51 Laramide Tectonics in the Southwest North American Cordilleran Interior II

Wednesday, 16 May 2018
3:30 PM - 5:15 PM
Humphreys/Mt. Elden Room

LITHOSPHERIC ARCHITECTURE AND TECTONIC EVOLUTION OF THE SOUTHWESTERN U.S. CORDILLERA: CONSTRAINTS FROM ZIRCON HF AND O ISOTOPIC DATA

Paper number: 51-4
Program: Theme Sessions
Time: 4:15 PM – 4:35 PM

Authors

Abstract

We present a regional dataset consisting of new zircon U-Pb, εHf(t) and δ18Ozrc data in 31 Triassic to early Miocene igneous rocks from a >1,600 km long transect in the southwestern U.S. Cordillera. This dataset is combined and compared with a compilation of whole rock isotopic data from the same transect. Orogen-scale spatial and temporal isotopic trends are identified and interpreted, both in terms of the underlying mechanisms that generated the trends and the tectonic processes that have shaped this part of the Cordillera. Most Cordilleran magmatism originates in the upper mantle and zircon εHf(t) primarily reflects the isotopic composition of the mantle source region. East of ~114 °W longitude in the southwestern U.S. Cordillera, the continental mantle lithosphere remained coupled to the crust until the late Miocene and zircon εHf(t) reflects the age of the lithosphere. Because the mantle lithosphere remained intact, zircon εHf(t) and δ18Ozrc of igneous rocks associated with low-angle to flat-slab subduction and crustal thickening during the Laramide orogeny are not significantly different from igneous rocks associated with Farallon slab rollback/foundering. Temporal isotopic trends identified in rocks east of ~114 °W longitude are related to migration of magmatism into lithospheric terranes of different age. West of ~114 °E longitude, in regions like the Mojave Desert in southern California, the continental mantle lithosphere is interpreted to have been partially removed and replaced by underplated Pelona-Orocopia-Rand schist and isotopically depleted asthenosphere or oceanic lithosphere during the Laramide orogeny. There is a temporal isotopic shift to more juvenile zircon εHf(t) and higher δ18Ozrc in igneous rocks west of ~114 °W, which is used to estimate the position of the western edge of intact North American continental mantle lithosphere before and after the Laramide orogeny.

Geological Society of America Abstracts with Programs. Vol. 50, No. 5

doi: 10.1130/abs/2018RM-314180

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