GEOCHRONOLOGY AND PALEOSTRESS ANALYSIS OF DIKES IN CENTRAL ARKANSAS: IMPLICATIONS FOR THE TECTONO-MAGMATIC EVOLUTION OF THE MISSISSIPPI EMBAYMENT

The Mississippi Embayment (ME) contains Upper Cretaceous and Cenozoic sediments unconformably overlying the Paleozoic Reelfoot Rift (RfR). It is bordered to the west by a suite of igneous intrusions known as the Arkansas Alkaline Province (AAP). Geochronologic ages for the AAP reveal a general NE-SW progression of Late Cretaceous ages and a complex range in igneous lithologies. Consequently, the relationship between the AAP and the ME’s structural evolution remains unclear. Researchers postulate that the ME’s Late Cretaceous evolution involved either passage of North America over the Bermuda hot spot (Cox and Van Arsdale 1997; 2002), or reactivation of RfR faults (Burke and Dewey, 1973; Ervin and McGinnis, 1975).

To better understand ME evolution, this study will utilize geochronology and thermochronology - U-Pb and (U-Th)/He, respectively - whole-rock geochemistry, and paleostress analysis in ArcMap 10.4 (Esri, 2016). Twelve rock samples were collected west of Little Rock, AR along the ME’s western border. Zircons were found in five samples and sent for analyses. Glass beads were analyzed via X-Ray Fluorescence (XRF). Geo- and thermochronologic data will be interpreted to determine timing of dike emplacement and implicated tectonic events, while geochemical data will provide context for petrogenesis. XRF analyses indicate that 7 of 12 samples exhibit SiO₂ ≥ 83% and are likely not derived from a melt. MORB-normalized trace element diagrams for the five igneous dikes (SiO₂ between 39% and 75%) reveal enrichment in incompatible elements. Preliminary rose diagrams developed in ArcMap 10.4 show a slight majority of E-W trending dikes, but dikes trend in nearly all directions. Whole-rock geochemistry data indicate that the five igneous samples experienced little crustal contamination and represent mantle-derived melts.

Preliminary results from XRF support intrusion of primitive magma during hot spot-related uplift, but zircon ages and further examination of Late Cretaceous stress regimes will allow for development of a robust evolutionary model. Results produced in this study will provide a greater understanding of the interplay between tectonics and magmatism during basin formation.