GEOLOGY OF THE INYO MOUNTAINS VOLCANIC COMPLEX - IMPLICATIONS FOR JURASSIC PALEOGEOGRAPHY OF THE SIERRAN MAGMATIC ARC IN EASTERN CALIFORNIA

An similar to 3.1-km-thick volcanic complex exposed in the southern In yo Mountains, east-central California, records Jurassic subaerial depo sitional environments along the east flank of the Sierran arc. This co mplex, which we name the Inyo Mountains Volcanic Complex, is subdivide d into lower, middle, and upper stratigraphic intervals, The 200-580-m -thick lower interval comprises predominantly epiclastic strata deposi ted on alluvial fans and adjacent river flood plains that were incline d 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 intermed iate to silicic lava flows and tuffs representing a major episode of v olcanism ending at ca. 169 Ma that is contemporaneous with emplacement of numerous plutons in the region. The greater-than 2260-m-thick uppe r interval is composed of epiclastic strata with minor intercalations of volcanic rock. Most of this interval accumulated on low-gradient fl ood plains that hosted evaporative lakes and that were episodically in vaded by alluvial fan complexes. Three new U-Pb age determinations con strain the lower half of the upper interval to have been deposited dur ing 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 complexe s that are the easternmost preserved Jurassic complexes of the Sierran arc. These complexes share sufficient similarities to suggest that th ey represent a distinctive arc-flank depositional province significant ly different from that represented by coeval volcanic complexes preser ved in roof pendants farther west, closer to the magmatic axis of the arc. Similarities among arc-flank complexes include predominantly to e xclusively subaerial settings, substantial (greater-than 30%) portions of epiclastic strata, and existence at times of north- to northeast-i nclined paleoslopes. We infer on the basis of the varying types and am ounts of volcanic rocks that whereas most complexes in the arc-flank p rovince were rarely if ever proximal to major eruptive centers, comple xes in two areas (White Mountains and eastern Mojave Desert) were at t imes located in or adjacent to such centers. These differences lead us to speculate that the east flank of the Jurassic arc consisted of eas tward-projecting volcanic salients separated by arc recesses-typified by the Inyo Mountains area-in which epiclastic deposition was dominant .