SPATIOTEMPORAL PATTERNS IN THE INCEPTION OF MIOCENE VOLCANISM AND UPPER PLATE EXTENSIONAL BLOCK ROTATION IN THE VICINITY OF THE WHIPPLE MOUNTAINS, CA AND AZ

The Whipple Mountains in the lower Colorado River Extensional Corridor is an archetypical example of a metamorphic core complex, with highly faulted and tilted supracrustal rocks in the upper plate separated from mid-crustal mylonitic rocks in the lower plate by the Whipple Detachment Fault (WDF) The precise timing of extension in this region and how it relates to local Miocene eruptive activity remain controversial. Previous efforts to date the inception of extension have relied mainly on inflections in footwall cooling ages, while dating of Miocene volcanic rocks has been hampered by potassic alteration. We report new 40Ar/39Ar and U-Pb LA-ICMPS ages from fresh, well-exposed volcanic successions in the eastern Mopah Range, SW Whipple Mountains, Aubrey Hills, Standard Wash, and Crossman Peak block (Mojave Mountains) that shed light on both the volcanic and extensional history of the region. The oldest volcanic rocks nonconformably overlie Precambrian crystalline basement and consist mainly of basaltic andesite and andesite lava flows with subordinate dacite ignimbrite and lava-dome complexes. These basal sections have uniformly steep dips (70° to overturned) and typically exceed 1 km in thickness. Stratigraphically consistent ages indicate local voluminous volcanic activity commenced no later than 20.5 Ma and persisted without pause until 19.0 Ma. Little evidence exists for angular unconformities or growth strata that would suggest syn-volcanic faulting during this early volcanic activity. A pronounced angular unconformity separates these steeply tilted sections from much more gently dipping (~30°) basalt and rhyolite lavas - the oldest of which yielded ages of 18.8 ± 0.1 Ma in the Crossman Peak block and 18.5 ± 0.1 Ma in the southwest Whipple Mountains. These relationships suggest that significant extensional faulting in the vicinity of the Whipple Mountans may not have begun until ~19.0 Ma (substantially younger than most published estimates) and constrain 60-70° of block rotation to have occurred within a 500,000 year period in these locations. This initial episode of rapid extension followed closely on the heels of a voluminous outpouring of mafic lavas and supports the idea that magmatic (thermal) weakening may have triggered extension in the region.