LOW-ANGLE ALLEGHANIAN CRUSTAL ACCRETION IN THE SOUTHERN APPALACHIANS (Invited Presentation)

The mode of crustal deformation during the late stages of lithospheric accretion in the southern Appalachian orogen has been long debated. In contrast to Himalayan-style crustal deformation, where shallowly dipping crustal detachments extend over hundreds of kilometers, some studies have inferred that the final (Alleghanian) collision between Laurentia and the Gondwanan Suwannee terrane occurred on a steeply dipping crustal suture, permitting collision models that are dominated by strike-slip motion. To address this question, we interrogated crustal structure using seismic phases recorded by the 85 broadband seismometers of the EarthScope SESAME array (which stretched from the Suwannee terrane in northern Florida to the southern Blue Ridge terrane in North Carolina), the EarthScope Transportable Array, and other stations. We applied common conversion point (CCP) stacking of S-to-P (Sp) converted waves. Unusually fine resolution of crustal structure was enabled by the close spacing of SESAME stations (~5 km above portions of the previously inferred suture) and the retention of periods from 1–33 s. The three-dimensional CCP stack reveals a low angle (<15°) southward-dipping interface that soles into a flat-lying mid-crustal detachment. We interpret this feature as the suture between the crusts of Laurentian and Suwannee lithosphere, consistent with its near-surface location which coincides closely with the northern limit of the Suwannee terrane reconstructed from its lower Paleozoic shelf strata (Boote and Knapp, 2016). The suture interface appears as positive-negative Sp phase pair, indicating a velocity increase with depth immediately underlain by a weaker velocity decrease with depth. Our preferred explanation for this structure is a layer of radial anisotropy due to mylonitization of the shear zone. The ~5 km thickness of this layer is consistent with surface observations of wide mylonite zones in western Georgia (Sears and Cook, 1984). The horizontal extent of the low-angle suture implies over 300 km of head-on shortening across a plate boundary structure similar in scale to the Himalayan mid-crustal detachment.