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)

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.