Abstract

In extensional geologic systems such as mid-ocean ridges, deformation is typically accommodated by slip on normal faults, where material is pulled apart under tension and stress is released by rupture during earthquakes and magmatic accretion. However, at slowly spreading mid-ocean ridges where the tectonic plates move apart at rates <80 km m.y.–1, these normal faults may roll over to form long-lived, low-angled detachments that exhume mantle rocks and form corrugated domes on the seabed. Here we present the results of a local micro-earthquake study over an active detachment at 13°20′N on the Mid-Atlantic Ridge to show that these features can give rise to reverse-faulting earthquakes in response to plate bending. During a 6 month survey period, we observed a remarkably high rate of seismic activity, with >244,000 events detected along 25 km of the ridge axis, to depths of ∼10 km below seafloor. Surprisingly, the majority of these were reverse-faulting events. Restricted to depths of 3–7 km below seafloor, these reverse events delineate a band of intense compressional seismicity located adjacent to a zone of deeper extensional events. This deformation pattern is consistent with flexural models of plate bending during lithospheric accretion. Our results indicate that the lower portion of the detachment footwall experiences compressive stresses and deforms internally as the fault rolls over to low angles before emerging at the seafloor. These compressive stresses trigger reverse faulting even though the detachment itself is an extensional system.

INTRODUCTION

Oceanic lithosphere is formed at mid-ocean ridges by a combination of magmatism and normal faulting, driven by far-field forces arising from processes including plate subduction and mantle convection (Lachenbruch, 1976). In these extensional settings, a portion of the strain is expected to be accommodated by slip on normal faults, which is reflected in the focal mechanisms of earthquakes observed near the spreading axis (Sykes, 1967). At slow-spreading ridges, accounting for large parts of the lithosphere accreted in the Atlantic, Indian, and Arctic Oceans, young lithosphere may be deformed by slip along large-offset normal faults called detachments (Cann et al., 1997; Tucholke et al., 1998; Dick et al., 2003; Escartín et al., 2008b). Detachment faults can exhume lower crustal gabbros and serpentinized mantle peridotites at the seabed and form kilometer-scale dome-shaped features called oceanic core complexes (Cann et al., 1997; MacLeod et al., 2002; Escartín et al., 2003; Grimes et al., 2008). The mechanical behavior of detachment faults is controversial because the domed fault surfaces emerge from the seafloor at low angles that are incompatible with the physics of extensional faulting (Buck et al., 2005). There is evidence for fault initiation on a steeply dipping, deeply penetrating rupture surface (MacLeod et al., 2009, 2011; Morris et al., 2009), but the mechanism by which the fault rolls over to low angles prior to seafloor exhumation is poorly understood. Local earthquake surveys with ocean bottom seismographs (OBSs) have the potential to address this issue; however, previous OBS deployments at oceanic detachments had insufficient aperture and instrument density to resolve the mechanics of fault rollover (deMartin et al., 2007; Collins et al., 2012; Grevemeyer et al., 2013). Intriguingly, a few reverse-faulting events were observed beneath the Logatchev core complex on the Mid-Atlantic Ridge at 14°40′N, but the relationship of these events to the extensional fault system was unclear, and they were attributed to volume expansion from serpentinization or magma-chamber filling rather than deformation on a detachment fault (Grevemeyer et al., 2013).

MICRO-EARTHQUAKE EXPERIMENT

In 2014, we conducted the largest micro-earthquake experiment to date at a slow-spreading ridge. A dense network of 25 short-period OBSs (instrument spacing of 2–3 km) was deployed for a 6 month period along ∼10 km of the ridge axis at 13°N on the Mid-Atlantic Ridge (Fig. 1). Detachment faults are common in this region, including two well-surveyed and sampled oceanic core complexes located at 13°20′N and 13°30′N (Smith et al., 2006; MacLeod et al., 2009; Mallows and Searle, 2012; Escartín et al., 2017). Both core complexes have well-developed, domed, corrugated surfaces and are accompanied by a high level of hydroacoustically recorded seismicity, suggesting that they are currently active or have been in the recent geological past (Smith et al., 2008; MacLeod et al., 2009; Mallows and Searle, 2012).

We recorded >244,000 events on more than three stations during the 198-day deployment, yielding a mean rate of ∼1240 micro-earthquakes per day (see Methods in the GSA Data Repository1), two orders of magnitude greater than that observed at the Logatchev core complex (Grevemeyer et al., 2013). This remarkably high rate of seismicity was fairly constant throughout the deployment period (Fig. 2B). There was no evidence for foreshock–main shock sequences, except for a small seismic swarm in the western band of events at Julian day 280 within a region extending 3 km south from the northern tip of the band. The locations and focal mechanisms of these events are indistinguishable from the rest of the seismicity in this area. Events have small local magnitude (ML), ranging between −1.0 and 2.7 and with a modal average of 0.3 (Fig. DR1 in the Data Repository). The high number of earthquakes, combined with the dense seismic network, allowed us to estimate hypocenters and focal mechanism solutions for a subset of 18,313 well-characterized events (see the Data Repository). These reveal that reverse faulting was the most common mode of deformation near the 13°20′N detachment during our deployment (Fig. 1). The compressional events define a distinct arc of intense seismicity that wraps around the detachment trace (on the eastern edge of the corrugated surface), at depths of 3–7 km beneath the seabed (Fig. 2; Fig. DR2). The slip directions (i.e., rake) of these events are typical of reverse and reverse-oblique faults. The compression (P) axes are dominantly subhorizontal, but there is no preferred orientation for the dip and strike of the fault planes (Fig. 2C). Events within the reverse-faulting band of seismicity have slightly smaller magnitudes than those in the normal-faulting band (Fig. DR1). In contrast, normal faulting is restricted to a narrow band of seismicity ∼3 km east of the reverse-faulting zone, at depths of 7–12 km beneath the seabed (Fig. 2; Fig. DR2). These event depths, which are comparable to the depth of normal-faulting seismicity observed at the Trans-Atlantic Geotraverse (TAG) detachment (deMartin et al., 2007), clearly show that extensional faulting extends beneath the crust. Focal mechanisms indicate steeply dipping (50°–70°) normal faults oriented subparallel to the near–north-south–trending spreading axis (Mallows and Searle, 2012) (Fig. 2C), and tension (T) axes indicating consistent extensional stress oriented parallel to the spreading direction (∼273°).

DISCUSSION

Our observations indicate that lithospheric extension at the 13°20′N detachment generates both compressional and extensional seismicity contemporaneously. The band of intense reverse faulting at 3–7 km depth is located directly beneath the hanging-wall cutoff, where the gently dipping corrugated surface emerges on the seafloor (Fig. 2E), and hence cannot lie on the detachment fault plane itself. Instead, this reverse faulting must be occurring within the detachment footwall. An active high-temperature vent field is located on the 13°20′N corrugated surface (Escartín et al., 2017), which could indicate footwall emplacement of magma bodies (Fig. 1); however, the vent site is located 2.3 km west of the band of reverse faulting (Fig. 2E), and cooling of a magma body should generate tensile, rather than compressive, stresses. Thermal contraction associated with heat extraction from a footwall magma body is therefore not a plausible source mechanism for the shallow band of compressive seismicity. Our observations instead support a model in which internal deformation of the lithosphere in response to flexural bending stress results in a high level of seismicity at the point of maximum bending (Lavier et al., 1999; Buck et al., 2005). The variability in the strike and dip of fault-plane solutions (P- and T-axes) in this zone indicates distributed, isotropic deformation of the deeper, internal portion of the detachment footwall (Fig. 2C). In contrast, toward the center of the axial valley and at greater depth (6–10 km), steep, ridge-parallel normal faulting accommodates extensional deformation on the active detachment as new material accretes into the footwall. The short distance between bands of reverse and normal faulting (∼2 km perpendicular to the fault plane) requires a rapid change in the footwall stress field over a short length-scale, from extensional stresses in the accretion zone to compressive stresses in the region of fault rollover. This observation, combined with the spatially restricted zone of reverse faulting, indicates that fault rollover may be a relatively abrupt, rather than gradual, process, with tightening of curvature at progressively shallower subsurface depths.

We have developed a simple model based upon the deflection of a bending plate with elastic-plastic rheology to reconcile our observations (see the Data Repository; McAdoo et al., 1978). The model is constrained by the location and dip of the corrugated fault surface at the seafloor, the spatial distribution and focal mechanisms of observed earthquakes, and a lithospheric slab thickness of 6 km inferred from the depth distribution of seismicity. We use the distribution of earthquakes in the footwall to define a stress profile, with “plastic” failure at depths where seismic events are observed (in elastic-plastic models, deformation from earthquakes is treated as bulk “plastic” yielding), and assume that the initiating fault is likely to have a maximum dip of ∼70°. We seek a bending profile that satisfies these constraints by varying the mechanical strength of the plate in terms of its flexural rigidity, or effective elastic thickness (Te). We find that a best fit is obtained if Te increases linearly from 0.7 km near the spreading axis to 0.9 km at the point where the footwall emerges at the seafloor (dashed line, Fig. 2E). This range in Te, which is a modeling parameter rather than a physical property of the lithosphere, is consistent with previous estimates from bathymetric profiles of detachment-faulted terrain (Schouten et al., 2010). Our simple model demonstrates that the location of the reverse faulting is consistent with that predicted by bending of the detachment footwall under a mechanically reasonable deflection profile (Fig. 3). Reverse faulting is not observed at the TAG detachment (deMartin et al., 2007), which is younger and less mature than the 13°20′N detachment. This lack of reverse faulting could be explained by a young system not having a sufficiently well-developed dome to produce compression in the footwall; however, without further observations at other young detachments, this suggestion remains speculative.

The two bands of seismicity show well-defined along-axis extents, the northern ends of which lie within the OBS network and are therefore well resolved. The extent of the western, compressional band roughly matches that of the corrugated surface between 13°15′ and 13°21′N, north of which the neovolcanic zone begins to develop (Fig. 1). The band of normal faulting extends ∼3 km further north to 13°23′N, beyond which the seismicity rate is remarkably low. These results demonstrate that the nature of seismically accommodated deformation changes significantly at the northern limit of the 13°20′N core complex, but the inability of our network to provide focal mechanism solutions in this area makes it difficult to interpret this change in the context of fault structure and deformation. A swarm of 276 events occurred over a 3–4 day period at 13°27′N, just south of the 13°30′N core complex, which is suggestive of magmatic activity; this interpretation, however, is necessarily tentative because we cannot obtain focal mechanism estimates from this area.

The apparent lack of seismicity on the upper surface of the detachment footwall at shallow crustal depths is enigmatic. Extensional bending stresses are clearly high in this region, and there must be slip between the footwall and hanging wall on the fault surface. Rock samples recovered from the 13°20′N detachment fault scarp are dominated by hydrothermal quartz-cemented basalt breccia, in addition to sheared serpentinites, talc schists, incohesive cataclasites, and hydrothermal deposits (MacLeod et al., 2009; Escartín et al., 2014, 2017). This assemblage provides evidence for significant hydrothermal alteration and mineralization in the fault zone, which may modify the rheology of these rocks and preclude the generation of detectable seismicity (Reinen et al., 1992; Escartín et al., 2008a). Alternatively, we cannot rule out the possibility that deformation in this zone occurs episodically over time intervals that are long compared to the duration of our observations.

CONCLUSIONS

We find that accretion and extension of oceanic crust at the 13°20′N detachment is accommodated by two distinct modes of deformation, reflecting two stages of lithospheric evolution. First, extensional faulting occurs at the point where the detachment fault initiates at depths of 6–10 km. These high-angle normal faults accommodate the far-field forces that drive plate separation as lower crustal and upper mantle rocks are incorporated into newly forming lithosphere in the rising fault footwall. Second, as the footwall rotates to lower angles, bending stresses lead to internal compression in the lower half of the plate. As a result, reverse faults initiate within the bending lithosphere at depths of 3–7 km below where the footwall emerges at the seafloor to form a domed, corrugated fault surface (Fig. 3). This evolution of footwall stress is consistent with kinematic models for detachment fault behavior (Buck, 1988) and with direct observations for reverse faulting in detachment fault footwalls (Pressling et al., 2012), suggesting that reverse faulting may be ubiquitous in mature, active oceanic detachments. Our results provide a new framework for interpreting detachment seismicity and suggest that reverse-faulting events reported at other core complexes may have been triggered by bending stresses rather than volume expansion (e.g., serpentinization). The mechanical regime we describe shows that plate bending associated with the exhumation and formation of oceanic core complexes can generate compressional stresses leading to reverse faulting, despite being situated in an extensional stress regime.

ACKNOWLEDGMENTS

This research was funded by Natural Environment Research Council (NERC) grants NE/J02029X/1, NE/J021741/1, and NE/J022551/1, and by U.S. National Science Foundation grant OCE-1458084. Instruments were provided by the NERC UK Ocean-Bottom Instrumentation Facility (Minshull et al., 2005). We are grateful to the officers, crew, technicians, and science parties of RRS James Cook cruises JC102, JC109, and JC132 for their hard work and professionalism. We thank V. Schlindwein and five anonymous reviewers for their constructive comments.

1GSA Data Repository item 2017309, additional information and figures describing the acquisition and processing of the microearthquake data set, and details of the elastic-plastic model, is available online at http://www.geosociety.org/datarepository/2017/ or on request from editing@geosociety.org.
Gold Open Access: This paper is published under the terms of the CC-BY license.