JOINTING IN PLUTONS AS EVIDENCE OF CONTINENTAL STRESS: JURASSIC TO NEOGENE

Rehrig and Heidrick (1972) demonstrated that mineralized joints in Laramide plutons of Arizona were systematically oriented with E to ENE strike, the result of parallel Sigma 1 compression from subduction of Farallon Plate beneath the North American (NA) continent. This indicated that Sigma 1 can exist hundreds of kilometers inboard from convergent plate boundaries causing extensional breakage/mineralization in plutons and considerable ~EW shortening and uplift of entire mountain blocks of NS trend as far east as the Rocky Mountain province. There were, however, several uplifts oriented east-west which caused some (i.e., Griese, 1983) to speculate later N-S oriented compression. This was elaborated by evidence of northerly striking extensional joints in post-Laramide plutons in the west U.S. (Rehrig and Price, 2021). Ongoing structural analysis within plutons from Jurassic to Neogene ages indicates that, on the basis of assumed uniaxial compression resulting in parallel (sheeted) extension jointing, each tectonic event records a distinct Sigma 1 vector over large regions.

The following compression orientations are: Jurassic – ESE; Laramide – E to NE; Eocene-Oligocene – NNE to NS; and Neogene NS to NNW, thus suggesting nearly 120 degree counter- rotation of Sigma 1 over some 140 m.y. Numerous but varying paleo-reconstructions deal with the interaction of Farallon - NA plates from Mesozoic through Neogene time. One study (Jurdy, 1984) is unique in reporting an approximately 90° degree rotation in plate motions between Farallon and NA crust from Cretaceous through Neogene times. Jurassic relative plate data suggests an additional 30 degrees of rotation between Middle Jurassic (ca. 160 Ma ) and Late Cretaceous time. Plutonic data support the hypothesis that a principal post-Laramide stress change to N-S orientation mirrors relative plate motions. Finally, the sheeted nature of joints argues that even the region of Tertiary extension through southwestern U.S. may have experienced some N-S compression.