March GEOLOGY and GSA TODAY media highlights

S. Mazzoli et al., Dipartimento di Scienze della Terra, Università di Napoli 'Federico II', Largo San Marcellino 10, 80138 Napoli (NA), Italy. Pages 243-246.

Mazzoli et al. examine the recent tectonic evolution of the southern Apennines, a key Italian area in terms of geological hazards, as well as for petroleum exploration and production. Integrated structural and thermochronological data indicate that, over the past one to five million years, the mountain belt underwent gravitational collapse. This process involved the shallow part (upper 5 kilometers) of the orogen, producing rock exhumation in the axial zone of the chain. Large horizontal extension dismembering the mountain belt was accompanied by forward (eastward) motion of thrust sheets. Linked low-angle extensional faults and frontal thrusts shared a common, shallow detachment. Below the detachment, coeval crustal shortening, though limited (less than 15 percent), led to the growth of large subsurface structures that form the hydrocarbon traps for the significant oil discoveries in the area. These subsurface structures also created gravity disequilibria in the overlying tectonic wedge, triggering further gravitational readjustments. Subsequently, as shortening ceased even at depth, crustal extension became dominant (from the Middle Pleistocene to the present).

Neogene extension and basin deepening in the West Antarctic rift inferred from comparisons with the East African rift and other analogs

Wesley LeMasurier, Institute of Arctic and Alpine Research, University of Colorado at Boulder, 1560 30th St., 450 UCB, Boulder, Colorado 80309-0450, USA. Pages 247-250.

The West Antarctic rift is a region of volcanic activity and crustal stretching that is roughly the size of the western United States (from Salt Lake City to the Pacific Ocean). About 98 percent of it is buried beneath glacial ice, up to 2.5 miles thick, and bedrock beneath the ice is 2000–3000 feet below sea level over large areas. All of this makes it a difficult region to study. It is interesting nevertheless, because volcanic eruptions beneath the ice could destabilize the ice sheet, leading to as much as 25 feet of sea-level rise. How likely is it that this could happen is a question scientists have debated for over a decade. LeMasurier addresses the question by comparing the West Antarctic rift with similar areas of crustal stretching elsewhere in the world. The comparison shows that volcanic activity in rifts is most common where the land is a mile or more above sea level, and rising, which can readily be seen in Antarctica along the Transantarctic Mountains, and in the Pacific coast mountains of Marie Byrd Land. The large sub-sea-level interior of the rift does not, therefore, seem to be a likely place for present-day volcanic activity. This is good news, because the sub-sea-level base of the West Antarctic ice sheet is already especially vulnerable to warming of the atmosphere and surrounding seas. However, this study also shows that the land in West Antarctica has been rising beneath the ice sheet in some areas and subsiding beneath it in others, over roughly the past 25 million years. Some areas have subsided to as much as 8500 feet below sea level. This tectonic restlessness contrasts markedly with the stability of the regions that lay beneath the northern hemisphere ice sheets of the recent geologic past, and its affect on the history of the West Antarctic ice sheet has not yet been evaluated.