Publications

Three-component seismograms, recorded by a small array of digital instruments in the northern Mississippi embayment, consistently show a high-amplitude phase on the vertical component that arrives approximately 0.8 s before the shear wave. On the basis of its timing and apparent velocity, this phase is identified as an S-P conversion from the boundary between the unconsolidated Cenozoic...

Mantling deposits on the Moon are considered to be pyroclastic units emplaced on the lunar surface as a result of explosive fire fountaining. These pyroclastic units are characterized as having low albedos, having smooth fine-textured surfaces, and consisting in part of homogeneous, Febearing volcanic glass and partially crystallized spheres. Mantling units exhibit low returns on...

Single crystals of Gd3Ga5O12 have been strained under confining pressure (1 5 GPa) at temperatures below 950°C. No evidence for macroscopic plasticity was found, but transmission electron microscopy revealed dislocation generation at crack tips. Deformation mechanisms are different from those operating in the high-temperature regime: extending stacking faults in {110} planes and the 〈010...

No abstract available.

For ground ice to exist on Mars, two conditions have to be met. One is the presence of permafrost; the second is the availability of water. Because the mean temperature of Mars’surface is − 80 C., permafrost 1–3 km thick occurs over the entire planet. Remote-sensing measurements suggest that water presently exists in the atmosphere and in the polar caps; frost has been observed at the...

The thick sequence of Miocene lava flows exposed on Steens Mountain in southeastern Oregon is well known for containing a detailed record of a reversed‐to‐normal geomagnetic polarity transition. Paleomagnetic samples were obtained from the sequence for a combined study of the directional and intensity variations recorded; the paleointensity study is reported in a companion paper. This...

We carried out an extensive paleointensity study of the 15.5±0.3 m.y. Miocene reversed‐to‐normal polarity transition recorded in lava flows from Steens Mountain (south central Oregon). One hundred eighty‐five samples from the collection whose paleodirectional study is reported by Mankinen et al. (this issue) were chosen for paleointensity investigations because of their low viscosity...

The 87Sr/86Sr ratios and the activity ratios of 234U/238U and 230Th/238U have been measured in dolomites from a 168-m-deep core taken on the island of San Salvador, Bahamas. These data suggest two periods of dolomitization. The first episode dolomitized Miocene age sediments during the latest Miocene, and the second dolomitized the Pliocene portion of the core and was still active as...

In contrast to the rarity of such observations a decade ago, seismic reflecting and refracting horizons are now being observed to Moho depths under continental shelves in a number of places. These observations provide knowledge of the entire crustal thickness from the shoreline to the oceanic crust on passive margins and supplement Consortium for Continental Reflection Profiling (COCORP)...

High-resolution, single-channel and multichannel seismic-reflection profiles in the New York Bight provide 7 crossings of a 50-km-long fault that trends north-northeast for 30 km from its southern end, then bends northeast, and may continue northward beneath Long Island. Displacement, which is consistently down to the west, decreases upsection and suggests a growth fault. Dip of the...

A major fault cutting through most of the crust can be identified and mapped on the Long Island platform using multichannel seismic reflection profiles and magnetic data. The fault, here called the Block Island fault (BIF), strikes north-northeast, dips westward at low angle, and does not resemble the thin-skinned thrust faulting observed in the foreland of the Appalachians. The BIF is...

Seismic profiles and bathymetric contours reveal a drowned barrier spit on Jeffreys Ledge off Cape Ann, Massachusetts. Seaward-dipping internal reflectors indicate that a regressive barrier formed during the early Holocene low sea-level stillstand. Preservation of the barrier spit may have been favored by its large size (as much as 20 m thick), by an ample sediment supply from...