Deformation History of Peridotites from Mariana Forearc, Conical Seamount, Leg 125

Abstract

Abundant serpentinite seamounts are found along the outer high of the Mariana forearc at the top of the inner slope of the trench. One of them, Conical Seamount, was drilled at Sites 778, 779, and 780 during Leg 125.The rocks recovered at Holes 779A and 780C, respectively, on the flanks and at the summit of the seamount, include moderately serpentinized depleted harzburgites and some dunites. These rocks exhibit evidence of resorption of the orthopyroxene, when present, and the local presence of very calcic-rich diopside in veins oblique to the main high-temperature foliation of the rock.The peridotites, initially well-foliated with locally poikiloblastic textures, show overprints of a two-stage deformation history: (1) a high-temperature (>1000°C), low-stress (0.02 GPa), homogeneous deformation that has led to the present Porphyroclastic textures displayed by the rocks and (2) heterogeneous ductile shearing at a much higher stress (0.05 GPa). This heterogeneous shearing probably describes a single tectonic event because it began at high temperatures, producing dynamic recrystallization of olivine in the shear zone, and ended at low temperatures in the stability field of chlorite and serpentine. In a few samples, olivine shows evidence of quasi-hydrostatic recrystallization at a very high temperature. Here, we propose that this recrystallization was related to fluid/magma percolation, a process that can also account for the resorption of the orthopyroxene and for the late crystallization of diopside veins in the rock.The impregnation by fluid or magma, development of the main high-temperature, low-stress deformation, and subsequent migration recrystallization of olivine probably occurred in a mantle fragment involved in the arc formation. In addition, this mantle has preserved structures that may have formed earlier in the oceanic lithosphere upon which the arc formed.Heterogeneous ductile shear zones in the peridotites may have developed during uplift. The "cold" deformation may have taken place during diapiric rise of hot mantle that underwent subsequent serpentinization or gliding along normal faults associated with the extension of the eastern margin of the forearc.

Significance of serpentinization of wedge mantle peridotites beneath Mariana forearc, western Pacific

“…Cleavable olivine, which has perfect cleavages on the (100), (010), and (001) crystal planes (Hawkes, 1946), were reported from peridotites of Conical Seamount by Ishii et al (1992) and Girardeau and Lagabrielle (1992). We found cleavable olivine in peridotites from Big Blue, Celestial, and South Chamorro Seamount (Figs.…”

Rheology and Biostratigraphy of the Mariana Serpentine Muds Unravel Mud Volcano Evolution

“…Rheological studies of the SMVs muds are scarce and mostly related to the identification of deformation structures inside the serpentine mud matrix (Girardeau & Lagabrielle, ), or to shear strength measurements conducted with a Wykeham‐Farrance torsion vane on core sections (Phipps & Ballotti, ). Two main aspects of the SMVs' evolutionary dynamics were identified in these papers: (i) pure‐ and simple‐shear deformation fabrics in the matrix are present only on serpentine muds on the flanks of SMV edifices, meaning they could be the result of gravitative sliding of mudflows emitted at the summits; (ii) the density and shear strength of serpentine muds are weaker than evaporites, known to behave diapirically, which implies that SMVs sediments must have risen through deep seated faults, in order to penetrate the forearc crust.…”

Serpentinite Mud Volcanism: Observations, Processes, and Implications

“…The variations in composition and mineralogy of materials from flow units in ODP cores (Fryer & Mottl 1992, Savov et al 2005a, however, suggest that discrete source regions can be distinguished. Girardeau & Lagabrielle (1992) examined the microtextures of serpentinized ultramafic clasts from Conical Seamount. They noted the initial formation of the peridotite under high-temperature asthenospheric conditions, followed by the remelting and infusion of the peridotite with a fluid phase, which was accompanied by a period of high-temperature/low-stress deformation of the peridotite that occurred either before subduction began or during the initial stages of subduction and proto-arc formation.…”