Oceanic corrugated surfaces and the strength of the axial lithosphere at slow spreading ridges

Abstract

We analyse the topography and gravity signature of 39 corrugated surfaces formed over the past 26 myrs in the footwall of axial detachment faults at the eastern Southwest Indian Ridge. These corrugated surfaces appear to have formed at a melt supply about half the global melt supply average for mid-ocean ridges, and we find that their presently elevated topography, relative to adjacent non-corrugated seafloor, was mostly acquired at the end of their formation, at the “termination stage”. This configuration, which also characterizes many off-axis corrugated surfaces in other oceans, suggests that the plate flexural rigidity was very low during the formation of the corrugated surface, and increased significantly at the termination stage. Following Buck (1988), we hypothesize that stresses related to bending of the plate cause internal deformation and damage in the footwall of the fault, which is associated with weakening. As a possible mechanism for enhanced footwall weakening while corrugated surfaces form, we propose the formation of weak shear zones coated with hydrous minerals such as talc, amphibole, chlorite and serpentine, in mantle-derived ultramafics next to gabbro intrusions. If this hypothesis is correct, the amount of footwall weakening and roll-over along axial detachment faults at slow spreading ridges may be controlled both by access to hydrothermal fluids in the footwall of the detachment, and by the abundance and distribution of gabbros intrusions in exhumed ultramafics.

Introduction

Oceanic corrugated surfaces are domal features characterized by spreading-parallel undulations of the seafloor. They were first discovered in the Atlantic (Cann et al., 1997), and interpreted as the exposed inactive footwall of large offset axial normal faults, also called detachment faults (Cann et al., 1997, Tucholke et al., 1998). Corrugated surfaces have since been dredged, cored and sampled with submersibles at a number of near-axis and off-axis locations, yielding variably deformed rock suites, including gabbros, serpentinized peridotites, and lesser volumes of basalt (MacLeod et al., 2002, Karson et al., 2006, Ildefonse et al., 2007, Dick et al., 2008). Deformed samples from the corrugated fault zone typically display low temperature to greenschist facies syntectonic minerals, and fabrics are dominantly brittle, with intervals of plastically deformed talc, amphibole, chlorite and serpentine (Escartin et al., 2003, Schroeder and John, 2004). Higher grade sheared gabbro and peridotite mylonites have also been sampled below exposed detachment surfaces (Cannat et al., 1991, Dick et al., 2000, Schroeder and John, 2004). The presence of gabbros confirms that, in spite of a thin crust inferred from seismic and gravity-data and suggesting reduced melt supply (Blackman et al., 1998, Canales et al., 2004), corrugated surfaces form during magmatically active stages of ridge spreading (Buck et al., 2005, Ildefonse et al., 2007).

Recent observations at 13°N in the Atlantic have shown corrugated domes originating in the footwall of active axial normal faults, with volcanic ridges in the hanging wall (Smith et al., 2006, Smith et al., 2008). Assuming fault dips of 40° or more at depth (e.g., (deMartin et al., 2007)), roll-over to dips less than 5° occurs within 5 km of footwall emergence (Smith et al., 2006), indicating a very low footwall flexural rigidity (Smith et al., 2006, Smith et al., 2008).

Corrugated surfaces have now been identified near most intermediate to ultraslow spreading ridges, predominantly, but not exclusively (Escartín and Cannat, 1999, Smith et al., 2008), in ridge-transform inside-corner settings. Their domal shape has been reproduced in numerical models by flexure of the footwall of large offset normal faults (Lavier et al., 1999). Numerical models of symmetrical spreading predict that such faults are most likely to form when about 50% of total extension is accommodated by magmatic accretion in the hanging wall (Buck et al., 2005, Tucholke et al., 2008). Corrugations, however, are not always present in the footwall of large offset axial normal faults. Near-axis domal structures devoid of corrugations, such as at the Trans Atlantic Geotraverse (TAG) (deMartin et al., 2007), are also interpreted as the footwall of detachments. It has been proposed that axial detachments, forming corrugated surfaces or not, are active along nearly 50% of the length of slow-spreading ridges (Escartin et al., 2008b). This is consistent with the estimated surface proportion of ultramafic and gabbroic seafloor exposures in the Atlantic (~ 25% ;(Cannat et al., 1995)), because these exposures form asymmetrically about the axis in the exhumed footwall of these detachments.

In this paper, we present a detailed description of the corrugated surfaces identified in ultraslow-spreading seafloor of the Southwest Indian Ridge (SWIR), east of the Melville Fracture Zone (Fig. 1). Our study area covers 630 km of ridge axis, and extends to ages of 28 myrs on both plates. Seismic data suggest that this eastern part of the SWIR has a very low average melt supply, equivalent to a 3 to 4 km-thick magmatic layer (Muller et al., 1999, Minshull et al., 2006). This is about half of the average melt supply estimated for the global mid-ocean ridge system (Chen, 1992). Corrugated surfaces occur throughout the mapped area (Fig. 1), although they cover only about 4% of the total surface (Cannat et al., 2006). This region of the SWIR also displays large expanses (~ 40% of the mapped area; Fig. 1) of non-corrugated «smooth seafloor», which have been interpreted as mostly mantle-derived ultramafic rocks exposed in the footwall of axial detachments (Cannat et al., 2006). The rest of the seafloor (“volcanic seafloor” in Fig. 1) displays volcanic cones, and spreading-perpendicular scarps. Many of these scarps have steep outward-facing slopes and may represent tectonically-rotated volcanic surfaces, as proposed by (Smith et al., 2008) for similar scarps formed by axial detachments in the 14°N area of the Mid-Atlantic Ridge (MAR). The prevalence of detachments in accretion processes at the eastern SWIR is thus probably greater than at the MAR (Cannat et al., 2006).

Previous work in the eastern SWIR has shown that smooth seafloor forms at minimal melt supply, and that the seafloor opposite the corrugated surfaces and across the ridge axis (conjugate seafloor) is primarily volcanic (Cannat et al., 2006). The eastern SWIR therefore has two assets for a study of how corrugated surfaces form: (1) it contains a large number of these surfaces in a relatively well defined spreading and regional melt supply context; and (2) some of these corrugated surfaces transition into seafloor that was formed at a minimal melt supply, with little to no volcanism. This is not the case in the Atlantic, where corrugated surfaces are surrounded by seafloor with morphological evidence for a volcanic carapace (e.g. (Smith et al., 2006, Smith et al., 2008)). Based primarily on the eastern SWIR data, we specifically discuss the topography and the conditions of termination of oceanic corrugated surfaces. We then propose a conceptual model in which apparent variations of the rigidity of the axial plate during and after the formation of corrugated surfaces, are explained by variable degrees of mechanical weakening of the footwall of axial detachment faults.