Fission track (FT) thermochronology studies on igneous and metamorphic rocks from the Ruby Mountains‐East Humboldt Range metamorphic core complex provide important constraints on the timing and nature of major mid‐Tertiary extension in northeast Nevada. Rocks from within the Ruby‐East Humboldt detachment (brittle‐ductile normal‐sense shear) zone were analyzed; the dominant lithology studied was variably mylonitic mafic orthogneiss. Nonmylonitic amphibolite from the top of the structurally lower migmatitic core and porphyritic biotite granodiorite from the Oligocene (circa 36 Ma) Harrison Pass pluton were also studied. FT ages on apatite, zircon, and sphene (except one 18.4‐Ma apatite from the basal portion of the detachment zone) are concordant and range in age from 26.5 Ma to 23.6 Ma; all ages overlap at 1 σ between 25.4 Ma and 23.4 Ma. These data suggest that rocks of the upper and middle portions of the detachment zone cooled rapidly from temperatures above ∼285°C (sphene closure temperature) to below ∼70°C (lower temperature limit of track stability in apatite) near the beginning of the Miocene (minimum cooling rate of 40°C/Ma). The length distribution of confined fission tracks in apatites from these exhumed middle crustal rocks is strikingly similar to those of rocks known to have cooled rapidly (Fish Canyon ash flow tuff). The lower part of the detachment zone as well as the underlying migmatitic core also cooled through sphene and zircon closure temperatures during this time interval, but apparently only cooled to a temperature within the zone of partial annealing of apatite (∼70°–130°C). Residence of these rocks in this zone led to a partial retention of fission tracks, thus resulting in a reduced age. Confined track length distributions from apatite provide independent evidence of very rapid cooling but also indicate that the rocks of the lower detachment zone experienced a more protracted cooling history (remained hotter longer) than structurally higher levels of the zone. FT data firmly establish the lower limit on the timing of mylonitization during detachment faulting in this area as 23.4 Ma. Rapid cooling of the region is considered to reflect large‐scale tectonic denudation (intracrustal thinning), the vertical complement to crustal extension. Rocks originating in the middle crust (10–15 km) were quickly brought near the surface along the Ruby‐East Humboldt detachment fault (brittle‐ductile simple shear zone) and juxtaposed against brittlely extended rocks deformed under upper crustal conditions.
The Brazilian Lithosphere Seismic Project (BLSP, a joint project by University of São Paulo and Carnegie Institution, 1992–1999) operated more than 20 temporary broadband stations in the southeastern Brazilian shield. The area, a transect ∼1000 km long and 300 km wide, covers different geological provinces: the Precambrian São Francisco craton, the adjacent Brasiliano (700–500 Ma) fold belts, and the Paraná basin of Paleozoic origin. Crustal thicknesses were estimated for 23 sites using receiver functions. For each station, receiver functions were stacked for different sets of earthquakes according to azimuth and distance. The P ‐to‐ S Moho converted phase was clearly identified at most sites. Crustal thicknesses were estimated using an average crustal P wave velocity of 6.5 km/s. Poisson's ratio of 0.23 ( Vp / Vs = 1.70) was used for the São Francisco craton and adjacent fold belt (based on travel times from small, local earthquakes) and 0.25 was used for the Paraná basin and coastal belt. Crustal thicknesses ranged from 35–47 km. Although there is a clear inverse correlation between topography and Bouguer gravity anomalies in the study area, Moho depths show the opposite pattern from that expected: areas of low topography and less negative Bouguer anomalies, such as the Paraná basin, have thicker crust (40–47 km) compared with the high elevation areas of the craton and fold belt (37–43 km). Two hypothesis are proposed to explain the data: (1) A lower density, by 30–40 kg/m 3 , in the lithospheric mantle under the Archean block of the São Francisco craton relative to the Proterozoic lithosphere is responsible for maintaining the high elevations in the plateau area. Relatively low density and high P wave velocity are compatible with a depleted (low FeO) composition for the Archean lithosphere. (2) Alternatively, if the density contrasts between Archean and Proterozoic lithospheres are smaller than the values above, then the crust beneath the Paraná basin must be more dense than that of the craton. Higher crustal density and high Poisson's ratio would be consistent with magmatic underplating in the lower crust beneath the Paraná basin, as inferred from other studies.
The Bear Mountain intrusive complex, Klamath Mountains, California, is a multiphase, ultramafic to silicic plutonic suite emplaced into the Rattlesnake Creek terrane during the Late Jurassic (ca. 151–147 Ma). The intrusive complex includes five plutonic units: (1) elongated, flanking bodies of ultramafic to gabbroic rocks (Blue Ridge, Clear Creek, and Cedar Creek intrusions); (2) biotite + two-pyroxene diorite/monzodiorite of the Buck Lake plutonic unit; (3) biotite-bearing hornblende ± pyroxene gabbro/diorite of the Punchbowl plutonic unit; (4) biotite + hornblende ± pyroxene (± quartz) diorite of the Doe Flat plutonic unit; and (5) minor biotite ± hornblende quartz diorite to tonalite/granodiorite...
The Bear Peak intrusive complex is a Late Jurassic (ca. 144 Ma) composite plutonic suite that ranges in composition from ultramafic to silicic. Clinopyroxene- and hornblende-rich ultramafic cumulate rocks form an intrusion breccia that is complexly intruded by multiple generations of crosscutting gabbroic to dioritic dikes. The bulk of the intrusive complex consists of mappable gabbroic to quartz dioritic to tonalitic/granodioritic units.
In January, 1962, The University of Wyoming published the first issue of Contributions to Geology (vol. 1, no. 1). That single-issue volume was followed by 31 two-issue volumes and four special papers. The final issue of Contributions to Geology appeared in March, 1998 (vol. 32, no. 2).
Contributions to Geology is now transformed to Rocky Mountain Geology , featuring a new format, a new editorial staff, and renewed emphasis on high-quality, refereed articles reporting original …
The Brazilian Lithosphere Seismic Project (BLSP: University of Sao Paulo / Camegie joint project, 1992-1996) operated temporary broad-band stations in 19 different sites in the southeastern Brazilian shield. Crustal thicknesses were estimated for 16 sites using receiver functions. For each station, receiver functions were stacked for different sets of earthquakes according to azimuth and distance. The Ps, Moho converted phase, was clearly identified in most stations. Crustal thicknesses were estimated using an average crustal P-wave velocity of 6.45 km/s. VpNs ratios were used as 1.70 for the stations in the Sao Francisco craton and adjacent fold belt (determined from small, local earthquakes), and 1.73 for the other stations. Crustal thicknesses ranged from 37 to 47 km. Although there is a clear normal correlation between topography and Bouguer anomaly in the study area, Moho depths show the opposite pattern from the expected: areas of low topography and less negative Bouguer anomaly, such as the Parana basin, have thicker crust (40 to 47 km) compared with the high elevation areas of the craton and fold belt (38 to 43 km). This may indicate that lower density (~p about -0.04 g/cm3) of the lithospheric mantle under the Archean block of the Sao Francisco craton is required for the regional isostatic compensation. In fact, surface wave dispersion seems to require slightly lower velocities in the upper mantle beneath the craton as compared with the Parana basin. Alternatively, if the density contrasts between Archean and Proterozoic lithospheres is smaller than the values above, then the crust beneath the Parana basin may be denser compared with the craton and fold belt. This would be consistent with magmatic underplating in the lower crust beneath the Parana basin. However, reasonable densities anomalies « 0.06 g/cm3) in the Parana crust would still require low density for the Archean lithosphere
