Deep‐tow studies of the Vema Fracture Zone: 1. Tectonics of a major slow slipping transform fault and its intersection with the Mid‐Atlantic Ridge
Authors: Macdonald Ken. C. , Castillo David A. , Miller Stephen P. , Fox Paul J. , Kastens Kim A. , Bonatti Enrico 
Published: February 2008

The Vema transform fault, which slips at a rate of 24 mm/yr, displaces the Mid‐Atlantic Ridge (MAR) 320 km in a left‐lateral sense. High‐resolution deep‐tow studies of the Vema ridge‐transform intersection (RTI) and the eastern 130 km of the active transform fault reveal a complex pattern of dip‐slip and strike‐slip faults which evolve in time and space. At the intersection, both the neovolcanic zone and the west wall of the MAR rift valley curve counterclockwise toward the transform fault along trends approximately 30° oblique to the regional north‐south trend of the spreading axis. The curving of extensional structures in the rift valley, such as normal faults and the axial zone of dike injection, appears to be related to transmission of transform related shear stresses into the spreading center domain. Intermittent locking of the American and African lithospheric plates across the RTI causes shear stresses to penetrate up to 4 km into the MAR axial neovolcanic zone where the lithosphere is relatively thin and up to 12 km into the block‐faulted west wall of the rift valley where the lithosphere is thicker. The degree of shear coupling across the RTI may vary with time due to changes in the thickness of the lithosphere along the axis (0–10 km), the strength of a “mantle weld” at depth, and the presence or absence of an axial magma chamber, so that extensional structures at the RTI may be either spreading center parallel when coupling is weak or oblique when coupling is strong. Oblique extension across the RTI in addition to other factors may account for some of the down dropping of lithosphere within the deep nodal basin. The easternmost 20 km of the active transform fault zone near the RTI displays a braided network of three to nine tectonically active grabens and V‐shaped furrows in a zone 2–4 km wide, interpreted to consist of interwoven Riedel shears, P shears, and oblique normal faults. Clay cake deformation experiments and deep‐tow observations suggest that P shears and R shears, which are 10°–20° oblique to the transform slip direction, develop during the initial stages of transform faulting near the RTI as the newly accreted lithosphere accelerates to full plate velocity. Some of the R shears propagate along strike and intercept the oblique normal faults resulting in sharply curving scarps at the RTI. Subsequent to this merging of the two fault types, some of the R shears develop a significant component of dip slip, while other R shears merge with P shears creating a complex anastomosing fault pattern up to 4 km wide. A continuous strand within this braided pattern of faults is interpreted to be the principal transform displacement zone near the RTI. Twenty kilometers west of the RTI the active transform fault zone narrows to a furrow generally less than 100 m wide with only a few short discontinuous splays. This narrow groove cuts through thinly sedimented basalt 20–40 km west of the RTI and continues as a narrow furrow (less than 100 m wide) through up to 1.5 km of layered turbidite fill most of the way to the western RTI. Such a narrow zone of deformation typifies the mature stages of transform faulting where the lithosphere on both sides of the transform fault is relatively old, thick, and rigid and has completed its acceleration to full plate velocity. The transform fault zone is closely associated with a partially buried median ridge and widens to 1–2 km where it transects exposed portions of the ridge. The transform parallel median and transverse ridges create the highest topography associated with the transform fault and may be serpentinized ultramafic intrusions capped by displaced crustal blocks of gabbro, metagabbro, and basalt.