April GEOLOGY and GSA TODAY media highlights

Folco et al. report on the discovery of a microtektite (microscopic impact glass particles) field from Victoria Land Transantarctic Mountains. Microtektites were found trapped in the local detritus accumulated in weathering pits and joints of several glacially eroded summits (approximately 2600 meters above sea level) distributed latitudinally for 520 kilometers. It is suggested that these microtektites define the southern extension of the Australasian tektite/microtektite strewn field. The margin of the Australasian tektite/microtektite strewn field is thus shifted southward by approximately 3000 kilometers, and the maximum distance from the putative parent impact site in Indochina increased by approximately 2000 kilometers. This emphasizes the paradox of the missing parent crater of the largest (more than 10 percent of the Earth’s surface) and youngest tektite-strewn field discovered on Earth.

Toroidal mantle flow through the western U.S. slab window

G. Zandt, University of Arizona, Department of Geosciences, Gould-Simpson Building, Building 77, Tucson, Arizona 85721, USA; and E. Humphreys, University of Oregon, Dept. of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA. Pages 291-294.

The circular pattern of anisotropic fast-axis orientations of split SKS arrivals observed in the western U.S. cannot be attributed reasonably to either preexisting lithospheric fabric or to asthenospheric strain related to global-scale plate motion. A plume origin for this pattern accounts more successfully for the anisotropy field, but little evidence exists for an active plume beneath central Nevada. Zandt and Humphreys suggest that mantle flow around the edge of the sinking Gorda–Juan de Fuca slab is responsible for creating the observed anisotropy. Seismic images and kinematic reconstructions of Gorda–Juan de Fuca plate subduction have the southern edge of this plate extending from the Mendocino triple junction to beneath central Nevada, and flow models of narrow subducted slabs produce a strong toroidal flow field around the edge of the slab, consistent with the observed pattern of anisotropy. This flow may enhance uplift, extension, and magmatism of the northern Basin and Range while inhibiting extension of the southern Basin and Range.

The roof of an axial magma chamber: A hornfelsic heat exchanger

Kathryn M. Gillis, School of Earth and Ocean Sciences, P.O. Box 3055 STN CSC, University of Victoria, Victoria, British Columbia V8W 3P6, Canada. Pages 295-298.

Mid-ocean ridges are dynamic features where magma generated within the mantle accumulates in axial magma chambers and builds the ocean crust. The heat released from these axial magma chambers drives the circulation of seawater-derived fluids through the porous crust. These fluids are ultimately released back into the ocean at hydrothermal vent sites. Gillis studied hornfelsic rocks that are the remnants of a critical boundary that separates the circulating hydrothermal fluids and axial magma chambers—the so-called conductive boundary layer. The characteristics of these rocks allow us to calculate the amount of heat transferred across the conductive boundary layer, and thus test theoretical models for how heat and mass are transported from Earth's interior to its surface.