Slab tears and intermediate‐depth seismicity

Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 40 (2013): 4244-4248, doi:10.1002/grl.50830.Active tectonic regions where plate boundaries transition from subduction to strike slip can take several forms, such as triple junctions, acute, and obtuse corners. Well‐documented slab tears that are associated with high rates of intermediate‐depth seismicity are considered here: Gibraltar arc, the southern and northern ends of the Lesser Antilles arc, and the northern end of Tonga trench. Seismicity at each of these locations occurs, at times, in the form of swarms or clusters, and various authors have proposed that each marks an active locus of tear propagation. The swarms and clusters start at the top of the slab below the asthenospheric wedge and extend 30–60 km vertically downward within the slab. We propose that these swarms and clusters are generated by fluid‐related embrittlement of mantle rocks. Focal mechanisms of these swarms generally fit the shear motion that is thought to be associated with the tearing process

Seismic evidence for a slab tear at the Puerto Rico Trench

Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 105 (2013): 2915-2923, doi:10.1002/jgrb.50227.The fore‐arc region of the northeast Caribbean plate north of Puerto Rico and the Virgin Islands has been the site of numerous seismic swarms since at least 1976. A 6 month deployment of five ocean bottom seismographs recorded two such tightly clustered swarms, along with additional events. Joint analyses of the ocean bottom seismographs and land‐based seismic data reveal that the swarms are located at depths of 50–150 km. Focal mechanism solutions, found by jointly fitting P wave first‐motion polarities and S/P amplitude ratios, indicate that the broadly distributed events outside the swarm generally have strike‐ and dip‐slip mechanisms at depths of 50–100 km, while events at depths of 100–150 km have oblique mechanisms. A stress inversion reveals two distinct stress regimes: The slab segment east of 65°W longitude is dominated by trench‐normal tensile stresses at shallower depths (50–100 km) and by trench‐parallel tensile stresses at deeper depths (100–150 km), whereas the slab segment west of 65°W longitude has tensile stresses that are consistently trench normal throughout the depth range at which events were observed (50–100 km). The simple stress pattern in the western segment implies relatively straightforward subduction of an unimpeded slab, while the stress pattern observed in the eastern segment, shallow trench‐normal tension and deeper trench‐normal compression, is consistent with flexure of the slab due to rollback. These results support the hypothesis that the subducting North American plate is tearing at or near these swarms. The 35 year record of seismic swarms at this location and the recent increase in seismicity suggest that the tear is still propagating

Seismic analysis of a slab tear in the Northeast Caribbean.

Active tectonic regions where plate boundaries transition from subduction to strike-slip can take several forms, such as triple junctions, acute and obtuse corners. The Caribbean-North American plate boundary is one such active margin, where subduction transitions from arc- to oblique-type off the northeast coast of Puerto Rico. Understanding mantle flow in this region will not only help determine the nature of tectonic activity and mantle dynamics that control these margins, but will also aid our understanding of the fate of subducting lithosphere. The forearc region of the northeast Caribbean plate north of Puerto Rico and the Virgin Islands has been the site of numerous seismic swarms since at least 1976. A six-month deployment of five ocean-bottom seismographs recorded two such tightly-clustered swarms, along with additional events. Focal mechanism solutions, calculated from a joint analyses of the OBS and land-based seismic data, reveal that the swarms generally have strike- and dip-slip mechanisms at depths of 50-100 km, while events at depths of 100-150 km have oblique mechanisms. A stress inversion reveals two distinct stress regimes, which supports the hypothesis that the subducting NOAM plate is tearing at or near these swarms. Global and well-documented slab tears associated with high rates of intermediate-depth seismicity are discussed here. Seismicity at these locations occurs, at times, in the form of swarms or clusters and various authors have proposed that each marks an active point of tear propagation. The swarms and clusters start at the top of the slab below the asthenospheric wedge and extend 30-60 km vertically downward within the slab. We interpret these swarms and clusters to be generated by dehydration embrittlement of mantle rocks. Focal mechanisms of these swarms generally fit the shear motion that is thought to be caused by the tearing process.Ph.D