ABRUPT 17-19 MA MIOCENE VOLCANIC EPISODE ALONG THE HUALAPAI PLATEAU MARGIN IN WESTERN ARIZONA: PROXY FOR THE ONSET OF LOCAL BASIN AND RANGE EXTENSION

In contrast to the documented younger interval of Basin and Range extension in the Lake Mead-Grand Wash Trough region (16-11 Ma), the Hualapai Plateau margin stretching south from the mouth of Grand Canyon experienced a brief interval of basaltic volcanism that closely brackets the Peach Spring Tuff eruption (18.66 Ma). These basaltic plateau eruptions coincided closely with the outburst of similar extensional volcanism near the proposed Peach Spring Tuff source (Silver Creek caldera) in the Black Mountains-Kingman-Oatman district 50 km to the southwest. During the 2.5 Ma interval from 17.4 to 19.9 Ma, basaltic eruptive vents from Peach Springs northward to the Colorado River migrated onto the present edge of the plateau. These eruptive centers take the form of thin dikes, small pipes, small shield-like flow centers, and one larger circular intrusion disconnected from any obvious surface flow. The largest visible basaltic intrusive pipe (300 m diameter) gently stoped its way up through the Peach Springs Tuff in Milkweed Canyon, 10.6 km east of the western plateau boundary fault. This newly dated intrusive (18.13 ± 0.04 Ma) preserves a normal secular paleomagnetic pole position of 47.3 North Latitude, 146 East Longitude. These local eruptive centers along the plateau margin contrast with slightly older basalts that flowed onto the plateau from more distant vents located west of the current plateau margin prior to development of an obstructing boundary fault(?) scarp. The brief age interval and plateau margin location of these basaltic eruptive centers is assumed to coincide approximately with the defining interval of structural differentiation at the abrupt geologic boundary between the plateau and the Basin and Range province, which lacks a typical transition zone in this region. No westerly derived basalts flowed onto the plateau following this early volcanic episode, although a few widely spaced, younger dikes and eruptive centers at or south of the Colorado River have ages ranging between 15.3 and 13.5 Ma. Thus, Basin and Range extension along the Colorado Plateau margin appears to have begun earlier to the south and progressed northward through time.