TRANSPRESSIONAL TECTONICS DURING NEOARCHEAN ASSEMBLY OF THE SOUTHERN WYOMING PROVINCE

The Wyoming Province consists of a core of granitic and tonalitic gneisses with an isotopic history that extends back to the Hadean (the Beartooth-Bighorn Magmatic Zone, BBMZ). A sequence of micro-contents and arc fragments with substantially younger crustal history (the Southern Accreted Terranes, SAT) was tectonically emplaced onto the Wyoming Province during the Neoarchean. The contacts between the SAT and the BBMZ are nowhere preserved, so the nature of the accretion is cryptic. The southernmost extension of the BBMZ is the Sacawee block, which is exposed across the Granite Mountains in central Wyoming and consists of Paleoarchean gneisses. The youngest structural element in the gneisses is foliation that strikes northeast-southwest. The gneisses are also cut by numerous northeast-southwest trending, sub-vertical shear zones. These shear zones have lineations that trend down-dip or obliquely to the northeast. They have both top to the northwest and top to the southeast sense of shear. We conclude that gneissic fabric and the shear zones formed during a transpressional event that was followed by extension.

We contend that these zones formed during the accretion of the SAT and record the structural elements of that accretion. The age of this deformation is constrained by U-Pb zircon ages of 2635.8 ± 1.6 Ma from a leucosome in a partially melted paragneiss and 2635± 6 Ma from granitic gneiss within the western Sacawee Block. Deformation had ceased by 2630 Ma based upon U-Pb ages of the undeformed Louis Lake and Wyoming batholiths. Because of the cryptic nature of the boundary between the BBMZ and the SAT, we place it through the Louis Lake and Wyoming batholiths. The apparent east-west orientation of the boundary belies the nature of the NE-SW structural fabric that produced the accretion, partially because the boundary has been obscured by Neoarchean batholiths and partially because the boundary runs across three uplifts that have been displaced by Laramide tectonics.

The Neoarchean fabrics may well have been reactivated during the Proterozoic, forming the NE-SW-trending Cheyenne belt and Fletcher Peak Shear Zone. These weaknesses may also have been reactivated during the Laramide, because NE-SW oriented fabrics are found in Laramide fabrics across South Pass, the Sacawee block, and the Ferris Mountains.