Emily Hopper

Reconstructing the end of the Appalachian orogeny

In contrast to crustal deformation observed in the actively forming Himalayas, where shallowly dipping crustal detachments extend over hundreds of kilometers, prior work on the Paleozoic southern Appalachian orogeny inferred that the final continental collision occurred on a steeply dipping crustal suture, permitting collision models that are dominated by strike-slip motion. Here, we use scattered seismic phases to instead reveal the Appalachian (Alleghanian) crustal suture as a low-angle (<∼15°) southward-dipping interface that soles into a flat-lying mid-crustal detachment. The observed suture geometry implies more than 300 km of head-on shortening across a plate boundary structure similar to the Himalayan mid-crustal detachment, indicating that this mode of deformation has been fundamental in continental collisions over hundreds of millions of years.

The relative roles of inheritance and long-term passive margin lithospheric evolution on the modern structure and tectonic activity in the southeastern United States

We perform inversions for the shear-wave velocity structure of the southeastern United States (SEUS) using Rayleigh-wave phase and amplitude data from the broadband stations of the South Eastern Suture of the Appalachian Margin Experiment (SESAME) and EarthScope Transportable Array (TA). Our tomographic images of shear-wave velocities in the upper mantle beneath the SEUS provide new constraints on the evolution of mantle lithosphere, both from the inheritance of structures from repeated Wilson cycles and from processes that have occurred while at a passive margin setting. Our images also allow us to correlate these structures to evidence of Eocene to recent tectonism observed at the surface. We find evidence for both inherited structures and more recently evolved structures, both of which bear some correlation to observations of ongoing tectonism. Our results suggest that lithospheric mantle continues to evolve while in a passive margin setting and that even relatively “stable” continental mantle lithosphere is subject to episodes of delamination, foundering, and erosion due to processes that are still not well under stood. Our results provide structural constraints on the types of processes that may be ongoing and on possible explanations for the numerous observations of comparatively recent tectonic activity occurring along this passive margin setting.

Imaging crustal structure beneath the southern Appalachians with wavefield migration

To constrain crustal structures in the southern Appalachians and the suture zone with the Gondwanan-affinity Suwannee terrane, we applied the 2D GRT wavefield migration method to the scattered incident P wavefield recorded by the EarthScope Southeastern Suture of the Appalachian Margin Experiment and adjacent Transportable Array stations. We resolve the root of thickened crust beneath the high topography of the Blue Ridge mountains and estimate its density contrast with the mantle to be only 104 ± 20 kg/m3. A weak velocity contrast across the crustal root Moho is observed and may be related to an ongoing crustal delamination event, possibly contributing to local tectonic rejuvenation. Beneath the Suwannee terrane, we confirm prior observations of a gently south-southeastward dipping crustal suture, indicating the terminal collision of Laurentia and Gondwana involved several hundred kilometers of overthrusting.