TECTONIC IMPLICATIONS OF THE VOLCANIC AND STRUCTURAL HISTORY OF THE SIERRA SANTA URSULA, SONORA MEXICO

Recent detailed mapping in the northern Sierra Santa Ursula (SSU) of Sonora has revealed a prolonged history of magmatism that spans the transition between mid-Miocene subduction and proto-Gulf extension in the Gulf Extensional Province (GEP).  Early Mesozoic or late Paleozoic quartz pebble conglomerates are intruded and metamorphosed by Cretaceous aged granodiorites.  These two units are unconformably overlain by a thin sequence of arkosic sandstones.  This sandstone of probable earliest Miocene age is intruded and overlain by a series of hornblende trachyandesite domes, flows, and their volcaniclastic aprons.  These early Miocene trachyandesite volcanic rocks interfinger, intrude, and are intruded by clinopyroxene-olivine basaltic andesite lava flows.  This mafic volcanic package is overlain by a thick sequence of turkey-track andesite, which is in turn overlain by large-volume, ~12 Ma aged dacite lava flows that greatly resemble volcanic rocks found throughout western Sonora.  These more silicic rocks are overlain by a thin sequence of basaltic andesite lava flows, which are in turn overlain by a thin sequence of crystal poor rhyolite lavas flows.  No ignimbrites were found anywhere in this well exposed volcanic section.

This continuous sequence of Miocene volcanic rocks has undergone minor amounts of normal faulting, yielding less than 20% ENE-WSW extension since earliest Miocene time.  Ar/Ar dating reveals that this faulting took place dominantly between 12 and 10 Ma.  This simple structural history is made to look more complex by a complicated map pattern largely due to complex volcanic features, as well as several buttress unconformities.

Except for the early andesite volcanism, the geologic history exposed in the SSU is remarkably similar to the Sierra Libre range to the north.  Significantly, these two continuous, well-exposed ranges span nearly 80 km subparallel to the Pacific-North American plate boundary.  The large volume of mid-Miocene volcanic material found within these ranges indicate that the arc must have last resided here after sweeping west from the Sierra Madre Occidental since Oligocene time.  The absence of large magnitude extension in presumably the weakest part of the North American crust at this time and latitude is surprising given the close temporal and spatial proximity of the GEP.