Cenozoic volcanic geology and probable age of inception of basin-range faulting in the southeasternmost Chocolate Mountains, California

Abstract

Research Article| February 01, 1978 Cenozoic volcanic geology and probable age of inception of basin-range faulting in the southeasternmost Chocolate Mountains, California BRUCE M. CROWE BRUCE M. CROWE 1Geosciences Group, Los Alamos Scientific Laboratory, University of California, Los Alamos, New Mexico 87545 Search for other works by this author on: GSW Google Scholar Author and Article Information BRUCE M. CROWE 1Geosciences Group, Los Alamos Scientific Laboratory, University of California, Los Alamos, New Mexico 87545 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1978) 89 (2): 251–264. https://doi.org/10.1130/0016-7606(1978)89<251:CVGAPA>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation BRUCE M. CROWE; Cenozoic volcanic geology and probable age of inception of basin-range faulting in the southeasternmost Chocolate Mountains, California. GSA Bulletin 1978;; 89 (2): 251–264. doi: https://doi.org/10.1130/0016-7606(1978)89<251:CVGAPA>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract A complex sequence of Oligocene-age volcanic and volcaniclastic rocks form a major volcanic center in the Picacho area of the southeasternmost Chocolate Mountains, Imperial County, California. Basal-volcanic rocks consist of lava flows and flow breccia of trachybasalt, pyroxene rhyodacite, and pyroxene dacite (32 m.y. old). These volcanic rocks locally overlie fanglomerate and rest unconformably on pre-Cenozoic basement rocks. They are intruded and overlain by rhyodacitic to rhyolitic plug domes, lava flows, volcaniclastic deposits, and locally thick unwelded and welded rhyolite ignimbrite. South and southeast of a prominent arcuate fault zone in the central part of the area, the rhyolite ignimbrite (26 m.y. old) forms a major ash-flow sheet. In the southwestern part of the Picacho area the rhyolite ignimbrite interfingers with and is overlain by dacite flows and laharic breccia derived from a probable cone complex, the deeply eroded center of which is marked by a dacite plug that forms the southern half of Picacho Peak. The rhyolite ignimbrite and the dacite of Picacho Peak are overlapped by lava flows and breccia of pyroxene andesite (25 m.y. old) that locally rest on pre-Cenozoic basement rocks. The rhyolite ignimbrite has a minimum volume of 40 km3 and is not associated with a collapse cauldron or major subsidence features.The volcanic rocks of the Picacho area form a slightly bimodal volcanic suite consisting chiefly of silicic volcanic rocks with subordinate andesite. The lavas have an alkali-lime index of 61, show no iron enrichment, and define a slightly potassic volcanic suite. Late Miocene augite-olivine basalt contains normative hypersthene but is most similar in major-element abundances to transitional alkali-olivine basalt of the Basin and Range province.Normal separation faults in the Picacho area trend northwest and north parallel to major linear mountain ranges in the region. The areal distribution of the 26-m.y.-old rhyolite ignimbrite and the local presence of megabreccia and fanglomerate flanking probable paleohighs suggest that the ignimbrite was erupted over irregular topography controlled by northwest- and north-trending probable basin-range faults. These relations date the inception of faulting in south-easternmost California at pre-26 and probably pre-32 m.y. ago. A transition to basaltic volcanism in the area is dated at 13 m.y. ago and is significantly later than the previously cited date of transition to fundamentally basaltic volcanism in the southwestern United States. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.