COMPARISON OF CONTINENTAL PALEO- AND NEO-RIFTS

Southern Oklahoma Aulacogen(SOA) is one arm of a plume-generated set of 3 rifts which broke apart southern Laurentia in Late Proterozoic/Early Cambrian. The other 2 arms formed the Paleozoic southern boundary of Laurentia making the Ouachita reentrant and marking the boundary where the Precordillera of Argentina left Laurentia. How does this older rift compare with present rifts, such as the Rio Grande Rift (RGR)?

Direct signature of SOA is a strong positive gravity anomaly with extent 400km by 50km, essentially from Dallas WNW through southern Oklahoma to Amarillo. This contrasts sharply with most neo-rifts where widths range from 40-200km but lengths are >1000km. Most neo-rifts have substantial flank uplifts, commonly exposing basement and generating sediments in rift-related basins. SOA has no known flank uplifts or sediments. In fact "fill" in stretched upper crust is igneous: volcanic or shallow-seated intrusives. Gravity shows significant basic material entered the crust at middle/lower levels and at upper levels. The largest volume of acidic material is extrusive (~40,000 km3 rhyolitic)with thin sheet-granite intrusives, all placed over gabbros. In contrast, Kenya Rift has 144,000 km3 of extrusives. Basic units are alkalic-tholeiitic transitional; acidic units are A-type silicic. Estimated SOA extension is ~20km; RGR is <24km, but for a much longer and wider rift. RGR isotopic data indicate two mantle sources for magma. SOA isotopics indicate both acidic and basic are mantle-derived, with some acidics showing some crustal comtamination. Most neo-rifts have a life span of ~20 Ma, although some have longer pre-histories and several stages. SOA's life may have been as short as ~10 Ma (~540-530 Ma).

What does this say about the SOA? It must have been a very hot, well-elevated topographic bulge. Igneous rocks "filled" rift-spaces rather than sediments, indicating much new crust was formed. Magmatic feeder systems must have been concentrated and mostly linear. There were no "competing" or overlapping tectonic events as in the RGR. Perhaps SOA represents a transition between Precambrian rift characer and those of today.