Diffuse Oceanic Plate Boundaries: Strain Rates, Vertically Averaged Rheology, and Comparisons With Narrow Plate Boundaries and Stable Plate Interiors

Diffuse plate boundaries occur in both oceanic and continental lithosphere and cover 15% of Earth's solid surface. The fastest plate speeds accommodated across diffuse oceanic plate boundaries are 15 mm/yr. The smallest strain rates averaged across narrow plate boundaries are at least 10 2 times larger than the largest strain rates across diffuse oceanic plate boundaries and at least 10 4 times larger than those across stable plate interiors, The effective viscosity (η eff ) of the lithosphere is estimated from the ratio of vertically averaged shear stresses to strain rates for three tectonic settings: (i) oceanic transform fault zones, for which η eff =3×10 16 to 5×10 19 Pa s, comparable to estimates for the asthenosphere. (ii) diffuse oceanic plate boundaries, for which η eff = 1×10 23 to 6×10 23 Pa S, 10 times larger than for diffuse continental plate boundaries, and (iii) stable plate interiors, for which η eff = 1×10 24 to 2×10 27 Pa s. The rheology of oceanic lithosphere over times longer than earthquake cycles is modeled as a plastic layer overlying a layer that deforms by creeping flow [Martinod and Davy, 1992]. Oceanic lithosphere deforms when the yield strength of the upper lithosphere is exceeded. The vertically averaged rheology of deforming oceanic lithosphere can be approximated by a power-law fluid for which e s (τ s ) n where e s is the rate of shear strain and τ s is the shear stress. If the ratio of the yield strength of the upper lithosphere to the force required to deform the lower lithosphere at a strain rate of 10 -16 s -1 is varied from 10 -2 to 10 2 , the calculated value of n varies from =3 to 300. The map-view aspect ratio of a deforming zone in a thin sheet of power-law fluid is proportional to n -1/2 [England et al., 1985]. A profile of displacement versus distance inferred from a seismic profile across the Central Indian Basin (India-Capricorn diffuse oceanic plate boundary), where the lithosphere is about 60-Myr old, indicates that the lithosphere is being deformed in response to a rigid moving boundary near the north end of the basin. If so, the vertically averaged rheology of the lithosphere in the Central Indian Basin is described by a power-law fluid with n8, which suggests that the yield strength of the upper lithosphere is a few times larger than the force required to deform the lower lithosphere. Owing to a lack of data, other diffuse oceanic plate boundaries can at best be interpreted qualitatively. Whereas the total aspect ratios (total along-strike length / total across-strike width) for diffuse oceanic plate boundaries in 60- to 90-Myr-old lithosphere are 1.25-1.8, they are 0.73-0.82 in young lithosphere adjacent to an active mid-ocean ridge, The latter aspect ratios suggest smaller values of n, indicating a relatively weaker brittle upper lithosphere relative to the force required to deform the lower lithosphere, similar to results for diffuse continental plate boundaries, for which n3 [England and Molnar, 1991]. In contrast, the East Africa Rift has a large aspect ratio, 2.3, which suggests its lithosphere behaves more like 60- to 90-Myr-old oceanic lithosphere than like other diffuse continental plate boundaries.