2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 6
Presentation Time: 9:15 AM

The Geometry and Mechanics of Active Oceanic Detachment Faulting from An Ocean-Bottom Seismicity Survey at the TAG Segment of the Mid-Atlantic Ridge (26°N)


DEMARTIN, Brian J., Department of Geological Sciences, Brown University, Box 1846, Providence, RI 02912-1846 and REVES-SOHN, Robert, Geology and Geophysics, Woods Hole Oceanographic Institution, Mail Stop 24, Woods Hole, MA 02543, Brian_deMartin@brown.edu

Long-term deformation along detachment faults has been recognized as a mechanism for exhumation of oceanic and continental core complexes. Previous results from a seismic refraction and microearthquake survey on the TAG segment of the Mid-Atlantic Ridge (26°N) revealed the subsurface dome-shaped morphology of the fault. Exhumation of the massif east of the spreading axis occurs along a low-angle (~20°) fault that rolls over to a steep (~70°) dip at a depth of ~3 km and extends into the upper mantle to depths of at least 7 km below the seafloor. Here we consider the detailed space-time behavior of 20,730 well-located microearthquakes detected over a 245-day period from June 2003 until January 2004. The seismicity rate is high and unusually stable, lacking mainshock-aftershock sequences that are characteristic of most active faults. Frequency-magnitude analysis yields a b-value of ~1.5 along the detachment fault, which is intermediate between standard values for tectonic (~1.0-1.2) and volcanic (~1.8-2.0) environments. Clustering analyses using waveform cross-correlation techniques suggest that seismic activity is characterized by repeated slip events on small fault patches. Recurrence intervals for individual fault patches range from 1 to 3 days. The moment release rate for detachment fault earthquakes observed during our study is roughly 1022 dyn-cm/yr, which is significantly less than the value of 1024 dyn-cm/yr calculated from a simple model of extension. This observation indicates that either: (1) slip along the detachment is predominately aseismic, or (2) extension rates were significantly less than the long-term geological average. The peculiar nature of seismicity on the detachment fault (high-rates of small events without mainshock-aftershock sequences) may result from a combination of the unique geometry (dome-shape), the composition (exhumed lower-crust and upper mantle), or the stress state (possible presence of elevated pore pressure) of the fault surface.