UNRAVELING THE HISTORY OF CRUSTAL THICKENING IN THE SEVIER OROGENIC BELT WITH LU-HF GARNET GEOCHRONOLOGY AND PRESSURE-TEMPERATURE PATHS FROM METAMORPHIC ROCKS

The hinterland regions of orogenic belts are typified by metamorphosed supracrustal rocks that reached depths of the middle and lower crust by stacking of thrust sheets or nappes, which can bury rocks to several times their original stratigraphic burial depths. Garnet that grows in pelitic schist develops chemical zonation that may be evaluated using differential thermodynamics (Gibbs method) and/or G-minimization to determine a pressure-temperature path. Paths that record a steep or isothermal increase in pressure indicate thrust loading during growth. In addition, garnet growth may be dated using the Lu-Hf method. By combining these two methods, we have directly determined the ages of tectonic burial in many areas of exhumed supracrustal rocks in the western U.S. Because exhumation has removed or obscured evidence of thrust faults in most of these areas, the ages provide a unique record and a basis for tying hinterland burial to growth of the retroarc wedge. In the hanging wall of the Basin-Elba thrust in the northern Albion Range of southern Idaho, burial ages of 132.1 ± 2.4 and 137.8 ± 0.5 Ma (2σ) were determined, and are similar to two Lu-Hf garnet ages of 132.1 ± 5.1 and 138.7 ± 0.7 Ma from the footwall in the Albion Mountains. An older burial age of 149.9 ± 1.2 Ma was determined from the western Raft River Mountains in NW Utah. In the northern Grouse Creek Mountains in NW Utah, a burial age of 85.5 ± 1.9 Ma was determined, and a similar age of 82.8 ± 1.1 Ma was obtained from the Wood Hills in eastern Nevada, interpreted to record a transition from synconvergent exhumation (core) to burial (rim). In the Funeral Mountains in Death Valley National Park in eastern California, a burial age of 158.2 ± 2.6 Ma was determined. The ages are biased toward the latter portions of the pressure-temperature paths due to the distribution of Lu in the garnets; more Lu is contained in rims than cores, even though Lu tends to have higher concentrations in cores due to Rayleigh fractionation of Lu during garnet growth. Generally, ages cluster into two distinct groups: Late Jurassic to Early Cretaceous burial associated with the initial crustal thickening phase of the orogenic wedge in the hinterland, and Late Cretaceous burial associated with renewed hinterland shortening.