GEOPHYSICAL INVESTIGATION OF THE CHESAPEAKE FAULT, SOUTHWEST MISSOURI

The Chesapeake Fault (CF) is one of several northwest-trending faults in southwest Missouri that affect both the Precambrian basement lithologies and the overlying Paleozoic strata. The CF extends for 325 miles from Western Kansas to Northern Arkansas, but the majority of the CF is located in Western Missouri. The CF is a part of a horst and graben fault system that extends across Missouri whose origin is basically unknown. Recent geological mapping and structural analyzes have implied that the horst and graben fault system in southwest Missouri is related to a transpressional adjustment of the Ozarks in the wake of the Late Paleozoic Ouachita orogeny. However geologic mapping in portions of the CF have shown that it is not one simple fault but could be a series of faults. In order to investigate the subsurface nature of the CF, a detailed geophysical study was conducted using gravity, magnetic and VLF-electromagnetic data. Regional gravity and magnetic anomaly maps including wavelength filtered anomaly maps, show that the CF is located within a northwest-trending gravity and magnetic minimum that is mostly caused by a thickening of the Paleozoic sedimentary units. To further investigate the CF, detailed gravity data were collected at a 0.1 mile interval. The resultant residual gravity anomaly constructed using wavelength filtering and horizontal derivative maps showed that central section of the CF in southwest Missouri consists of one major northwest-trending fault with two smaller northwest-trending faults to the east that are connected by a transfer fault within an isolated graben. To further understand the nature of the faults a series of east-trending, two-dimensional gravity models were constructed that showed that the basement consists of granite, rhyolite and gabbro with thicker sedimentary units closer to the main CF fault. To confirm the gravity results, detailed VLF-EM data were collected along the gravity profiles. Two-dimensional modeling of the filtered VLF-EM data showed conductive regions were the gravity data inferred the faults to exist. The geophysical analysis showed that the CF is more complicated than previously thought and additional studies are planned along other segments of the fault to completely understand its nature and to help in determining its origin.