PRESERVATION OF PALEOTOPOGRAPHY OF THE GRANITE-RHYOLITE PROVINCE OF THE MIDCONTINENT, USA: A BURIAL AND EXHUMATION HISTORY

In the Granite-Rhyolite Province (GRP) of Midcontinent USA, an assemblage of rhyolite and epizonal granite yielding dates in the range of ~1.47 and 1.37 Ga erupted onto or intruded into older basement. Rocks of the GRP lie directly beneath the Great Unconformity across a large swath of the Midcontinent. Examination of a collection of cuttings and core from 2769 drill penetrations constrains the spatial distribution of granite relative to rhyolite beneath in this region. Coupling this data set with field relationships visible in the St. Francois Mountains, at the northeast corner of the Ozark Plateau in Missouri, provides an image of the GRP, just prior to the Cambrian. Notably, rhyolite beneath the Great Unconformity clusters in regions that are topographic highs on the Great Unconformity. In the St. Francois Mountains, Cambrian strata onlap these paleohighs at buttress unconformities. These paleohighs are composed of rhyolite flows and tuffs that intercalate with volcaniclastic sediment. Moreover, epizonal granite often forms the core of many of the highs. These relationships have been used to argue that the St. Francis Mountains preserve a series of calderas with original topography. Well penetrations also demonstrate that rhyolite centers elsewhere occur in regions with high topographic variability at the Great Unconformity. Preserving relatively pristine caldera topography at the surface for ~900 m.y. (the time between eruption of the rhyolite at ~1.4 Ga and the deposition of Cambrian strata at ~ 0.5 Ga) is untenable, given typical erosion rates. Therefore, the region must have been buried and exhumed prior to the deposition of Cambrian strata. The sediments that initially buried the volcanic centers of the GRP may have been deposits of the foreland basin of the Grenville Orogen. Detrital zircon analysis suggest that these sediments became the source of sediments filling Neoproterozoic rift basins east of the GRP. Potentially, Neoproterozoic glaciation also contributed to removal of these sediments. Prior to the middle Cambrian transgression, the paleotopography of the basement was exposed at the surface. In effect, topography controlled by the distribution of rhyolite eruptive centers was formed at ~1.47 Ga, was exhumed at ~0.5 Ga, and is now being re-exhumed again.