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

Abstract

We studied the contributions of plagioclase, clinopyroxene, and amphibole to the P‐wave velocity properties of gabbroic mylonites of the Godzilla Megamullion (site KH07‐02‐D18) in the Parece Vela Rift of the central Parece Vela Basin, Philippine Sea, based on their crystal‐preferred orientations (CPOs), mineral modes, and elastic constants and densities of single crystals. The gabbroic mylonites have been classified into three types based on their microstructures and temperature conditions: HT1, HT2 and medium‐temperature (MT) mylonites. The P‐wave velocity properties of the HT1 mylonite are dominantly influenced by plagioclase CPOs. Secondary amphibole occurred after deformation in the HT1 mylonite, so that its effect on P‐wave velocity anisotropy is minimal due to weak CPOs. Although the HT2 mylonite developed deformation microstructures in the three minerals, the P‐wave velocity properties of the HT2 mylonite are essentially isotropic, resulting from the destructive interference of different P‐wave velocity anisotropy patterns produced by the distinct CPOs of the three constituent minerals (i.e., plagioclase, clinopyroxene, and amphibole). The P‐wave velocity properties of the MT mylonite are influenced mainly by amphibole CPOs, whereas the effect of plagioclase CPOs on P‐wave velocity anisotropy becomes very small with a decrease in the intensity of plagioclase CPOs. As a result, the gabbroic mylonites tend to have weak P‐wave velocity anisotropy in seismic velocity, although their constituent minerals show distinct CPOs. Such weakness in the whole‐rock P‐wave velocity anisotropy could result from the destructive contributions of the different mineral CPOs with respect to the structural framework (foliation and lineation). These results show that amphibole has a high potential for P‐wave velocity anisotropy by aligning both crystallographically and dimensionally during deformation in the hydrous oceanic crust. The results also suggest that the effect of a hydrous phase on P‐wave velocity anisotropy within the detachment shear zone in a slow‐spreading oceanic crust varies depending on the degree of deformation and on the timing of hydrothermal activity.

Crustal Accretion in a Slow Spreading Back‐Arc Basin: Insights From the Mado Megamullion Oceanic Core Complex in the Shikoku Basin

“…Oxide gabbros only present amphibolite‐facies mylonitic recrystallization of clinopyroxene porphyroclasts. Granulite‐facies mylonitic shear zones are widespread at modern OCCs—Godzilla Megamullion (Harigane et al, 2008, 2010, 2011, 2019; Michibayashi et al, 2016), Mid‐Atlantic Ridge (Blackman et al, 2006, 2011; Coogan et al, 2001; Dick et al, 2008; Hansen et al, 2013; Sanfilippo et al, 2018; Schroeder & John, 2004), and Southwest Indian Ridge (Dick et al, 2000; Dick, Kvassnes, et al, 2019; Dick, MacLeod, et al, 2019; MacLeod et al, 2017; Mehl & Hirth, 2008; Miranda & John, 2010)—and in other fossil examples of OCC such as in the Alpine ophiolites (e.g., Sanfilippo & Tribuzio, 2011; Tribuzio et al, 2020). They were interpreted as the early expression of a growing detachment fault and formed at temperatures as high as 1,000°C (e.g., Mehl & Hirth, 2008; Sanfilippo et al, 2018).…”

“…Extensive multibeam bathymetric mapping by Japan's continental shelf survey revealed the potential presence of OCCs in the Shikoku and Parece Vela Basins (Ohara et al, 2015). Accordingly, the Godzilla Megamullion was documented in 2001 in the Parece Vela Basin (~16°N; Harigane et al, 2008, 2010, 2011, 2019; Loocke et al, 2013; Michibayashi et al, 2014, 2016; Ohara, 2016; Ohara et al, 2001, 2003, 2011; Ohara & Snow, 2009; Sanfilippo et al, 2013, 2016; Spencer & Ohara, 2014; Tani et al, 2011). It is the largest OCC structure in the world (55 km along‐segment, 125 km ridge‐perpendicular) and presents topographic corrugations indicative of long‐lived detachment faulting and heterogeneous lithologies varying from exposed mantle peridotites to massive gabbroic bodies, diabase, and basalts (e.g., Ohara, 2016).…”

Upper Mantle Anisotropic Shear Velocity Structure at the Equatorial Mid‐Atlantic Ridge Constrained by Rayleigh Wave Group Velocity Analysis From the PI‐LAB Experiment

“…The shear strain could either have occurred in the past, with anisotropy currently frozen‐in to the lithosphere (fossil anisotropy) or the strain and associated anisotropy could be currently accumulating, reflecting present day absolute plate motion (APM). The strength of anisotropy owing to olivine LPO beneath the oceanic plate varies depending on the physical conditions at the bottom of the cooling plate and a spreading rate (Michibayashi et al., 2016). However, particularly near ridges melt generation and segregation processes (Holtzman & Kendall, 2010; Phipps Morgan et al., 1987) may also play an important role for controlling the seismic anisotropy through shape preferred orientation (SPO)of melt aligned cracks.…”

ゴジラメガムリオン掘削から明らかにする背弧海盆海洋下部地殻と上部マントルの組成と構造

“…成したデタッチメント断層に伴う流体の関与が 重要であることが示された (Harigane et al, 2008(Harigane et al, , 2010(Harigane et al, , 2011a(Harigane et al, , 2019 2) (Harigane et al, 2008(Harigane et al, , 2010(Harigane et al, , 2011a(Harigane et al, , 2019 (Harigane et al, 2011a, b;Tani et al, 2011;Loocke et al, 2013;Sanfilippo et al, 2013Sanfilippo et al, , 2016Michibayashi et al, 2014Michibayashi and Snow, 2016;Ohara, 2016Ohara, , 2017) Synthesis of oceanic crustal structure from twodimensional seismic profiles. Reviews of Geophysics,…”