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A seismic refraction/wide-angle reflection survey for the Trans-Alaska Crustal Transect program reveals a thin, reflective crust beneath the southern Yukon-Tanana terrane (YTT) in east central Alaska. These data are the first detailed refraction survey of the southern YTT and compose a 130-km-long reversed profile along the Alaska and Richardson highways. Results from this study indicate that low-velocity (-• 6.4 km/s) rocks extend to approximately 27 km in depth. Based on these low velocities and an average Poisson's ratio of 0.23 determined for depths of -• 27 km, an overall silicic composition is interpreted for this portion of the crust beneath the Yukon-Tanana terrane. From approximately 8 to 27 km depth the crust exhibits an increase in reflectivity. This middle to lower crustal reflectivity is modeled as alternating high-and low-velocity lamellae with an average velocity of 6.1 km/s at 10 km depth to an average velocity of 6.4 km/s at 27 km depth. Beneath these reflective, low-velocity rocks a 3-to 5-km-thick, 7.0 km/s basal crustal layer produces a prominent reflection that extends to offsets of up to 280 km. The crust-mantle boundary, modeled at an average depth of 30 km, produces a variable PmP reflection, which may indicate lateral heterogeneity of this boundary, and a weak and emergent Pn refraction with a velocity of 8.2 km/s. We interpret the crustal section as follows: the low-velocity rocks of the southern YTT extend from the surface to depths of approximately 10 km; underthrust Mesozoic flysch of the Kahiltna terrane, rocks of the Gravina arc, and basement of the Wrangellia(?) terrane extend from 10 to 27 km depth; a 3-to 5-km-thick layer of mantle-derived mafic rocks, relic oceanic crust, or Wrangellia(?) terrane lower crust extends from 27 to approximately 30 km depth; a tectonically young Moho beneath the southern YTT is found at an average depth of 30 km; and it is underlain by a mantle that may be relatively cool and/or olivine rich. In this interpretation, the Yukon-Tanana terrane is a thin-skinned terrane. Our results indicate that tectonic, and possibly magmatic, underplating has played a significant role in crustal growth for central

Abstract. As the Mendocino Triple Junction migrates northward along the California margin it is widely presumed to leave a "slab-free" or "asthenospheric" window in its wake. A 250-km-long south-north seismic refraction-reflection profile crossing the transition from transform to subduction regimes allows us to compare and contrast crust and upper mantle of the North American margin before and after it is modified by passage of the Mendocino Triple Junction. From the seismic data we have determined that (

The Transantarctic Mountains (TAM), which separate the West Antarctic rift system from the stable shield of East Antarctica, are the largest mountains developed adjacent to a rift. The cause of uplift of mountains bordering rifts is poorly understood. One notion based on observations of troughs next to many uplifted blocks is that isostatic rebound produces a coeval uplift and subsidence. The results of an over-snow seismic experiment in Antarctica do not show evidence for a trough next to the TAM but indicate the extension of rifted mantle lithosphere under the TAM. Furthermore, stretching preceded the initiation of uplift, which suggests thermal buoyancy as the cause for uplift.

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A 140‐km‐long refraction/wide‐angle reflection profile recently recorded by Stanford University and the U. S. Geological Survey imaged the subducting Gorda slab beneath northern California. The profile, which is subparallel to the coast from Cape Mendocino northward, indicates that the North American plate is 13‐ to 14‐km‐thick along the coast north of Cape Mendocino. The crust is characterized by relatively uniform, low velocities of ≤6 km/s interpreted as Franciscan rocks. Two strong reflections define the upper and lower boundaries of the subducting Gorda crust. Our data indicate that the subducting Gorda crust thickens northward from Cape Mendocino from 7‐km‐thick just north of Cape Mendocino to 10‐km‐thick 120 km to the north. This change in thickness is coincident with a change in velocity from 6.7 km/s south to 6.2 km/s north. Mantle velocities of 7.7 km/s are observed for offsets greater than c. 80 km. We interpret our model to indicate that the Gorda slab is not imbricated on a crustal scale beneath our profile, that sediments and/or a tectonically thickened oceanic layer 2 are present to the north but not in the vicinity of Cape Mendocino, and that proximity to the Mendocino triple junction affects the way sediments are subducted.

Earthquakes recorded by a dense seismic array at Nanga Parbat, Pakistan, provide new insight into synorogenic metamorphism and mass flow during mountain building. Microseismicity beneath the massif drops off sharply with depth and defines a shallow transition between brittle failure and ductile flow. The base of seismicity bows upward, mapping a thermal boundary with 3 km of structural relief over a lateral distance of 12 km. Anomalously low seismic velocities are observed at the core of the massif and extend to depth through the crust. The main locus of seismicity and low velocities correlates with a region of high topography, rapid exhumation, high geothermal gradients, young metamorphic and igneous ages, and crustal fluid flow. We suggest a genetic link between these phenomena in which hot rocks, rapidly advected from depth, are pervasively modified at relatively shallow levels in the crust.

Appendix 1. Abbreviated list of airgun shotpoint locations, FFID numbers, and shot times Appendix 2. Reftek and OBS station locations and elevations Appendix 3. List of timing used for Reftek stations 56 Appendix 4. List of stations having problems with spurious 12-second shifts Appendix 5. List of location of Reftek station data on archival tapes