The Southern Oklahoma Aulacogen is a failed rift, or failed rift arm (aulacogen), of the triple junction that became the Iapetus Ocean spreading ridges. It is a significant geological feature in the Western and Southern United States. It formed sometime in the early to mid Cambrian Period and spans the Wichita Mountains, Taovayan Valley, Anadarko Basin, and Hardeman Basin in Southwestern Oklahoma. The Southern Oklahoma Aulacogen is primarily composed of basaltic dikes, gabbros, and units of granitic rock.
Description
The Southern Oklahoma Aulacogen extends roughly 500 miles long (805 km) by ~80–90 miles wide (129–145 km). The two remaining continental plate boundary arms of the triple junction from which the Southern Oklahoma aulacogen formed became spreading zones for the spreading of the Iapetus Ocean during the breakup of the supercontinent, Rodinia, estimated to have occurred in the Cryogenian Period, approximately 750 million years ago. These arms closed in the Pennsylvanian Period (~323.2–298.9 Ma) and formed part of the Ouachita orogenic belt. The Southern Oklahoma Aulacogen is estimated to contain over 250,000 km3 of igneous rock. The aulacogen is inverted: rather than extending across the surface it penetrates into the North American craton, and is aligned with the northern edge of a deeply buried Proterozoic basin of uncertain origin which may have formed through igneous layering or deposition. The aulacogen terminates on contact with the Ouachita orogenic belt. The Southern Oklahoma Aulacogen is associated with a widespread anomalous area in which seismic waves travel more slowly. A common comparison is drawn from this aulacogen to the Dniepr-Donets Aulacogen in Baltica because both are significant intracratonic rifts.
The Southern Oklahoma Aulacogen contains numerous igneous rocks. Among these rocks are a multitude of gabbros, including anorthosite, titanium-rich, iron-rich, phosphorus-rich, and biotite gabbros. Also included are rhyolites and granites. This assemblage is very similar to the mid-Proterozoic age anorthosite-mangerite-charnockite-granite (AMCG) complexes of North America, but for the lack of coarse massif anorthosites. This is significant in that AMCG complexes tend to form at huge depths in the Earth's crust and thus cool more slowly, allowing the massif anorthosites to form coarse-grained. The similar igneous assemblage suggests that the magmas that formed the igneous rocks of the Southern Oklahoma Aulacogen quickly cooled to at or near their crystallization point, much more quickly than the magmas of AMCG complexes, thus resulting in finer-grained anorthosites.
More recently, different interpretations of seismic and outcrop data, as well as stratigraphy in the area have led some studies to postulate that this formation may not be an aulacogen after all, but a system of transform faults.
Tectonic evolution
The Southern Oklahoma Aulacogen formed sometime in the Late Proterozoic Eon, between 525 and 550 million years ago, during the rifting of the Laurentia supercontinent or North American Craton, the geological core of North America. Its formation and bimodal igneous activity occurred simultaneously, with two definite episodes of magma activity, mafic and felsic, the former of which being chiefly composed of gabbro-heavy magma and the latter phase being primarily composed of rhyolitic magma. It is hypothesized that between the mafic and felsic stages of magmatic activity substantial uplift occurred, which correlates to the lack of coarse-grained massif anorthosites presented previously. The remaining two arms of its original triple-junction became spreading zones for the nascent Iapetus Ocean. The aulacogen penetrated the craton, causing normal faults to form in what became the Anadarko Basin.
The aulacogen underwent crustal shortening and inversion sometime in the Mississippian Period to the Early Permian Period, roughly 330–280 million years ago. This coincides with the closing of the Iapetus spreading zones and the overthrusting of the Ouachita uplift over the Anadarko Basin, forming the Wichita Mountains. This also resulted in the reactivation of Cambrian rift faults, often becoming reverse or listric thrust faults. Anticlines formed in the sedimentary rock layers of the basin, contributing to the formation of the deep hydrocarbon reservoir of the Anadarko Basin. The formation of these listric faults and anticlines indicates that the crustal shortening was significant, up to 10–15 km or more.