Causes and consequences of variations in faulting style at the Mid‐Atlantic Ridge

Abstract

Both volcanism and faulting contribute to the rugged topography that is created at the Mid‐Atlantic Ridge (MAR) and preserved off‐axis in Atlantic abyssal hill terrain. Distinguishing volcanic from fault‐generated topography is essential to understanding the variations in these processes and how these variations are affected by the three‐dimensional pattern of mantle upwelling, ridge segmentation, and offsets. Here we describe a new quantitative method for identifying fault‐generated topography in swath bathymetry data by measuring topographic curvature. The curvature method can distinguish large normal faults from volcanic features, whereas slope methods cannot because both faults and volcanic 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 MAR between 28° and 29°30′N. The fault styles mapped in this way are strongly correlated with their location within the ridge segmentation framework: long, linear, small‐throw faults occur toward segment centers, while shorter, larger‐throw, curved faults occur toward ends; these variations reflect those of active faults within the axial valley. We investigate two different physical mechanisms that could affect fault interactions and thus underlie variations in abyssal hill topography at the MAR. In the first model only one fault is active at a time on each side of the rift valley. Each fault grows while migrating away from the volcanic center due to dike injection; extension across the fault causes a flexural rotation of nearby inactive faults. The amount of stress necessary to displace the fault increases as the fault grows. When reaching a critical size the fault stops growing as fault activity jumps inward as a new fault starts its growth near the rift 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 faults (e.g., 10° reached 20 km from the active fault). Fault spacing 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 considers multiple active faults; each fault relieves stresses as it grows and inhibits the growth of nearby faults, causing a characteristic fault spacing. Such fault interactions would occur in a region of necking instability involving deformation over an extended area. This mode of extension would drive a feedback mechanism that would act to regulate the size of nearby faults. We hypothesize that this mechanism may be active in the relatively weak regions of strong mantle upwelling near segment midpoints, causing the homogeneous abyssal hill fabric in these regions.

The geological record of oceanic crustal accretion and tectonism at slow-spreading ridges

“…Such a direct relation was shown by Shaw (1992) and Shaw and Lin (1993). They observed that large-throw, widely-spaced faults interpreted from multibeam bathymetry occur near segment ends where residual mantle Bouguer anomaly (RMBA) highs suggest thin crust.…”

“…However, the mechanism that transforms the steep relief of the inward-facing rift-valley wall into subsiding ridge-flank abyssal hills is poorly understood. Hypotheses include systematic development of normal faults dipping away from the ridge axis, reverse faulting, and/or progressive backtilting of fault blocks along existing inward-facing faults (e. g., Needham and Francheteau, 1974;Harrison and Stieltjes, 1977;Macdonald and Luyendyk, 1977;Macdonald and Atwater, 1978;Shaw and Lin, 1993). Although recent microseismicity studies suggest that faulting can occur up to 15 km off-axis (Wolfe et al, 1995), it is unclear how far off-axis faulting contributes to the formation of ridge topography.…”

Topographic controls on dike injection in volcanic rift zones

“…Temporal and spatial variations in the rate of magma accretion are frequently invoked to explain differences in faulting and axial morphology at slow-spreading ridges [29,35,36]. Regions with deep rift valleys are interpreted to reflect lower rates of magma injection, while shallower and more distributed rift morphology is taken to imply enhanced magmatism [13,37].…”

“…Near the segment center, the rift valley is typically shallow, formed by closely spaced faults with relatively small throw [6,35,44].…”

Megamullions and mullion structure defining oceanic metamorphic core complexes on the Mid‐Atlantic Ridge

“…Theoretical models [e.g., Sparks et al, 1993] and observational geophysics [Kuo and Forsyth, 1988;Lin et al, 1990; Tolstoy et al, 1993] indicate that the least magmatic portions of slow spreading ridge segments are at segment ends where the ocean crust tends to be thin, the lithosphere is thick, and brittle deformation and fault throw are maximized [Shaw and Lin, 1993;Escartfn et al, 1997a]. Crustal morphology, gravity, and seafloor sampling also show that there is a consistent, strong asymmetry between inside-corner (IC) and outside-corner (OC) tectonic settings across the rift axis near segment ends [Tucholke and Lin, 1994].…”