TECTONIC EVOLUTION OF THE NORTHERN SIERRA NEVADA BATHOLITH A DISSERTATION SUBMITTED TO THE DEPARTMENT OF GEOLOGICAL AND ENVIRONMENTAL SCIENCES AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Nicholas James Van Buer December 2011 © 2011 by Nicholas James Van Buer. All Rights Reserved. Re-distributed by Stanford University under license with the author. This work is licensed under a Creative Commons Attribution- Noncommercial 3.0 United States License. http://creativecommons.org/licenses/by-nc/3.0/us/ This dissertation is online at: http://purl.stanford.edu/xb187vq0064 Includes supplemental files: 1. Plate 1. Geologic Map of the Jayhawk Well 7.5' Quadrangle, Pershing County, Nevada (jayhawkwell.pdf) 2. Plate 2. Geologic Map of the Juniper Pass 7.5' Quadrangle, Pershing County, Nevada (Juniperpass.pdf) 3. Plate 3. Geologic Map of the Tohakum Peak NE 7.5' Quadrangle, Pershing County, Nevada (TohakumpkNE.pdf) 4. Plate 4. Geologic Map of the Tunnel Spring 7.5' Quadrangle, Pershing County, Nevada (tunnelspr.pdf) 5. Plate 5. Geologic Map of the Bob Spring 7.5' Quadrangle, Pershing County, Nevada (bobspring.pdf) 6. Plate 6. Geologic Map of the Tohakum Peak SE 7.5' Quadrangle, Pershing County, Nevada (TohakumpkSE.pdf) 7. Plate 7. Geologic Map of the Sage Hen Spring 7.5' Quadrangle, Pershing County, Nevada (SageHenSpr.pdf) 8. Plate 8. Geologic Map of the Bluewing Spring 7.5' Quadrangle, Pershing County, Nevada (BluewingSpr.pdf) ii I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Elizabeth Miller, Primary Adviser I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Trevor Dumitru I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Martin Grove I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Gail Mahood Approved for the Stanford University Committee on Graduate Studies. Patricia J. Gumport, Vice Provost Graduate Education This signature page was generated electronically upon submission of this dissertation in electronic format. An original signed hard copy of the signature page is on file in University Archives. iii iv ABSTRACT About a third of the Mesozoic Sierra Nevada Batholith actually extends northward out of the Sierra Nevada proper into the Basin and Range Province of northwest Nevada, where it is obscured by late Cenozoic volcanic rocks, extensional faulting, and sediment-filled grabens. This segment differs from the main Sierra Nevada batholith because it is intruded into marginal terranes that are not underlain by old continental crust. The region is also characterized by a widespread unconformity overlain by Eocene to Miocene volcanic and sedimentary strata, which are not present above the southern Sierra Nevada Batholith. Although Basin and Range normal faulting has disrupted the structural continuity of the northernmost Sierra Nevada Batholith, breaking it into tilted fault blocks, this has also exposed depth sections into the upper crust, supplying 3-D structural information on the upper crust of the batholith. Although the Sierra Nevada Batholith serves as a worldwide model for arc tectonics, the northern segment of this arc has received little study in the past. We address fundamental questions not thoroughly explored in the literature: Where exactly does the batholith go in the NW Basin and Range? How does the structure, age, petrology and geochemistry of the intrusions compare to those in the Sierra Nevada mountain range, and what can this tell us about the generation of arc batholiths? When did the batholithic roots of the magmatic arc become exposed at the surface, and where did the eroded material go? What can this tell us about the paleogeography of the ancestral Sierra Nevada and its evolution to the modern range? The basal Tertiary unconformity provides an important datum for reconstructing the pre-extensional geology of the area. Restored for Neogene extension, the geology v beneath this unconformity was compiled to construct an early Tertiary paleogeologic map. This map outlines the extent of granitic rocks of the Sierra Nevada Batholith and its country rocks across the NW Basin and Range. The paleogeologic map also illustrates the rock types eroded across the region prior to the development of the Tertiary unconformity and suggests deeper exhumation along the axis of the arc (compared to further east) in the late Cretaceous to early Tertiary. To understand the structural and petrological architecture and history of this portion of the arc, eight 7.5´ quadrangles covering a distinct intrusive suite in the Sahwave and Nightingale Ranges were mapped at 1:24,000 scale. Together, the Sahwave and Nightingale ranges form a normal-fault-bound horst block that is warped into a gentle syncline. This is expressed in the dips of Miocene volcanic and sedimentary strata that unconformably overlie the Mesozoic batholith and its wallrocks. These basement rocks are composed of concentrically arranged granodiorite intrusions with an area of ~1000 km2, referred to as the Sahwave Intrusive Suite, intruded into older plutonic and metasedimentary rocks. The Sahwave Intrusive Suite youngs inward from equigranular hornblende biotite granodiorite to more felsic, K-feldspar megacrystic granodiorite. Zircon U-Pb dating by SHRIMP shows that the Sahwave Intrusive Suite spans ~ 93–88.5 Ma, and intrudes older plutonic rocks crystallized at 97–110 Ma. Sample transects analyzed for modal mineralogy, trace and major element geochemistry show variation similar to that measured in coeval large zoned intrusions along the Sierra Nevada crest, such as the Tuolumne Intrusive Suite, which represent the last and biggest pulse of magmatism in the Mesozoic arc. One substantial difference between the Sahwave Intrusive Suite and coeval Sierran rocks are the more primitive Sr and Nd vi 87 86 isotopic ratios of the Sahwave Intrusive Suite ( Sr/ Sri ~ 0.7047 and εNd ~ –0.2). This is consistent with the inference that part of the arc in the central and southern Sierra Nevada straddles the edge of continental crust, whereas part of the arc in NW Nevada, which intrudes deep marine strata, is underlain by transitional or oceanic crust. The similarity of the Sahwave Intrusive Suite to its southern counterparts, despite this important difference, suggests arc flare-up events, as represented by the ca. 90 Ma intrusive suites along the Sierra Nevada crest, are controlled by a subcrustal process. Therefore, high magmatic flux might be caused by fluid release from the downgoing slab and/or an increase in subducted sediment rather than by backarc crustal thickening. To constrain the detailed history of exhumation of the northern Sierra Nevada Batholith, nine transects of samples were collected for thermochronologic study. Across the NW Basin and Range, normal faults tilt the batholithic rocks, exposing material that once lay up to 7 km depth beneath the early Tertiary unconformity. Apatite and zircon U-Th/He, apatite fission track, biotite 40Ar/39Ar, and K-feldspar 40Ar/39Ar multi- diffusion-domain modeling were used together to understand the thermal history of the northern Sierra Nevada Batholith. Cooling and exhumation began promptly after intrusion in the Late Cretaceous, and continued at a roughly exponentially decreasing rate into the mid-Tertiary, consistent with previous work in the NW Basin and Range as well as the cooling history determined for the northern Sierra Nevada proper. The timing of exhumation is correlated with the local intrusive age of the batholith, and continues until later in the younger, eastern part of the arc. Correlations between magmatism and later exhumation suggest that residual arc heat acted to focus topographic relief and denudation. vii ACKNOWLEDGEMENTS Many people have supported me during the course of this research. Primary thanks go to my advisor, Elizabeth Miller: a constant source of no-nonsense geologic logic; excellent advice on which way to turn next; enthusiastic, foot-stamping motivation; and an appreciation for art and beauty in science. I thank Trevor Dumitru for training me in the intricacies of fission track dating and his finely honed mineral separation procedures. Joe Wooden has been an excellent mentor, both in the use and interpretation of ion microprobe data and also in introducing me to the visitors and new ideas always streaming through the SHRIMP lab. Gail Mahood has helped keep me honest on the workings of magmatic systems, and Norm Sleep has given me good advice on matters of heat flow. Marty Grove deserves credit for directing me in the use of 39Ar/40Ar dating and diffusion-domain modeling in K-feldspar, Jeremy Hourigan for help with U-Th/He dating at UC Santa Cruz, Bettina Wiegand for help measuring Sr and Nd isotopic data, and Bob Jones and Brad Ito for other lab help. I have also had fruitful collaborations with Sandra Wyld and Jim Wright, as well as excellent advice from Chris Henry and other folks at the Nevada Bureau of Mines and Geology. Special thanks go to Matt Coble, true master of the argon line, and to Julie Fosdick for teaching me bomb digestion tactics for zircon. Thanks also to Joe Colgan for sharing data and samples from NW Nevada, to Carrie Whitehill for getting me interested in the Sahwave and Nightingale Ranges, and to the excellent companionship of many other grad students. Thanks to my field assistants Spencer Craven (2007), Laainam “Best” Chaipornkaew (2008), Silas Stafford (2008), and Sarah Dasher (2009).