MOUNTAIN ROOTS, LITHOSPHERIC LAYERING, AND MANTLE FLOW: IMAGING P-WAVE REFLECTIVITY BENEATH THE EASTERN US

We use reflections generated by distant earthquakes to image P-wave reflectivity beneath Transportable Array (TA) stations across the eastern half of the US. The study area encompasses a region bounded by latitudes 33°- 42° N and longitudes 70°- 93° W. Structures imaged include the Moho, discontinuities within the lithosphere, the base of the lithosphere, and structure within the asthenosphere. Features imaged within the upper crust include the Appalachian Basin, the Cincinnati Arch, structures associated with the Midcontinent Geophysical Anomaly, and contrasts in basement structure across the New York-Alabama Lineament. The methodology is capable of imaging structure over a wide range of scales including high-frequency reflections from the base of unconsolidated sediments in the coastal plain and the Mississippi Embayment to lower-frequency features within the upper mantle.

For single earthquakes as well as stacks, reflections interpreted as the Moho increase in two-way travel time from 10 s beneath the Georgia coastal plain to 16-17 s beneath the Blue Ridge Mountains, providing evidence for a localized crustal root beneath the highest elevations of the Appalachians, in agreement with recent results using wide-angle reflections and P-S conversions. Currently, we are investigating the extent of crustal thickening along strike. Northwest of the Blue Ridge, the crust thins by several kilometers before thickening again towards the continental interior. Multicyclic reflections that dip westward from 32-45 s (100-170 km) are interpreted as a layered complex associated with the LAB. Reflections interpreted as layering in the asthenosphere are found at 58-64 s (210-230 km) for single earthquakes as well as stacks. These observations agree with recent interpretations of SESAME data. Reflections interpreted as the base of Mississippi Embayment sediments are observed at 2-4 s. Results in the northern half of the study area show evidence of the basement surface within the Appalachian Basin at 1-2 s. Similar reflections extend westward beneath the Cincinnati Arch and Illinois Basin. Reflections observed beneath the Midcontinent gravity high include a prominent negative polarity event at 5 s. Current work is focusing on characterization of the basement on either side of the NY-AL Lineament.