The northern Sierra Nevada and adjacent Basin and Range, western United States, are marked by a widespread regional unconformity at the base of the Eocene-Miocene volcanic and sedimentary section that overlies the Mesozoic batholith and its wall rocks. To help address controversial questions about the origin, uplift, and erosion of the batholith, we compiled a subcrop geologic map of the unconformity prior to Tertiary extension. This simple but underutilized technique reveals the distribution of rock units that could have contributed detritus to the Eocene-Miocene river channels crossing the Sierra Nevada and demonstrates that the Mesozoic Sierra Nevada batholith was continuous to the northeast across the northwestern Basin and Range. More speculatively, the subcrop map implies that Late Cretaceous-Eocene erosional stripping may have been greatest above the axis of the batholith and decreased to the east; thus, drainage in this area may have been eastward and then switched westward in Eocene-Miocene time.*
Detailed mapping and sensitive high-resolution ion microprobe (SHRIMP) U-Pb geochronology centered around the Nightingale and Sahwave Ranges, ~100 km northeast of Reno, Nevada, reveal that most of the Mesozoic basement in this area is composed of predominantly granodiorite-composition plutonic rocks intruded ca. 110-88.5 Ma. These rocks are similar in age, petrology, and composition to the mid-Cretaceous eastern part of the Sierra Nevada Batholith, and are likely related. The youngest plutonic rocks, ca. 93-88.5 Ma, form a large, compositionally zoned intrusive suite, referred to as the Sahwave intrusive suite. This suite is composed of a set of nested, inward-younging intrusions, varying between mafi c, equigranular granodiorite around the periphery to more felsic, K-feldspar-megacrystic granodiorite in the center. The Sahwave intrusive suite is coeval with the Cathedral Range intrusive event along the crest of the Sierra Nevada, including the Tuolumne intrusive suite. The geochemistry and petrology of this intrusion also support similar magma genesis and emplacement. Intrusions of the Cathedral Range intrusive event in the Sierra Nevada were emplaced along the margin of North American continental crust, whereas the Sahwave intrusive suite was intruded into a thick package of basinal metasedimentary rocks that were likely underlain by transitional crust. More primitive initial 87 Sr/ 86 Sr and ε Nd values (ca. 0.7047 and -0.2, respectively) refl ect this difference. In light of this likely fundamental difference in lower-crustal character, other factors, possibly related to subducted, water-rich material, must be responsible for creating similar melting conditions among the series of large intrusions that represent the last magmatic fl are-up of the Cretaceous arc.
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