Morphology and tectonics of the Romanche Transform and its environs

Abstract

Abstract.We describe the results of GLORIA surveys of the Romanche Transform and neighbouring areas of the Equatorial Atlantic Ocean. These surveys have imaged the whole of the Romanche Transform Fault, the eastern end of the Saint Paul Transform, the spreading centre joining them, and the northern part of the spreading centre between Romanche and Chain transforms. They have also imaged representative parts of the off-axis seafloor structure in these areas, and have facilitated preparation of a new bathymetric map, which is presented here. Contrary to previous descriptions of the area, we have found that there is 'normal' spreading fabric (volcano-tectonic lineaments trending NNW-SSE orthogonal to the spreading direction) between these major equatorial fracture zones. This implies that normal seafloor spreading processes have operated there. The Romanche Transform Domain, within which normal spreading fabric is either absent or heavily modified, and transform-related structures predominate, is about 100 km wide. There are multiple transform faults within Saint Paul Fracture Zone, of which we have imaged the active traces of the two southernmost ones. These transforms are joined by short but otherwise normal spreading segments. The Romanche Transform is shorter than previously thought, with a length of 840 km. This is equivalent to an age offset of about 50 Ma. The transform has a complex system of short normal and oblique spreading centres at its eastern intersection, forming an overall oblique section of ridge which may have recently cut off the corner of an earlier, simple, orthogonal ridge-transform intersection. The recent complex system appears to have developed at about 1.5 Ma. An oblique valley is mapped south of the western end of Romanche Transform, and is thought to be the fracture zone trace of an old transform offset between Romanche and Chain transforms. This transform disappeared at about chron 8 (26.9 Ma), as its trace is not seen between 20°W and the spreading centre.

Locations of mid‐oceanic earthquakes constrained by seafloor bathymetry

“…The RFZ consists of complex zones of alternating ridges and troughs [Gorini, 1981;Thomas et al, 1985;Searle et al, 1994] and includes two inactive limbs of ridge crests at the west and east ends, linking the RFZ to the St. Paul and Chain transforms, respectively. The bathymetry features mostly E-W trending transverse valleys as deep as 8 km below sea level.…”

“…If the plate boundary is a great circle, the plates move either toward or away from each other and in the ocean the boundary is the ridges. However, ridges may display oblique spreading (i.e., not normal to ridge axis) [Searle et al, 1994]. Anomalous plate motions and deeper mantle dynamics may cause deviations from ideal linear topography features on the seafloor [Bergman and Solomon, 1984;Macdonald et al, 1991;Wolfe et al, 1993].…”

“…Bonatti [1978] noted factors such as horizontal thermal conduction across a transform valley; viscodynamic forces operating in the ridge -transform intersections; compressional and tensional horizontal stresses imposed on the fracture zones by changing plate motions [Wolfe et al, 1993]; and random reheating or excess volcanism during the evolution of the lithosphere that can create secondary bathymetry signatures and seismic events, some of which might be large. Recent bathymetric mapping, side scan imaging, and other observations reveal complex geometries and structures within the transform domain [Fox and Gordon, 1984;Searle et al, 1994;Smith and Sandwell, 1997].…”

“…We associate and relocate them to the appropriate bathymetry features based on their focal mechanisms. We first use the 2 min global bathymetry map [Smith and Sandwell, 1997] and sometimes higher resolution regional bathymetry map [Searle et al, 1994] to sample and digitize the ridges or transform faults. We assume that plate boundary is at the deepest bathymetric lows along slow-spreading ridge axes [Bonatti, 1978;Solomon et al, 1988;Bergman and Solomon, 1990;Small, 1994] and at the bathymetry highs along fast spreading ridges (axial ridges [Small, 1994]).…”

“…The brittle lithosphere fails along linear boundaries and generates earthquakes as well as linear topography. With the improvement in resolution and accuracy of the bathymetry [Searle et al, 1994;Smith and Sandwell, 1997], as well as gravity anomalies, it becomes possible to better constrain event locations with the related mid-oceanic ridges/transforms.…”