin.

Correlation of Mississippian Shelf-to-Basin Strata, Eastern California

Mississippian rocks in eastern California comprise two distinct facies: a siliceous clastic facies to the northwest and a carbonate facies to the southeast, separated by a transition zone within which the two facies apparently interfinger (Fig. 1). Carbonate-facies rocks of the Cordilleran miogeocline represent shelf-margin and platform deposits (Rose, 1976; Gutschick and others, 1980), whereas clastic-facies rocks have been inferred to have been deposited in a foredeep basin located along the southeast margin of the mid-Paleozoic Antler orogenic belt (Poole, 1974; Poole and Sandberg, 1977). Through most of Mississippian time, the axis of this foredeep basin must have been located northwest of the area shown in Figure 1 (Stevens and Ridley, 1974). Although studies by Pelton (1966) and Randall (1975) clarified many aspects of the Mississippian stratigraphy in this region, only a somewhat generalized correlation of units across the transition zone has been developed (Fig. 2). Analysis and interpretation of platform, shelf-margin, and basin relationships of the type carried out by Rose (1976) in areas farther northeast have not been possible in eastern California because of abrupt lateral changes in lithology and thickness, scarcity of age-diagnostic fossils, and paucity of complete Mississippian sections in the transition zone. The Lee Flat Limestone, defined by Hall and MacKevett (1958) for exposures in the southern Santa Rosa Hills (Fig. 1), has been especially difficult to correlate with other formations. As defined, this formation poses two stratigraphic problems. First, the depositional top of the Lee Flat Limestone is not exposed in the type area, nor at other exposures of this unit mapped by Hall and MacKevett (1962), and no age-diagnostic fossils were recovered. Thus, the age and relationships to other Mississippian units (Rest Spring Shale and Perdido Formation) proposed by Hall and MacKevett (1962; Fig. 2A) have remained in doubt. Second, the lithology of the Lee Flat Limestone in the type area (thin-bedded, medium to dark gray, silty limestone) is distinctly different from the very light gray, massive-appearing, crinoidal limestone or marble mapped by Stadler (1968), Hall (1971), Johnson (1971), Randall (1975), Holden (1976), and Moore (1976) as Lee Flat Limestone in the carbonate facies region to the east and southeast. This latter lithology was identified as Lee Flat Limestone by these workers on the basis of its stratigraphic position between the Perdido and Keeler Canyon Formations, a position similar to that assumed by Hall and MacKevett (1958) for the Lee Flat Limestone at its type locality.