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.

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