GEOLOGIC MAP OF THE WESTERN WHIPPLE MOUNTAINS AND EASTERN MOPAH RANGE, SOUTHEAST CALIFORNIA: UNRAVELING THE ERUPTIVE AND STRUCTURAL HISTORY OF A SYNEXTENSIONAL MIOCENE VOLCANIC PROVINCE

This study focuses on the magmatic and structural history of a major Miocene eruptive center in the western Whipple Mountains metamorphic core complex and Mopah Range, in the lower Colorado River Extensional Corridor. This area excellently exposes a faulted and tilted sequence of Miocene volcanic and sedimentary rocks in the hanging wall of the Whipple Detachment Fault. To understand the structural and stratigraphic development of this highly deformed region, we mapped an ~150 km² area at scales ranging from 1:4000 to 1:10,000 and complied at 1:20,000 as part of a USGS EdMap project. This new mapping, ⁴⁰Ar/³⁹Ar geochronology, and major and trace element geochemistry have unraveled a detailed geologic history of this continental rift system.

Local eruptions commenced ~20.0 Ma with the effusion of voluminous mafic lavas, including olivine basalt, cpx-olv-plag basaltic andesite and plag-rich “turkey track” andesite. This was followed by the emplacement of hornblende-biotite dacite lava domes in the central part of the area with continued effusion of aphyric to sparsely porphyritic andesite in the flanking regions. The early hornblende-biotite dacites were followed by effusion of voluminous and somewhat more tabular, high temperature two pyroxene dacite lavas, gradually giving way to late stage hornblende-rich dacite lava domes. Throughout the development of this voluminous dacite lava-dome field between ~19.7 and 19.2 Ma, mafic and intermediate lavas continued to be erupted in adjacent areas to the east interfingering with the distal dacite lavas. This early (pre-extensional) episode of volcanism concluded with small volume eruptions of olv-cpx basalt at ~19.0 Ma.

At approximately 19.0 Ma, large magnitude extension began in the Western Whipple Mountains. A broad regional tablelands underlain by approximately 1 km of mainly lavas was broken up by a system of NW-trending, closely spaced normal faults and extended NE-SW by at least a factor of two. Rapid block rotation associated with this extension tilted the older volcanic successions ~55˚ to the SW and formed local half-grabens that accumulated coarse clastic sediments. By 18.5 Ma, the Peach Springs Tuff flowed across this irregular landscape and was followed by small volume scattered eruptions of olivine basalt and basaltic andesite at ~18.4 and 16.5 Ma.