32-17 Worlds(?) Largest(?) Sill(?) Discovered(?) A Hypothesis

Session: Latest Research Advances in Structural Geology and Tectonics (Posters)

Presenting Author:

Author:

Abstract:

The Balcones Fault is a 15-mile-wide zone with total displacement of over 2,000’ just NW of San Antonio. It extends 200 miles to the north and 60 miles to the southwest. It is frequently referred to as “down-to-the coast’. Relative to the Edwards Plateau it is but relative to sea level it is not.

The faulting is one of three intertwined events. The other two are uplift of the Edwards Plateau, and the Rio Grande Rift. All were active in late Oligocene through mid-Miocene time, or 25-15 Mya.

As a reference, Socorro, New Mexico, is within the rift at an elevation of 4,600’. The east flank has an elevation above 9,000’. It drops rapidly at first; to 3,200’ near Carlsbad, NM. From there to the Balcones Fault Zone, elevations gradually decrease from 6 or 7 to 4 ft/mile. That 400-mile stretch is the Edwards Plateau.

I propose that the uplift of the Edwards Plateau was caused by a gigantic sill; beginning within the bowels of the rift and riding on the aesthenosphere all the way to the Balcones fault zone without being burdened by having to carry the crust on its back. 

Take a big step back in time. About 300 Mya, Pangaea/Gondwana collided with the Texas portion of Laurentia. The boundary is on the surface in SW Arkansas and continues in the subsurface in Texas where it is known as the ‘Ouachita facies’.  At the base of the Cretaceous, the boundary is about 15 miles northwest of the Balcones Fault Zone. But at the asthenosphere, maybe 30-40 miles deeper, it is most likely Southeast of the Balcones Fault system. If the aesthenosphere is deeper on the Gondwana block the sill would encounter the equivalent of a brick wall. The Eastward movement of the sill would stop, the uplift of the Edwards Plateau would stop, and the Balcones Fault system would grind to a halt.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7580

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