Revision of lithological logs for boreholes penetrating the volcanic center at Pahute Mesa, Nevada, has led to a thorough review of the volcanic stratigraphy and geologic structure. We have combined this review with a compilation of old and newly acquired gravity and seismic travel time data, producing a unified interpretation along a northwest to southeast profile. The analysis supports a new interpretation of the Silent Canyon caldera complex. The caldera is found to be more asymmetric than previously suggested, with the southeastern boundary formed by linear, high‐angle normal faults and a more gently sloping northwestern boundary. The total thickness of volcanic units within the caldera complex does not appear to exceed 5 km. The shallow structure at Pahute Mesa could have a profound effect on the seismic response for regional and teleseismic signals from this nuclear test site. The Silent Canyon caldera complex is actually a set of nested calderas first filled by thick (>1 km) postcaldera lavas and subsequently buried by outflow sheets of the Timber Mountain caldera to the south. Thick, postcaldera lavas filled a half‐graben structure formed west of the West Greeley fault, dropping the tops of the youngest caldera‐forming units to depths in excess of 2 km. Therefore the western boundary of the caldera complex is poorly defined. East of the West Greeley fault, two overlapping calderas are defined, and stratigraphic data suggest the presence of even older calderas. The youngest caldera, the calc‐alkaline Area 20 caldera, is well defined from drill hole data. The Area 20 caldera overlaps the 13.6 Ma peralkaline Grouse Canyon caldera, which is less well defined, but apparently collapsed in trap‐door style along the Almendro fault. For both these calderas, collapse continued after the main caldera‐forming eruption, concurrent with the accumulation of thick (>1 km) lavas within the peripheral collapse zones. The geophysical interpretation indicates that the major structural boundary of the caldera complex corresponds to the NNE trending Scrugham Peak and Almendro faults, which offset the pre‐Tertiary contact more than 1 km but have less than 200 m offset in rocks of 11 Ma age. Drill hole data show that offsets along these faults increase systematically within older (up to 15 Ma) units, which are commonly rotated eastward in a style similar to units at the surface. Abrupt changes in the subsurface thickness of the caldera‐forming units occur across the faults, indicating that these linear features served as caldera boundaries.

Five Pleistocene basaltic volcanoes in Crater Flat (southern Nevada) demonstrate thc complexity of eruption processes associated with small-volume basalts and the cffccts of initial emplacement characteristics on post-eruptive geomorphic evolution of thc volcanic surfaces. The volcanoes record eruptive processes in their pyroclastic facies ranging from "classical" Strombolian mechanisms to, potentially, violent Strombolian mechanisms. Cone growth was accompanied, and sometimes disrupted, by effusion of lavas from the bases of cones. Pyroclastic cones were built upon a gently southwardsloping surface and were prone to failure of their down-slope (southern) flanks. Early lavas flowed primarily southward and, at Red and Black Cone volcanoes, carricd abundant rafts of cone material on the tops of the flows. These resulting early lava ficlds eventually built platforms such that later flows erupted from the eastern (at Rcd Cone) and northern (at Black Cone) bases of the cones. Three major surface features -scoria cones, lava fields with abundant rafts of pyroclastic material, and lava fields with littlc or no pyroclastic material -experienced different post-eruptive surficial processcs. 1Contrary to previous interpretations, we argue that the Pleistocene Crater Flat volcanoes are monogenetic, each having formed in a single eruptive episode lasting months to a few years, and with all eruptive products having emanated from the area of the volcanocs' main cones rather than from scattered vents. Geochemical variations within thc volcanoes must be interpreted within a monogenetic framework, which implies preservation of magma source heterogeneities through ascent and eruption of thc magmas.

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Source parameters determined from interferometric synthetic aperture radar (InSAR) measurements and from seismic data are compared from four moderate size (less than M 6) earthquakes in southern California. The purpose is to verify approximate detection capabilities of InSAR, assess differences in the results, and to test how the two results can be reconciled. First, we calculate the expected surface deformation from all earthquakes greater than magnitude 4 in areas with available InSAR data. A search for deformation from the events in the interferograms yielded 4 possible events with magnitudes less than 6. The search for deformation was based on a visual inspection as

Corrections for terrain effects are required for virtually all gravity measurements acquired in mountainous areas, as well as for high‐precision surveys, even in areas of low relief. Terrain corrections are normally divided into two parts, one part being the correction for terrain relatively close to the gravity station (the “inner‐zone” correction) and the other part being the correction for more distant, say, >2 km, terrain. The latter correction is normally calculated using a machine procedure that accesses a digital‐terrain data set. The corrections for terrain very close to the gravity station are done manually using Hammer’s (1939) procedures or a similar method, are guessed in the field, or simply are neglected. Occasionally, special correction procedures are used for the inner‐zone terrain corrections (e.g., LaFehr et al., 1988); but such instances are uncommon.