You remember Rodinia, the supercontinent that came together during the Proterozoic, and then rifted apart toward the end of the Proterozoic and in the early Cambrian? Today I want to talk about one chapter in that rifting apart that failed.
Oklahoma’s Wichita Mountains are all that’s left of that failed rift, called the Southern Oklahoma Rift (called an aulacogen on the map). In early Cambrian time, about 540 to 525 million years ago, this area was much like the Red Sea, or East Africa. The continent was pulling apart, extending, and the cracks allowed diverse kinds of magmas, from granitic to basaltic, to ascend from deeper in the earth. It pulled apart enough that there was even a general rise in the earth’s mantle below what is now southern Oklahoma. The break was trying to turn into a new oceanic basin, but although the mantle was involved, and lavas flowed out, it never completely rifted. It’s kinda like the Mid-Continent Rift of Kansas, Iowa, and Minnesota that we talked about a half billion years ago, although this break is almost perpendicular in orientation to that older one.
The crust subsided along two major west-northwest trending fault zones, and lavas as well as thick sediments went into the basin that formed. These rocks are exposed today in the Wichita and Arbuckle Mountains.
But wait, you say – that was half a billion years ago. I thought erosion would have leveled the mountains by now. Well, you’re right. It would, and it did. But the break in the crust created a weak zone, still weak millions of years later. Fast forward to this coming July and August, when we’ll talk about the Pennsylvanian time period when Africa was colliding with eastern North America. Those forces were great enough to affect what is now Oklahoma too, and the formerly downdropped zone became active again – and this time, because of the squeeze play driven by Africa’s collision, things popped up. We call this a rejuvenation of the old fault zone, and it was in the opposite sense to the original rift structure. So that’s 250 million years ago or so – and the Wichita and Arbuckle Mountains are the low, eroded remnants of that uplift. But the surface rocks, granites and their volcanic equivalents, rhyolite, and other igneous rocks, date back to the original rift in the Early Cambrian.
This zone probably extended west of Oklahoma, across what is now the Texas Panhandle (which has some complex geology beneath its flat surface), and into northeastern New Mexico, southwestern Colorado, and even into present-day Utah and maybe beyond. It has been rejuvenated in various ways over time. It was (and is) a big-time weak zone in the North American crust. It’s also been studied a lot, because there’s oil and gas trapped in some of these rocks and structures.
At about the same time, but perpendicular to the Southern Oklahoma Rift, and a few hundred miles to the northeast, another rift was trying to break the continent apart. This is called the Reelfoot Rift or Mississippi Embayment. It runs from northeastern Arkansas and western Tennessee up the Mississippi River to southwestern Indiana. This zone, old as it is, is still active. The famous New Madrid earthquakes of 1811 and 1812, some of the most powerful earthquakes known in North America, reflect the presence of this ancient rift. We’ll talk more about those earthquakes another day.
—Richard I. Gibson
Map from Van Schmus, W. R., Bickford, M. E., and Turek, A., 1996, Proterozoic geology of the east-central mid-continent basement; in, Basement and Basins of Eastern North America, B. A. van der Pluijm, and P. A. Catacosinos, eds.: Geological Society of America, Special Paper 308, p.