UC Santa Barbara

A suite of Miocene dikes, collectively termed the Chambers Well dike swarm are exposed the southwestern footwall of the Whipple Detachment fault (WDF) and provide key insight into the evolution of the Whipple Mountains metamorphic core complex. New geologic mapping, U-Pb zircon geochronology, and whole-rock geochemistry allow for the assessment of: 1.) The ages, compositions, and volume of the dikes in the context of the local volcanic and extensional history 2.) The magnitude and timing of footwall rotation. 3.) The amount of slip on the southwestern WDF. The dikes intruded an approximately 40 km2 portion of the footwall of the WDF, comprised of an assemblage of Proterozoic gneisses and amphibolite bodies. Dikes can be broadly divided into two distinct groups; an andesite-rhyolite series (61-78 wt. % SiO2) ranging in age from 18.75 to 20.1 Ma, and a subordinate group of younger diabase dikes (55 wt. % SiO2). In the central portion of the dike swarm, dike-to-wall rock ratios range from 0.93 to 2.60 and imply ~100 to 250% WNW-ESE extension that was accommodated by intrusive dilation. Dike dips vary systematically from sub-vertical in the eastern portion of the swarm to gently east-dipping (~20-30˚) in the west, and take the form an inward-dipping fan. The combined field observations, geochronology, and geochemistry from the Chambers Well dikes and lava flows in the hanging wall of the WDF indicate that the western Whipple Mountains was a major Miocene eruptive center with local magmatic activity that began ~20 Ma. Early stages of extension (20.2 and 18.75 Ma) were dominated by intrusive dilation, but transitioned to large scale extensional faulting and tilting at 19.0-18.5 Ma, and may have been the result of thermal weakening of the crust. The asymmetric, fan-shaped geometry of the Chambers Well dike swarm can be explained by the combination of emplacement from an elongate, compositionally-zoned pluton at depth followed by ~40˚ of SW rotation about a horizontal axis, parallel to strike of upper plate strata (310˚). Restoring this rotation brings the mean dike orientations to sub-vertical, with strikes ranging from 310˚ to 002˚, and suggests an extension direction between 040˚ and 090˚. This interpretation also implies that the WDF initiated with a much steeper dip (~40-50˚), in agreement with classical Andersonian fault mechanics. Similarities in compositions and ages between the Chambers Well dikes and upper plate lava flows suggest that the dikes are the feeders to the thick volcanic sequence exposed in the western Whipple Mountains and Mopah Range. This correlation implies that there has been little-to-no NW-directed slip on the southwestern potion of the WDF since ~20 Ma.