February Geology and GSA Today media highlights

Ridge reorientation mechanisms: Macquarie Ridge Complex, Australia-Pacific plate boundary
Sharon Mosher, Department of Geological Sciences, The University of Texas at Austin, 1 University Station C1100, Austin, Texas 78712-0254, USA; and Christina Symons, Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093, USA. Pages 119-122.

Mosher and Symons analyzed a unique, complete seafloor record to document the processes associated with the transition of the Australia-Pacific tectonic plate boundary south of New Zealand from an oceanic spreading center at 40 million years ago into the present-day transform fault plate boundary. Although the geologic record shows that most plate boundaries have changed from one type of plate boundary to another over time, rarely is a record of the transition preserved. During this drastic 60°-90°change in spreading direction, individual spreading ridge segments propagated and failed while undergoing reorientation, resulting in the shortening of spreading segments and, in most cases, their complete disappearance. Moreover, newly created crust was modified and transferred between plates during the gradual reorientation of the spreading axes. The entire transition was accommodated by non-rigid plate deformation along the plate boundary prior to the cessation of magmatism.

Probable low-angle thrust earthquakes on the Juan de Fuca–North America plate boundary
Anne Tréhu et al., Oregon State University, CAOS Administration Building 104, Corvallis, Oregon 97331, USA. Pages 127-130.

In 2004, two clusters of earthquakes occurred in the central part of the Cascadia forearc. Tréhu et al.'s analysis suggests that both events, which had magnitudes of 4.9 and 4.8, resulted from low-angle thrust motion between the North America and Juan de Fuca plates in the zone that is generally thought to be locked. Paleoseismic data indicate that the locked part of the plate boundary ruptures in large earthquakes every several 100 years, most recently in 1750. Tréhu et al.'s observations represent the first instrumentally recorded events on the Cascadia plate boundary, with the possible exception of the Petrolia Earthquake of 1992 at the southern end of the subduction zone.

Elliptical mud volcano caldera as stress indicator in an active compressional setting (Nirano, Pede-Apennine margin, northern Italy)
Marco Bonini, Consiglio Nazionale Delle Ricerche, Istituto di Geoscienze e Georisorse, Via G. La Pira, 4, 50121 Firenze, Italy. Pages 131-134.

Active fluid release occurs through numerous vents located within an elliptical depression developed over the crest of an anticline associated with the active Pede-Apennine thrust front of the Northern Apennines (Italy). Bonini interprets this depression as a caldera collapse structure that may have developed in response to the deflation of a shallow mud chamber, triggered by eruption and sediment fluid evacuation events. Bonini concludes that the mud volcano caldera reflects the regional tectonic stress axes, and exhibits mechanical similarities to the igneous homologues.