https://doi.org/10.1130/G48420.1 Manuscript received 23 April 2020 Revised manuscript received 22 September 2020 Manuscript accepted 22 September 2020 © 2020 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 4 November 2020 Seismicity trends and detachment fault structure at 13°N, Mid-Atlantic Ridge R. Parnell-Turner1, R.A. Sohn2, C. Peirce3, T.J. Reston4, C.J. MacLeod5, R.C. Searle3 and N.M. Simão3 1 Scripps Institution of Oceanography, University of California, San Diego, California 92037, USA 2 Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA 3 Department of Earth Sciences, Durham University, Durham DH1 3LE, UK 4 School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham B15 2TT, UK 5 School of Earth and Ocean Sciences, Cardiff University, Cardiff CF10 3AT, UK ABSTRACT Escartín et al., 2017; Parnell-Turner et al., 2017; At slow-spreading ridges, plate separation is commonly partly accommodated by slip on Peirce et al., 2019, 2020; Searle et al., 2019; long-lived detachment faults, exposing upper mantle and lower crustal rocks on the seafloor. Simão et al., 2020). The presence of two closely However, the mechanics of this process, the subsurface structure, and the interaction of these spaced OCCs led to the conflicting hypotheses faults remain largely unknown. We report the results of a network of 56 ocean-bottom seismo- that they might represent either the exposed graphs (OBSs), deployed in 2016 at the Mid-Atlantic Ridge near 13°N, that provided dense part of a single, more extensive undulating de- spatial coverage of two adjacent detachment faults and the intervening ridge axis. Although tachment (e.g., Smith et al., 2008), or two me- both detachments exhibited high levels of seismicity, they are separated by an ∼8-km-wide chanically distinct, locally controlled structures aseismic zone, indicating that they are mechanically decoupled. A linear band of seismic (Smith et al., 2008; MacLeod et al., 2009). The activity, possibly indicating magmatism, crosscuts the 13°30′N domed detachment surface, first micro-earthquake survey was an approxi- confirming previous evidence for fault abandonment. Farther south, where the 2016 OBS mately 6 month experiment from April to Oc- network spatially overlapped with a similar survey done in 2014, significant changes in the tober 2014, with 25 short-period ocean-bottom patterns of seismicity between these surveys are observed. These changes suggest that oce- seismographs (OBSs) deployed along ∼10 km anic detachments undergo previously unobserved cycles of stress accumulation and release of the ridge axis, which yielded new insight into as plate spreading is accommodated. the internal deformation of the fault footwall (Parnell-Turner et al., 2017). The second sur- INTRODUCTION expanses of seafloor (Cannat et al., 2006; Sauter vey, conducted 15 months later in early 2016, At spreading ridges with a low or variable et al., 2013; Reston, 2018). was a shorter, ∼11 day, experiment employing magma supply, faulting is commonly heteroge- Here we present the results of a local earth- a network of 56 OBSs distributed along ∼30 km neous, giving rise to a variety of deformation quake survey conducted in 2016 at the 13°N of the ridge axis, including both the 13°20′N styles, including long-lived detachment faults segment of the Mid-Atlantic Ridge that encom- and 13°30′N OCCs. Stations were arranged in (Cannat et al., 1995; Blackman et al., 1998; Es- passes two detachments at different stages of a grid with 2–5 km inter-element spacing and cartín et al., 2003; Ildefonse et al., 2007; Ma- the faulting life cycle. The observed seismic- an aperture covering the domes and footwalls of cLeod et al., 2009). Recognition of this detach- ity patterns provide new insight into the me- both OCCs and the adjacent neovolcanic zone ment mode of spreading is considered to be chanical evolution of OCCs and their along-axis (Fig. 1A). Although the duration was shorter one of the most important recent advances in structure. Our 2016 experiment is located in the (limited by the gaps in an active-source survey plate tectonics (Mutter and Karson, 1992; Can- area of a similar survey undertaken in 2014. The shot into the OBSs), the high seismicity rate nat et al., 1995; Cann et al., 1997; Dick et al., combined results of the two surveys allow us to (23 events per day per kilometer of ridge axis; 2003; Escartín and Canales, 2011; Reston and assess temporal variations in detachment fault Parnell-Turner et al., 2017) and larger footprint McDermott, 2011). We now know that detach- seismicity for the first time. of the second survey allowed the identification ment faults initiate at steep angles (∼70°) at of primary fault structures associated with the depths ≥∼10 km, rotate to low angles (∼15°) in APPROACH two OCCs and the intervening portion of the the shallower crust, and can slip for several mil- We conducted repeat micro-earthquake sur- ridge axis. lion years (Cann et al., 1997; Dick et al., 2003; veys over and between the 13°20′N and 13°30′N deMartin et al., 2007; Smith et al., 2008; Morris OCCs at the Mid-Atlantic Ridge, chosen because RESULTS et al., 2009). These faults can bring lower crustal these OCCs have been extensively mapped, im- During the 2016 experiment, we detected and upper mantle rocks to the surface in domes aged, and sampled over the past decade (Smith 21,332 events on four or more OBSs using a known as oceanic core complexes (OCCs) or et al., 2008; MacLeod et al., 2009; Mallows and standard triggering algorithm, giving an event generate gently undulating peridotite-dominated Searle, 2012; Craig and Parnell-Turner, 2017; rate of >82 per hour. Of these events, 5511 could CITATION: Parnell-Turner, R., et al., 2021, Seismicity trends and detachment fault structure at 13°N, Mid-Atlantic Ridge: Geology, v. 49, p. 320–324, https://doi.org/10.1130/G48420.1 320 www.gsapubs.org | Volume 49 | Number 3 | GEOLOGY | Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/3/320/5236661/320.pdf by guest on 17 March 2021 of ∼6 km heading south from the 13°20′N de- ABtachment (Fig. 2D), suggesting the fault surface deepens where it encounters thicker or cooler lithosphere. This interpretation is tentative due to reduced hypocentral resolution in this region, which is beyond the network aperture. High lev- els of persistent seismicity along the basal por- tion of the detachment surface have also been observed at the Trans-Atlantic Geotraverse (TAG) detachment on the Mid-Atlantic Ridge at 26°N (deMartin et al., 2007), suggesting that this type of activity may be common to active oceanic detachment faults. Between the 13°30′N and 13°20′N OCCs, there is an ∼8 km zone (from 13°22′N to 13°25′N) that was effectively aseismic during both the 2014 and 2016 surveys (Fig. 1). This aseismic zone is much longer than the lateral uncertainties in the hypocenter estimates, and it is located near the center of the 2016 OBS net- work, where detectability bias is negligible. We thus find that the 13°30′N and 13°20′N OCCs are separated by an ∼8 km length of ridge axis that did not experience significant seismic de- formation during either observation interval. The 2016 micro-earthquake survey imaged a linear band of micro-earthquakes that cuts the 13°30′N OCC dome on a trend of ∼355° and at a depth of ∼6–7 km bsf. Focal mecha- nism estimates are not available for this band of micro-earthquakes due to network geometry, but remotely operated vehicle (ROV) surveys of the 13°30′N dome surface have shown that it is disrupted by normal faulting, fissuring, and mass Figure 1. Bathymetry (Searle et al., 2019) and seismicity near 13°20′N, Mid-Atlantic Ridge. (A) wasting (Escartín et al., 2017). These observa- Inset shows study site (red box) and plate boundaries (black lines). Black dots are relocated tions suggest that the 13°30′N OCC is being micro-earthquakes recorded by ocean-bottom seismographs (OBSs) (triangles) over ∼11 days dissected by a new fault surface. The band of in 2016; red line is neovolcanic zone (NVZ; Parnell-Turner et al., 2017); red stars are hydrother- seismicity extends to a set of linear volcanic mal vents. Locations of oceanic core complexes (OCCs) are shown by 13°20′N and 13°30′N labels; cross size is average 68% confidence level in horizontal location uncertainty (0.9 km). ridges and a seamount south of the dome that are (B) Same area as A, with brown dots indicating micro-earthquakes recorded over 198 days in known to have been recently magmatically ac- 2014 (squares are OBSs; Parnell-Turner et al., 2017). tive (Mallows and Searle, 2012; Escartín et al., 2017; Searle et al., 2019) and that generated a be reliably located using P- and S-wave arrival Seismic moment and local magnitudes were es- swarm of 276 events over ∼3 days during the times and a velocity model derived from the timated using displacement spectra (2–40 Hz) 2014 survey. The new fault surface dissecting active-source experiment (Baillard et al., 2014; recorded by the vertical OBS channel, yielding the OCC may, therefore, be associated with Peirce et al., 2019; Simão et al., 2020). The a magnitude of completeness, MLC, = 0.7 (Fig. magmatic processes, including possibly lateral methods used here, including the velocity mod- S1 in the Supplemental Material). dike propagation either into or out of the OCC el, are the same as those used for the 2014 ex- The 13°30′N and 13°20′N OCCs have high interior (Mallows and Searle, 2012).