Date of Final Oral Examination (Defense)
Type of Culminating Activity
Doctor of Philosophy in Geosciences
Clyde J. Northrup, Ph.D.
Mark D. Schmitz, Ph.D.
Walter S. Snyder, Ph.D.
New geologic mapping, high-precision U-Pb zircon geochronology, and isotope geochemistry provide insight into the early Mesozoic paleogeography and magmatic, stratigraphic, and structural evolution of the Jackson Mountains in the Black Rock Desert region of northwest Nevada. The magmatic history of the Jackson Mountains records Late Triassic to Early Jurassic (~215-195 Ma) marine deposition of the Boulder Creek Beds adjacent to the arc followed by the Early Jurassic (~193-189 Ma) eruption and emplacement of the Happy Creek Igneous Complex and associated plutons of the Early Mesozoic Intrusive Suite. Pb, Sr, and Nd isotopic data from Early Jurassic intrusive rocks of the Jackson Mountains plot along a binary mixing relationship of DMM and EMII end-member mantle components and suggest some contribution of enriched continental material in the production of melts. Detrital zircon from the Boulder Creek Beds show primarily unimodal Late Triassic-Early Jurassic age distributions implying marine deposition adjacent to an active Triassic and Jurassic volcanic arc with minimal input from other sources. Together with regional geologic and tectonic relationships, these data are consistent with Triassic-Early Jurassic paleogeographic reconstructions of the Jackson Mountains as a fringing-arc separated from the continent by a deep-marine basin.
Though of shorter duration, the timing of magmatism in the Jackson Mountains overlaps with the timing of magmatism in the Olds Ferry terrane of the Blue Mountains Province. Pb, Sr, and Nd isotopic data from intrusive rocks of the Jackson Mountains consistently plot within the field of intrusive rocks from the Olds Ferry terrane. Comparison of detrital zircon ages in the Boulder Creek Beds and Blue Mountains Province is consistent with this relationship and paleogeographic setting. Combined with similarities in lithology and paleogeography, these new data provide compelling evidence for the correlation of Triassic-Jurassic arc-affinity rocks of the Jackson Mountains and the Olds Ferry terrane as fragments of the same arc separated and displaced by ~400 km of Cretaceous dextral translation. Furthermore, considering recent correlations of the Olds Ferry and Quesnellia terranes, these data indicate that the Jackson Mountains, Olds Ferry, and Quesnellia terranes may represent fragments of an early Mesozoic fringing-arc system.
Two main phases of Mesozoic NW-SE shortening are documented in the Jackson Mountains. Between the middle Early Jurassic and early Late Jurassic (post ~189 Ma and pre ~162 Ma) NW-SE shortening resulted in the production of thrust faults, folding, and cleavage development. Given the geographic proximity to the Luning-Fencemaker fold-and-thrust belt (LFTB), strong similarities in timing, structural style, and shortening direction suggests that Jurassic shortening in the Jackson Mountains was associated with the development of the LFTB and closure of the early Mesozoic marine basin that culminated in the eastward thrusting of the arc terrane over the basinal terrane. Isotopic data for intrusions from the Early Mesozoic Intrusive Suite provide further insight into this structural event. While initial 87Sr/86Sr and εNd exhibit no significant variation with emplacement age, initial Pb ratios are consistently elevated in Late Jurassic plutons. These changes are likely the result of crustal shortening and eastward movement of the upper crust during the development of the LFTB whereby younger intrusions are sourced from and travel through a more eastern crustal column that may include small fragments of enriched continental material.
In Early Cretaceous time (~118 Ma), NW-SE shortening resulted in movement along the NW-directed Deer Creek thrust and syncontractional deposition of the King Lear Formation. The timing and kinematics of this structural event relate to several significant, temporally overlapping changes in the forearc, arc, and retroarc regions that correspond with a significant change in plate motions. As such, Early Cretaceous shortening in the Jackson Mountains may be in response to a change in the Pacific-Farallon-North American velocity structure and reflect internal shortening of the hinterland. In this model, the Deer Creek thrust represents an out-of-sequence back thrust that overall aided in shortening the orogenic wedge.
Colby, Thomas Anthony, "Investigations on the Mesozoic Geologic and Tectonic History of the Jackson Mountains, Northwest Nevada" (2017). Boise State University Theses and Dissertations. 1245.
Available for download on Saturday, May 25, 2019