Eclogite breccias in a subducted ophiolite: A record of intermediate-depth earthquakes?

Deep plate serpentinization triggers skinning of subducting slabs

“…Oceanic fragments are common features of all modern and ancient accretionary complexes (Kimura and Ludden, 1995) and have been attributed to the progressive stacking of coherent ophiolitic thrust slices against the leading edge of the continental crust (Coleman, 1971) or the offscraping and underplating of smaller ophiolitic slivers from the upper oceanic crust (Kimura and Ludden, 1995), but examples may also be found in collisional orogens. Exhumed ophiolitic terranes in the Western Alps are believed to represent fossil examples of oceanic crust brecciation under eclogite facies conditions (~23 kbar) and suggest deep burial (~80 km) prior to fluid-assisted fragmentation and subsequent emplacement at crustal levels (Angiboust et al, 2012). Brecciation in the middle part of the oceanic crust has been suggested to invoke intermediate-depth earthquakes, consistent with eclogite breccias in ophiolite complexes (Angiboust et al, 2012).…”

“…Exhumed ophiolitic terranes in the Western Alps are believed to represent fossil examples of oceanic crust brecciation under eclogite facies conditions (~23 kbar) and suggest deep burial (~80 km) prior to fluid-assisted fragmentation and subsequent emplacement at crustal levels (Angiboust et al, 2012). Brecciation in the middle part of the oceanic crust has been suggested to invoke intermediate-depth earthquakes, consistent with eclogite breccias in ophiolite complexes (Angiboust et al, 2012). The exhumation of oceanic crust and associated mantle is discontinuous and short lived, but may occur early with respect to the subduction zone cycle (e.g., Chilean subduction zone; Franciscan subduction zone in California, USA; possibly Makran subduction zone in Iran), at the midst of convergence (e.g., Zagros, Himalaya, Andes) or late in the subduction cycle (e.g., Western Alps; New Caledonia, southwest Pacific Ocean) (Agard et al, 2009).…”

Geological map of the Monviso massif (Western Alps)

“…The eclogitic Fe-Ti metagabbros are mainly made up of garnet and omphacite (Figure 3(d)), and consist of both dam-sized masses associated to the Mg-Al metagabbros (NW of Lago Chiaretto) and msized rounded bodies scattered in the antigorite schists. Peculiar blocks of brecciated eclogites that occur at Punta Forcion, SW of Testa Rossa and at Colle di Luca, have been interpreted as fault-rocks resulting from deep intra-slab seismicity (Angiboust, Agard, Yamato, & Raimbourg, 2012b). The metaperidotites show well-preserved relics of pyroxene, and particularly consist of a minor m-sized level interlayered in the Mg-Al metagabbros (SW of Lago Fiorenza), and a major dam-sized body (E of Colle Armoine) embedded in the antigorite schists.…”

Seismic visibility of a deep subduction channel – insights from numerical simulation of high-frequency seismic waves emitted from intermediate depth earthquakes

“…Schertl et al, 1991;Stöckhert et al, 1997;Reinecke, 1998;Groppo et al, 2007;Groppo and Castelli, 2010;Krebs et al, 2011;Angiboust et al, 2012), geochronological results pose narrow time windows for typical timescales of exhumation, asking for displacement rates comparable to plate velocity (e.g. Gebauer et al, 1997;Rubatto and Hermann, 2001;Baldwin et al, 2004;Little et al, 2011).…”

“…2011; Grigull et al, 2012), ranging from map scale (e.g Angiboust et al, 2012) down to the scale of the individual outcrop (e.g. Federico et al, 2007).…”