Joint sets of the Llano Uplift, Texas
Alnes, Joel R
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Rock units of the Llano Uplift of central Texas, which range in age from Precambrian through Cretaceous, display well-developed joint systems. A total of 3i88l joints at 40 exposures across the region are analyzed in the manner of Hoist and Foote (1981). From this analysis, three regional joint sets, with average median strikes of N46E, N88W, and N46W, respectively, are delineated in rocks of Precambrian through Pennsylvanian age. Rocks of Cretaceous age display a different and irregular pattern of jointing. Jointing of rocks of Precambrian through Pennsylvanian age apparently is of post-Atokan - pre-Cretaceous age and developed in response to regional stresses, while joints in the Cretaceous rocks appear to be a product of a different and very localized stress regimes. The fracture pattern in the rocks of Precambrian through Pennsylvanian age may be explained by strains induced by NW-SE stresses. These stresses are interpreted to be a product of converging plates during the Ouachita orogeny, which would date jointing as post-Atokan to Permian in age. Apparent abruptness of doming and faulting, followed by jointing of the region, suggests plate collision along the Ouachita tectonic belt.