Shearing within lower crust during progressive retrogression: Structural analysis of gabbroic rocks from the Godzilla Mullion, an oceanic core complex in the Parece Vela backarc basin

Abstract

Microstructural and petrological analyses of gabbroic rocks sampled from the Godzilla Mullion, located along the Parece Vela Basin spreading ridge (Parece Vela Rift), Philippine Sea, reveal the development of a ductile shear zone in the lower crust. The shear zone is interpreted to represent a detachment fault within an oceanic core complex. Microstructures indicative of intense deformation, characterized by porphyroclastic textures consisting dominantly of coarse plagioclase porphyroclasts and lesser clinopyroxene porphyroclasts in a fine-grained matrix, are observed within samples of gabbroic rocks dredged near the breakaway area of the Godzilla Mullion (dredge site D6). Samples are classified into three types based upon the grain size of fine-grained plagioclase in the matrix: coarse (80-130µm), medium (25µm), and fine (~10µm). Although the chemical composition of plagioclase porphyroclasts is consistently An 40-50 among all sample types, the compositions of fine grains in the matrix vary with decreasing grain size, being An 40-50 for the coarse-type, An 30-40 for the medium-type, and An 5-30 for the fine-type. This finding implies that the composition of fine-grained plagioclase in the matrix is related to the following retrograde reaction that occurred during deformation: clinopyroxene + plagioclase + Fe-Ti oxide + fluid → hornblende + plagioclase. Plagioclase crystal-preferred orientations also show a gradual change with grain size, varying from a (010)[100] pattern for the coarse-type, (010)[100] and (001)[100] patterns for the medium-type, and a weak (001)[100] pattern or random orientations for the fine-type. These patterns are interpreted to result from a change in the deformation mechanism of plagioclase from dislocation creep to grain-size-sensitive creep with decreasing temperature, thereby leading to strain softening and localization during cooling. Although secondary amphibole occurs ubiquitously within all samples, the chemical composition of amphibole varies from pargasitic hornblende (i.e. brown hornblende) to actinolite (i.e. green hornblende) within each of the sample types. However, amphibole in the coarse-type shows no evidence of deformation, whereas brown hornblende in the medium-and fine-types is plastically deformed. As a consequence, we argue that the microstructural development of the gabbroic rocks occurred during uplift-related cooling of the gabbro body and that a primary shear zone developed near the breakaway area at depth under anhydrous conditions at high temperatures above 850 °C; the shear zone subsequently evolved during progressive retrogression in association with hydration of the shear zone, possibly resulting in the development of the detachment fault that gave rise to the Godzilla Mullion.

Fabric Development in a Middle Devonian Intraoceanic Subduction Regime: The Careón Ophiolite (Northwest Spain)

“…High-temperature experiments, studies of preferred orientations in naturally deformed high-grade rocks, and simulations all suggest that plagioclase dominantly deforms by dislocation glide on (010) [001] accompanied by (010)[100] slip (Lafrance et al 1998;Ji et al 2000Ji et al , 2004Stü nitz et al 2003;Gó mez Barreiro et al 2007a;Harigane et al 2008). Slip on (001)[100] has been also identified in naturally deformed rocks at medium-to high-grade conditions (Kruhl 1987;Siegesmund et al 1994;Terry and Heidelbach 2006;Harigane et al 2008). Both slip systems have been observed under TEM (Marshall and McLaren 1977;Olsen and Kohlstedt 1984;Montardi and Mainprice 1987).…”

“…The (010)[001] system has been documented as the principal slip system under amphibolite and granulite facies conditions (Olsen andKohlstedt 1984, 1985;Ague et al 1990;Kruse and Stü nitz 1999;Kruse et al 2001). Slip on (010)[100] has been proposed in high-temperature deformation experiments and natural mylonites (Ji et al 2000(Ji et al , 2004Gó mez Barreiro et al 2007a;Harigane et al 2008). Experiments by Ji et al (2005) suggest a transition from [001] to [100] with increasing temperature, strain rate, and water content.…”

The Godzilla Megamullion, the largest oceanic core complex on the earth: a historical review

“…With the completion of the YK11‐08 cruise, 37 expeditions have been devoted to the study of the Godzilla Megamullion and the Parece Vela Basin to date (as of 2015) (Table ). Studies using the samples from these post‐discovery cruises include Harigane et al (, , , ), Tani et al (), Loocke et al (), Sanfilippo et al (), and Michibayashi et al ().…”

“…After the discovery of the Godzilla Megamullion (Ohara et al ), more than 20 cruises explored the OCC and the Parece Vela Basin (Table ). Important results from these cruises were published in Ohara et al (, ), Harigane et al (, , ), Tani et al (), Loocke et al (), Sanfilippo et al (), Michibayashi et al (), and Spencer and Ohara (). The aim of this article is to provide a sound background for studying the Godzilla Megamullion, summarizing the necessary information that will help accelerate ongoing studies by the ocean lithosphere community.…”

The effect of a hydrous phase on P‐wave velocity anisotropy within a detachment shear zone in the slow‐spreading oceanic crust: A case study from the Godzilla Megamullion, Philippine Sea

“…The Godzilla rock samples underwent similar plastic deformation at temperatures of 650–850 °C, and experienced various degrees of hydrothermal activity during exhumation at the Parece Vela Rift (Harigane et al . , , ; ). More recently, Michibayashi et al .…”