Experimental deformation of serpentinite and its tectonic implications

Abstract

Experimental investigation into the strength and ductility of serpentinite at temperatures to 700øC and confining pressures to 5 kb has yielded results important to the understanding of the role of serpentinite in orogenesis. Sealed specimens of antigorite-chrysotile serpentinite, with ultimate strength comparable to that of granite at room temperature, showed a marked weakening above 500-600øC; a mesh-textured serpentinite containing lizarditc, chrysotile, and a minor amount of brucite showed a similar loss of strength at 300-350øC. Brittleness always accompanied the high-temperature weakening, although the samples showing high strength at lower temperatures were often ductile. Petrographic and X-ray examinations reveal that serpentine in the weakened and embrittled specimens has undergone partial dehydration to forsteritc and talc. The embrittlement and weakening is attributed to a reduction in the effective confining pressure due to the pore pressure of the water released during dehydration and to a loss in cohesive strength due to changes in the structure upon dehydration. The hypothesis of tectonic emplacement of serpentinites of the alpine type thus becomes highly plausible at temperatures great enough for dehydration weakening, while being difficult to accept at lower temperatures where the strength of the serpentinite is high. Weakening upon heating to the appropriate dehydration temperature in the range 300-600øC of a partially serpentinized oceanic lower crust or upper mantle should also serve to concentrate deformation in the heated belt, thus facilitating mountain building. The embrittlement associated with dehydration extends the maximum theoretical depth for brittle fracture in the mantle to that of the deepest hydrated phases.

Thermal structure and intermediate-depth seismicity in the Tohoku-Hokkaido subduction zones

“…A number of processes have been suggested for causing local weakening. These include shear instabilities (Ogawa, 1987;Kelemen and Hirth, 2007), thermal runaways (John et al, 2010), and local weakening and embrittlement of the crust due to dehydration (e.g., Raleigh and Paterson, 1965;Kirby et al, 1996;Seno and Yamanaka, 1996;Hacker et al, 2003;Yamasaki and Seno, 2003). Possible consequences of dehydration include stress changes to the change in volume of the oceanic crust , pore pressure changes (Wong et al, 1997) or just the release and transport of water (e.g., Jung et al, 2004;John and Schenk, 2006).…”

Structural and petrological constraints on the tectonic evolution of the garnet‐lherzolite facies Higashi‐akaishi peridotite body, Sanbagawa belt, SW Japan

“…[43] It is generally accepted that serpentine is important for dynamic processes in subduction zone because of wide stability in mantle lithosphere [e.g., Ulmer and Trommsdorff, 1999;Seno et al, 2001;Bostock et al, 2002] and relatively weak mechanical properties [e.g., Raleigh and Paterson, 1965;Gerya et al, 2002]. Particularly its contribution to exhumation processes has been discussed on the basis of numerous geological and structural studies [e.g., Takasu, 1989;Hermann et al, 2000;Guillot et al, 2000].…”

Rock Deformation and Formation of LPO of Minerals in the Upper Mantle: Implications for Seismic Anisotropy

“…이것은 아직까지 미스터리로 남 아있다. 중간 깊이(약 60−300 km)의 지진발생 메커니 즘으로는 함수광물(예, 사문석)의 탈수약화(dehydration embrittlement) 현상으로 인해 단층이 형성될 수 있음이 고압고온 실험을 통해 제안되었고 (Jung et al, 2009b;Raleigh and Paterson, 1965), 심부(~300-670 km)에서의 지진 발생 메커니즘으로는 감람석−스피넬의 상변이로 생기 는 앤티크랙(anticrack)이 단층을 만들기 때문이라고 제안된 바 있다 (Green and Burnley, 1989;Green et al, 1990 2. An example of seismic anisotropy observed in NE Japan (after Karato et al, 2008).…”