Lithospheric flexure at fracture zones

Abstract

Bathymetric profiles across six major fracture zones (FZ's) in the North Pacific are used to demonstrate the absence of vertical slip on the fossil fault planes. The scarp heights on these FZ's are constant with age and equal to the initial vertical offsets at the ridge‐transform fault‐FZ intersections. Because of the frozen‐in scarp and the differential subsidence of lithosphere far from the FZ, the lithosphere bends in the vicinity of the FZ. This flexure results in a characteristic ridge‐trough topographic FZ signature. The flexural amplitude, which is the difference between the scarp height and the overall change in depth across the FZ, increases with age. Good fits to the bathymetric profiles across the Mendocino and Pioneer FZ's are obtained by modelling the topography as the flexure of a thin elastic plate with an age‐dependent effective elastic thickness. Results of the modelling indicate that the base of the elastic lithosphere is approximately defined by the 450°C isotherm. Maximum bending stresses at FZ's are on the order of 100 MPa, substantially less than the stresses encountered at subduction zones. Because the Mendocino and Pioneer FZ's are separated by less than a flexural wavelength, they are elastically coupled.