Variations in magmatic and tectonic extension at the Chile Ridge

Abstract:

We measure normal fault characteristics at the intermediate-spreading Chile Ridge to investigate how tectonomagmatic processes vary along and between spreading segments of different lengths and offsets. Multibeam bathymetry and bathymetry gradients are used to locate fault scarps along spreading flowlines and to measure fault displacement and spacing. We estimate the fraction of plate separation taken up by prominent, lithosphere-scale normal faults by summing the horizontal offsets of individual faults along selected across-axis transects and dividing by the total extension. We attribute the remaining fraction of extension, M, primarily to magmatic accretion. We find that M ranges from 0.80 to 0.95, and systematically increases from first- and second-order segment offsets towards segment centers. This is accompanied by a strong anti-correlation of M with depth of the ridge axial valley relative to the axial flanks. No significant correlation between M and segment length or offset distances is found. Further, we find that fault spacing does not correlate with ridge morphology, geometry or M. Thus, the observed increase in tectonic strain toward segment ends is primarily achieved through increasing slip on approximately equally spaced faults, rather than uniform slip on more numerous and closely-spaced faults. Analyses of the seafloor fabric suggest an evolutionary cycle whereby small faults form in the axial valley during periods of diffuse tectonic extension. This phase ends when a few larger faults accumulate enough strain to rapidly link along-axis and transition into axially continuous abyssal hills. Finally, we assess potential correlations between M and previously published geochemical proxies for magma supply. We focus on estimates of the extent of partial melting F, which is expected to increase with mantle melt supply, and the MgO content of seafloor basalts, which is expected to decrease in melt-rich crustal storage zones due to fractional crystallization. Estimates of M, MgO, and F exhibit similar wavelengths of variability (30-50 km) shorter than the length scale of first-order segmentation. This may indicate that the extension not accounted for by faulting, M, is more closely related to the magmatic segmentation of Chile Ridge spreading centers than to first-order tectonic segmentation.