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Large-magnitude Miocene extension of the Eocene Caetano caldera, Shoshone and Toiyabe Ranges, Nevada

¹ U.S. Geological Survey, Menlo Park, California 94025, USA
² Nevada Bureau of Mines and Geology, Reno, Nevada 89557, USA
³ U.S. Geological Survey, Menlo Park, California 94025, USA

View larger version (68K): [in this window] [in a new window]   Figure 1. Geologic map of north-central Nevada, showing location of study area (Figs. 2, 9, and Plate 1) and other features discussed in the text. Geology simplified from Stewart and Carlson (1978).

View larger version (68K): [in this window] [in a new window]   Figure 2. Shaded-relief map of study area (same extent as Fig. 9 and Plate 1), showing location of cross sections, geographic features, and mineral deposits discussed in the text. Dashed gray line is approximate extent of Caetano caldera. Gray lines are improved gravel (bold) and dirt roads (thin, solid and dashed). Base map same as Plate 1.

View larger version (39K): [in this window] [in a new window]   Figure 3. Schematic stratigraphic column for rocks in and around the Caetano caldera. Thickness of units not to scale. Unit colors and symbols same as Plate 1. Modified from John et al. (2008).

View larger version (25K): [in this window] [in a new window]   Figure 4. Restored cross sections A–A' (western part only), B–B', and C–C'. Sections reproduced directly from 1:100,000 scale geologic map (Plate 1). Lithologic units, colors, and symbols same as Plate 1 and Figure 3. FLT—fault; CYN—canyon. If you are viewing the PDF of this paper or reading it offline, please visit http://dx.doi.org/10.1130/GES00115.S2 or the full-text article on www.gsajournals.org to access the full-size file of Figure 4.

View larger version (129K): [in this window] [in a new window]   Figure 5. (A) View south of high-angle west-dipping normal fault at the mouth of Moss Creek Canyon. (B) View north of west-dipping Greystone fault in roadcut (10 m high) 2 km north of Greystone Mine. (C) View east of west-dipping Rocky Pass fault, 2 km south of Rocky Pass. Ridge is 325 m above valley floor.

View larger version (133K): [in this window] [in a new window]   Figure 6. (A) View east of north-dipping Crescent fault at the Cortez Mine. (B) View north of west-dipping fault north of Stone Cabin Basin. (C) View east of Red Mountain fault on east side of Carico Lake Valley; note prominent fault scarp. Ridge along skyline is 300 m above valley floor. U—upthrown; D—downthrown.

View larger version (25K): [in this window] [in a new window]   Figure 7. Lower hemisphere, equal-area plot of poles to bedding planes and compaction foliation from 35–25 Ma rocks within the Caetano caldera. Square is average pole of all measurements; great circle is plane to this pole (N11°E, 40°E). Gray arrows are inferred extension direction (N80°W).

View larger version (20K): [in this window] [in a new window]   Figure 8. Plot of assumed initial fault dip versus percent strain (from equation 1) and resulting initial east-west width of the caldera, assuming 42 km present width. Range of probable initial fault dips (60°–70°; Table 1), yields 22 ± 3 km extension and an original east-west caldera width of 20 km.

View larger version (61K): [in this window] [in a new window]   Figure 9. Map showing approximate extent of Miocene sedimentary rocks in the study area (same extent as Fig. 2 and Plate 1). Tephra correlations and 40Ar/39Ar data presented in Tables 2 and 3. Simplified from Plate 1.

View larger version (130K): [in this window] [in a new window]   Figure 10. (A) Silty sandstone packages (each roughly 5 m thick) in the hanging wall of the Toiyabe Mine fault, lower Wood Spring Canyon. (B) Miocene sandstone beds, lower Wilson Canyon. Tephra interbedded with these deposits yielded an 40Ar/39Ar age of 15.03 ± 0.10 Ma (Tables 2 and 3). Pencil is 14 cm. (C) Folded tephra beds in Miocene sedimentary rocks east of Moss Creek Canyon with 40Ar/39Ar ages of 15.84 ± 0.15 Ma and 15.68 ± 0.14 Ma (Tables 2 and 3). (D) Matrix-supported angular conglomerate in the hanging wall of the Toiyabe Mine fault. Note crude inverse grading. Clasts are predominantly Paleozoic quartzite. Hammer in C and D is 27 cm.

View larger version (40K): [in this window] [in a new window]   Figure 11. Illustration of Miocene extension of the Caetano caldera. Sections are east-west through the center of the caldera. No vertical exaggeration. (A) Pre-extensional Caetano caldera, modified from Figure 20D of John et al. (2008). (B) Middle Miocene extension on closely spaced, domino-style normal faults, with sedimentation in small half-graben basins. (C) Widely spaced, high-angle normal faulting with broad sedimentary basins.

View larger version (75K): [in this window] [in a new window]   Plate 1. Colgan, J.P., Henry, C.D., and John, D.A., Geologic map and cross sections of the Caetano caldera, Lander County, Nevada, scale 1:100:000. If you are viewing the PDF of this paper or reading it offline, please visit http://dx.doi.org/10.1130/GES00115.S1 or the full-text article on www.gsajournals.org to access the full-size file of Plate 1.