PRE-CRETACEOUS TERRANES, BASINS, AND FAULTS BENEATH THE ATLANTIC COASTAL PLAIN: ANALYSIS OF SUBSURFACE SAMPLES AND BOREHOLE DATA IN RELATION TO MAGNETIC AND GRAVITY ANOMALIES

Subsurface terranes and basins beneath Atlantic Coastal Plain (ACP) sediments are among the last frontiers of regional geology in the eastern U.S. We interpret the basement geology based on integration of magnetic and gravity data, and analysis of >1400 basement boreholes (AL to NY), including >1100 with descriptions or samples of pre-Cretaceous bedrock. The data provide insights into crustal evolution, resource potential, passive-margin structure, and basement influence on aquifers and neotectonics. Magnetic anomalies reflect magnetic susceptibility, and to a lesser degree remanent polarization, in crystalline rocks that underlie relatively non-magnetic sedimentary strata. Magnetic gradients also provide source-depth information. Digital processing produces wavelength-filtered and derivative maps that enhance effects of different geologic sources. For example, high-pass-filtered magnetic and gravity maps accentuate high-frequency anomalies due to shallow basement sources; tilt-derivative magnetic maps locate edges and are useful for mapping faults. Bouguer gravity anomalies reveal subsurface geometries of faults, plutons, and sedimentary basins. The combined results provide insights into kinematics of buried faults, basement-rooted ACP faults, seismic zones, and influence of Paleozoic fault systems on Mesozoic rift structures.

From VA to NJ, buried extensions of Piedmont terranes and early Mesozoic basins are flanked successively eastward by the concave-east Chesapeake Bay suture(?) zone and Sussex mafic complex, Neoproterozoic and Paleozoic rocks of the Chesapeake zone (mostly greenschist-facies), and amphibolite-facies rocks of the Hatteras (Neoproterozoic) and Cape May (Mesoproterozoic?) zones. Circular to elliptical gravity lows commonly coincide with Paleozoic granites. The main part of the Taylorsville rift basin coincides with a gravity low and broad-wavelength magnetic anomalies. Other Mesozoic basins are better constrained by boreholes and seismic data, and existence of a Delmarva basin is not supported by borehole samples. Tectonic inheritance is exemplified by the Hylas fault zone, where late Paleozoic transpression was followed by early Mesozoic normal faulting, and then by recurrent Cretaceous to Cenozoic movements on ACP faults.