CAUSES AND CONSEQUENCES OF VARIATIONS IN FAULTING STYLE AT THE MID-ATLANTIC RIDGE

Both volcanism and faulting contribute to the rugged topography that i s created at the Mid-Atlantic Ridge (MAR) and preserved off-axis in At lantic abyssal hill terrain. Distinguishing volcanic from fault-genera ted topography is essential to understanding the variations in these p rocesses and how these variations are affected by the three_dimensiona l pattern of mantle upwelling, ridge segmentation, and offsets. Here w e describe a new quantitative method for identifying fault-generated t opography in swath bathymetry data by measuring topographic curvature. The curvature method can distinguish large normal faults from volcani c features, whereas slope methods cannot because both faults and volca nic constructs can produce steep slopes. The combination of curvature and slope information allows inward and outward facing fault faces to be mapped. We apply the method to Sea Beam data collected along the MA R between 28-degrees and 29-degrees-30'N. The fault styles mapped in t his way are strongly correlated with their location within the ridge s egmentation framework; long, linear, small-throw faults occur toward s egment centers, while shorter, larger-throw, curved faults occur towar d ends; these variations reflect those of active faults within the axi al valley. We investigate two different physical mechanisms that could affect fault interactions and thus underlie variations in abyssal hil l topography at the MAR. In the first model only one fault is active a t a time on each side of the rift valley. Each fault grows while migra ting away from the volcanic center due to dike injection; extension ac ross the fault causes a flexural rotation of nearby inactive faults. T he amount of stress necessary to displace the fault increases as the f ault grows. When reaching a critical size the fault stops growing as f ault activity jumps inward as a new fault starts its growth near the r ift valley. This model yields a realistic terracelike morphology from the rift valley floor into the rift mountains; the relief is caused by the net rotation accumulated in the lithosphere from the active fault s (e.g., 10-degrees reached 20 km from the active fault). Fault spacin g is controlled by lithospheric thickness, fault angle, and the ratio of amagmatic to magmatic extension. We hypothesize that this mechanism may be dominant toward ridge segment offsets. An alternative model co nsiders multiple active faults; each fault relieves stresses as it gro ws and inhibits the growth of nearby faults, causing a characteristic fault spacing. Such fault interactions would occur in a region of neck ing instability involving deformation over an extended area. This mode of extension would drive a feedback mechanism that would act to regul ate the size of nearby faults. We hypothesize that this mechanism may be active in the relatively weak regions of strong mantle upwelling ne ar segment midpoints, causing the homogeneous abyssal hill fabric in t hese regions.