Volcanic-Tectonic Structure of the Mount Dent Oceanic Core Complex in the Ultraslow Mid-Cayman Spreading Center Determined From Detailed Seafloor Investigation.

G A Haughton,B J Murton,T Le Bas,N W Hayman,R C Searle

doi:10.1029/2018gc008032

Abstract

The flanks of the ultraslow‐spreading Mid‐Cayman Spreading Center (MCSC) are characterized by domal massifs or oceanic core complexes (OCCs). The most prominent of these, Mount Dent, comprises lower‐crustal and upper‐mantle lithologies and hosts the Von Damm vent field ~12 km west of the axial deep. Here, presented autonomous underwater vehicle‐derived swath sonar (multibeam) mapping and deep‐towed side‐scan sonar imagery lead to our interpretation that: (i) slip along the OCC‐bounding detachment fault is ceasing, (ii) the termination zone, where detachment fault meets the hanging wall, is disintegrating, (iii) the domed surface of the OCC is cut by steep north‐south extensional faulting, and (iv) the breakaway zone is cut by outward facing faults. The Von Damm vent field and dispersed pockmarks on the OCC's south flank further suggest that hydrothermal fluid flow is pervasive within the faulted OCC. On the axial floor of the MCSC, bright acoustic backscatter and multibeam bathymetry reveal: (v) a volcanic detachment hanging wall, (vi) a major fault rifting the southern flank of Mount Dent, and (vii) a young axial volcanic ridge intersecting its northern flank. These observations are described by a conceptual model wherein detachment faulting and OCC exhumation are ceasing during an increase in magmatic intrusion, brittle deformation, and hydrothermal circulation within the OCC. Together, this high‐resolution view of the MCSC provides an instructive example of how OCCs, formed within an overall melt‐starved ultraslow spreading center, can undergo magmatism, hydrothermal activity, and faulting in much the same way as expected in magmatically more robust slow‐spreading centers elsewhere.

Highlights

Mid‐ocean ridges accommodate seafloor spreading via a combination of magmatic and tectonic processes (Cann, 1968; Macdonald & Luyendyk, 1977; Mutter & Karson, 1992; Shaw & Lin, 1993; Smith & Cann, 1990; Sykes, 1967)
Where the magmatic component of seafloor spreading is low and tectonic extension is high, the oceanic basement may be characterized by large‐offset detachment faults that dip shallowly at the surface, yet accommodate significant seafloor spreading resulting in the exhumation of lower‐crustal and upper‐mantle rocks at the seafloor to form oceanic core complexes (OCCs; Cann et al, 1997; Cannat, 1993; Cannat et al, 2006; Escartin et al, 2008; Ildefonse et al, 2007; Karson & Dick, 1983; Schouten et al, 2010; Tucholke & Lin, 1994; Tucholke et al, 1998, 2008)
Based on shipboard and autonomous underwater vehicle (AUV)‐derived multibeam bathymetry, deep‐towed side‐scan sonar imagery (30 kHz TOBI), and near‐bottom video surveying and sampling, we can divide the Mid‐Cayman Spreading Center (MCSC) into three distinct segments (Figures 1b and 2a): (1) a northern segment containing circular volcanoes and a ridge of hummocky lavas that extends into the nodal deep basin marking the intersection with the Oriente Fracture Zone, (2) a central segment dominated by the Mount Dent massif, and (3) a southern segment comprising, from north to south, several smooth floored basins, divided by a number of prominent NW‐SE trending morphological ridges and a field of hummocky and sheet flow lavas, respectively

Summary

Introduction

Mid‐ocean ridges accommodate seafloor spreading via a combination of magmatic and tectonic processes (Cann, 1968; Macdonald & Luyendyk, 1977; Mutter & Karson, 1992; Shaw & Lin, 1993; Smith & Cann, 1990; Sykes, 1967). Observations along the slow‐spreading Mid‐Atlantic Ridge (MAR), combined with geodynamic modeling, suggest that OCCs evolve via a “rolling hinge,” wherein the OCC detachment fault initiates at a higher angle and as a result of flexure and exhumation of the lower crust and/or upper mantle, is back‐tilted to emerge as a domal footwall (deMartin et al, 2007; Garces & Gee, 2007; Lavier et al, 1999; Morris et al, 2009). Following this exhumation, at some point OCCs are rendered inactive and are passively transported off axis. Could such magmatic controls on OCC development be important in ultraslow spreading centers that are thought to be generally magma poor (e.g., Dick et al, 2003)? what roles might hydrothermal activity play in OCC evolution via mechanical linkages with faulting (e.g., Hirose & Hayman, 2008) and cooling of magmatic bodies within OCCs (e.g., Canales et al, 2017)?

Methods

Results

Discussion

Conclusion