August 2009 Geosphere highlights

The August issue of GEOSPHERE, The Geological Society of America's online-only journal is available now. This issue covers tectonics, river channels, and waterfalls in Arizona and the controversy over the impossible existence of low-angle normal faults in Baja California.

Tectonic and structural control of fluvial channel morphology in metamorphic core complexes: The example of the Catalina-Rincon core complex, Arizona

Jon D. Pelletier et al., Dept. of Geosciences, University of Arizona, Gould-Simpson Building, 1040 East Fourth Street, Tucson, Arizona 85721, USA.

River channels can become oriented along with the direction of tectonic extension even if that extension has long since ceased. In this paper, Pelletier et al. examine the strange behavior of rivers in a certain part of Arizona where the rivers are all aligned in the same direction and are dominated by waterfalls despite the lack of active tectonics in the area. These authors use field measurements and computer modeling to understand how the rivers got that way.

Patterns of Quaternary deformation and rupture propagation associated with an active low-angle normal fault, Laguna Salada, Mexico: Evidence of a rolling hinge?

John M. Fletcher, Departamento de Geologia, CICESE, Ensenada, Baja California Mexico C.P. 22860; and Ronald M. Spelz.

This paper presents new observations from an active low-angle normal fault that bear upon several long-standing controversies in the geosciences. Until very recently, low-angle normal faults have been considered impossible to exist. Classic theories of rock mechanics have been modified to explain the incompatibility of these structures, but direct field evidence to corroborate the models has been lacking. This paper presents observations that conclusively demonstrate that a major low-angle normal fault in northern Baja, California, has been seismically active for the past ~250 thousand years, which thus eliminates the controversy of their existence. Our data show how the geometry of large faults evolves when continents are in the process of rifting apart, and we argue that these geometric changes are consistent with a specific mechanical explanation for the genesis of low-angle normal faults -- the rolling-hinge model. Additionally, our observations are the first to demonstrate how seismic ruptures propagate to the surface along active low-angle normal faults.

Source: Geological Society of America