Oceanic corrugated surfaces and the strength of the axial lithosphere at slow spreading ridges
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.
Section snippets
Size, age and map distribution of eastern SWIR corrugated surfaces
Bathymetric data used for this study were acquired with a Thomson TMS 5265B multibeam system, and have a horizontal resolution of 1 to 2% of seafloor depth. At depths of 4000 m and more, these data therefore do not properly image seafloor structures less than 100 m across. They are sufficient, however, to map the corrugated surfaces, and to derive some basic characteristics of their transition to non-corrugated seafloor.
Individual corrugated surfaces in the eastern SWIR cover areas up to 880 km2 (
Gravity and topographic signature of eastern SWIR corrugated surfaces
Processing of the gravity data and calculation of residual topography for the study area are presented in (Cannat et al., 2006). Modelling of both residual seafloor depths and gravity anomalies involves a correction for plate cooling with age, which is most pronounced and probably more inaccurate, in near-axis regions. Because the near axis region in Fig. 1 also comprises few corrugated surfaces, we have limited our study to the gravity and topographic signature of crust formed prior to
Transition of eastern SWIR corrugated surfaces to adjacent volcanic and smooth seafloor
Corrugated surfaces typically form at the transition between volcanic domains and smooth seafloor domains. Fig. 1 shows that only seven corrugated surfaces (number 22, 24-28 and 39) occur within wide expanses of volcanic seafloor, and none within a wide expanse of smooth seafloor. This is consistent with the intermediate gravity signature of corrugated-volcanic conjugate pairs (Fig. 3a), which suggests that they formed at intermediate melt supply, in the low regional melt supply context of the
Fault topography and the rigidity of the axial plate
Dome-shaped corrugated surfaces that form presently at 13°N in the Mid Atlantic Ridge show evidence for a large rotation of the detachment fault footwall (at least 35°) over remarkably short distances (less than 5 km; Smith et al., 2006). This requires a very low flexural rigidity (equivalent to an elastic thickness < 1 km; (Smith et al., 2006, Smith et al., 2008)). Very low rigidity is also indicated by the subdued topography of some off-axis corrugated surfaces in our SWIR study area, relative
Concluding remarks
In this paper, we document the location, size, approximate age, gravity signature and residual topography of 39 corrugated surfaces in the melt-poor eastern region of the SWIR. We find that these corrugated surfaces formed at a melt supply about half the global melt supply average for mid-ocean ridges (Chen, 1992). This is consistent with the model of (Buck et al., 2005), which was recently refined by (Tucholke et al., 2008). In this model, which does not address asymmetric spreading during the
Acknowledgments
We warmly thank our three reviewers for their extremely helpfull comments. Most data we used are from the «SWIR 61–63» cruise of RV Marion Dufresne (2003), and processing was partly supported by a research grant from CNRS-INSU. This is IPGP publication number 2545.
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