INTEGRATED GEOPHYSICAL AND GEOLOGICAL STUDIES OF SELECTED MAJOR TECTONIC FEATURES IN SOUTH-CENTRAL U.S.

Abstract

The current dissertation includes three separate chapters, each utilizing the power of the integration of different geophysical datasets with geology to investigate tectonic and structural processes responsible for the geological evolution of selected major tectonic features in south-central U. S. These tectonic features are; the Arkoma basin of Oklahoma and Arkansas, the Llano uplift of central Texas, and the Meers fault of the southwestern Oklahoma. Although, the study covers three different tectonic features, these features share a similar tectonic history and evolution. These tectonic features are located along the front of the Ouachita orogenic belt, and they have been subjected to major tectonic events that dominated the south-central U.S. beginning with the Late Precambrian-Early Cambrian rifting through the Late Paleozoic convergent Ouachita orogeny.

The Arkoma basin is an arcuate structural feature that extends from the Gulf coastal plain in central Arkansas westward 400 km to the Arbuckle Mountains in south-central Oklahoma. The study area lies at the transition zone between the Arkoma basin and the Ouachita orogenic belt and could also be associated with northwest trending faults associated with the Southern Oklahoma aulacogen. The interpretation of the 3-D seismic data reveals an E-W zone of crustal weakness in the northern part of the study area, which could be a Late Paleozoic tectonic inversion of the extension faulting that developed during Cambrian rifting and later foreland basin development. The seismic interpretation reveals a compressive deformation of the Late Paleozoic strata related to the Ouachita orogeny. Magnetic boundaries such as faults and\or body edges extending

E-W, NE-SW and NW-SE have been delineated using magnetic edge detector techniques in the northern, southeastern, and western parts of the study area, respectively. The Euler magnetic depth estimation method delineated the same faults determined using magnetic edge detector techniques. The maximum depth to faults dominating the basement and/or the intrabasement features determined by the Euler's method is about 3850 m. The fault trends delineated by the seismic interpretation and those determined by the Euler's method and the edge detector techniques show a very clear correlation.

The Llano Uplift is a broad structural dome in central Texas with 2 to 3 km of structural relief relative to the subsurface Fort Worth and Kerr basins to the northeast and southwest. The Llano uplift appears like an island of Precambrian granitic and metamorphic rocks surrounded by Paleozoic and Cretaceous rocks. The current Llano uplift developed due to a variety of tectonic and structural processes. The initial uplift due to an arc-continent collision was followed by a continent-continent collision between the Laurentia and a southern continent during the Grenville orogeny. The extensional tectonism associated with the Cambrian rifting and the opening of the Gulf of Mexico played a pronounced role in the evolution of the Llano uplift. The compressional tectonism of the Late Paleozoic Ouachita orogeny as well as the Ouachita related foreland basins contributed to the rise of the Llano uplift area. The complete Bouguer gravity and reduced to pole total magnetic intensity (RTP) maps of the Llano uplift show anomalously high values. A number of short wavelengths maxima superimposed on a relatively broad, high gravity anomaly coincide with Llano uplift

area. The sources of the short wavelength anomalies can be related to shallow mafic bodies that were intruded into the uppermost crust during subduction of the Laurentia (North America continent) beneath a southern continent during the Grenville orogeny. The source of the broad, circular gravity anomaly appears to be related to a deeper geologic body situated in the middle crust. The RTP map reveals NW-SE trending magnetic highs that coincide with metamorphic rock exposures. Based on the gravity signature, the Llano uplift is interpreted to be independent terrane with physical and geological properties that differ distinctly from its surroundings.

The Meers fault is the southernmost element of the complex and frontal fault zone which separates the uplifted igneous rocks of the Wichita Mountains, and the Anadarko basin in southwest Oklahoma. Motion on the Meers fault represents continued activity on one of the largest structural features in North America. The Wichita uplift and the Anadarko basin, which are separated by the Meers fault and related subparallel fault strands, indicate significant intra-plate deformation along the trend of the Southern Oklahoma aulacogen. The interpretation of the gravity and magnetic data reveals clearer variations in the magnetic properties than densities of the rocks on both sides of the Meers fault. The high magnetic contrast on both sides of the Meers fault is mostly due to the Late Paleozoic movement, which juxtaposed Early Cambrian igneous rocks against Cambrian-Ordovician carbonate rocks. The magnetic expression of the Meers fault suggests that the fault can be traced in the subsurface for 21 km southeastward further beyond the clearly defined scarp and for 51 km northwestward where it branches and strikes more northerly. Thus, its total subsurface

extension is ~95 km. The seismic data provide impressive images of the structural complexity of the Wichita Thrust Front, and suggest that the Meers fault is a deep-seated high-angle thrust fault.