Geology of the Southernmost Exposures of the Brevard Zone in the Red Hill Quadrangle, Alabama

Abstract

The southernmost exposures of the Brevard zone were mapped to characterize its lithologic makeup and structural character and to explore its significance to the tectonic evolution of the southernmost Appalachians. Two important findings were made. First, the Lake Martin duplex (LMD) was discovered. The LMD comprises repeated panels of lower-greenschist to lower-amphibolite-facies siliciclastics, cherts, carbonate-bearing pelites, and volcanics (?) of the Jackson's Gap Group (JGG) that are separated by earlier-formed Acadian or early-Alleghanian (?) oblique-right-slip, D1, thrust zones. The structural top of the LMD is interleaved with units of the Inner Piedmont (hanging wall) along D1 shear zones, whereas the base is in contact with units of the eastern Blue Ridge (EBR). Retrogressive D2 semi-brittle shear zones locally shear and weakly stretch the duplexed panels and S0/S1 schistosity and gneissosity. S2 shear bands and F2 folds formed as a result of D2 deformation. Rocks of the LMD, therefore, are principally devoid of the D2 retrogressive, semi-brittle, right-slip shear zones that elsewhere typify the Brevard zone. The present study area, thus, preserves the orogen's only location where JGG lithologies and their associated D1 structures and fabrics are preserved in a relatively unaltered structural state. Relatively low metamorphic grade and low strains allow for the preservation of primary sedimentological structures and other features that help to clarify the depositional, structural, and tectonic settings of the JGG. The geology of the LMD is lithologically, geometrically, kinematically, and structurally comparable to that of the Hollins Line duplex of the Talladega Belt and the Pine Mountain imbricate zone that borders the Inner Piedmont and Pine Mountain window. Second, previously undescribed igneous units were discovered within panels of the LMD. A bimodal suite of metatonalite and greenstone, the Lake Martin Dam metatonalite (LMDM) and Eagle Creek greenstone (ECG), respectively, were mapped and characterized using petrographic and whole-rock geochemical methods. Major oxide discrimination diagrams indicate that the LMDM and ECG are tholeiitic tonalite and calc-alkaline basalt, respectively, and exhibit a bimodal nature similar to that of the Hillabee Greenstone in the Talladega Belt. Chondrite-normalized REE patterns indicate the LMDM originated as a relatively shallow intrusion within a volcanic arc whereas the ECG formed in a continental rift setting similar to those of the Laurentian rift basalts of the western Blue Ridge. The protolithic assemblage (i.e., orthoquartzite, quartz-pebble conglomerate, and carbonate-rich shale) and geochemical nature of the LMDM and ECG, clearly indicate that the JGG was deposited in a shallow-marine shelf setting off of the ancient, rifted margin of Laurentia.