Shear stresses on megathrusts: Implications for mountain building behind subduction zones

Abstract

[1] Shear stresses t on a subduction megathrust play an important role in determining the forces available for mountain building adjacent to a subduction zone. In this study, the temperatures and shear stresses on megathrusts in 11 subduction zones around the Pacific rim (Hikurangi, Tonga, Izu-Ogasawara, western Nankai, northeastern Japan, Aleutians, western Alaska, Cascadia, northern Chile, southern Chile) and SE Asia (northern Sumatra) have been determined. The main constraint is that vertical normal stresses beneath the highlands behind the subduction zone are nearly equal to horizontal normal stresses, in the plane of a trench-or arc-normal section. For a typical brittle and ductile megathrust rheology, frictional shear stress t = mrgz, for depth z, and ductile shear stress t = A exp (B/RT) at temperature T, where m, A, B are rheological parameters treated as constants. Rheological constants common to all the megathrusts (m crust , m mantle , B) are determined by simultaneously solving for the force balance in the overlying wedge and megathrust thermal structure, using a simplex minimization algorithm, taking account of the induced mantle corner flow at depth (65 ± 15 km (2s)) and constant radiogenic heating (0.65 ± 0.3 mW m À3 (2s)) throughout the crust. The A constants are solved individually for each subduction zone, assuming that the maximum depth of interplate slip earthquakes marks the brittle-ductile transition. The best fit solution shows two groupings of megathrusts, with most subduction zones having a low mean shear stress in the range 7-15 MPa (m crust = 0.032 ± 0.006, m mantle = 0.019 ± 0.004) and unable to support elevations >2.5 km. For a typical frictional sliding coefficient $0.5, the low effective coefficients of friction suggest high pore fluid pressures at $95% lithostatic pressure. Tonga and northern Chile require higher shear stresses with m crust = 0.095 ± 0.024, m mantle = 0.026 ± 0.007, suggesting slightly lower pore fluid pressures, at $81% lithostatic. Ductile shear in the crust is poorly resolved but in the mantle appears to show a strong power law dependency, with B = 36 ± 18 kJ mol À1 . A mantle values are sensitive to the precise value of B but are in the range 1-20 kPa. The power law exponent n for mantle flow is poorly constrained but is likely to be large (n > 4). The brittle-ductile transition in the crust occurs at temperatures in the range 370°C-512°C, usually close to the base of the crust and in the mantle at much lower temperatures (180°C-300°C), possibly reflecting a marked change in pore fluid pressure or quasi ductile and subfrictional properties. In subduction zones where the subducted slab is older than 50 Ma, a significant proportion of the integrated shear force on the megathrust is taken up where it cuts the mantle and temperatures are 300°C. In much younger subduction zones, the stress transmission is confined mainly to the crust. The shear stresses, particularly in the crust, may be kept low by some sort of lubricant such as abundant water-rich trench fill, wh...

Estimates of stress drop and crustal tectonic stress from the 27 February 2010 Maule, Chile, earthquake: Implications for fault strength

“…We can then determine the minimum value of stress in the crust applied by tectonic forces required to both offset the stress exerted by topography and to maintain a stress field orientation consistent with that of the stress released in the earthquake. This idea is similar to previous studies that have estimated tectonic stress by balancing the loading of topography [e.g., Lamb, 2006], but it has the advantage of deriving its estimate from the short-wavelength topography immediately above the ruptured megathrust region. In this way, the estimate more directly assesses the stress in the crust at the seismogenic portion of the subduction zone, rather than relying on stress beneath the high Andes being transmitted several hundred kilometers back to the shallow megathrust region.…”

“…More recently, Tassara [2010] determined that fore-arc topography is spatially correlated with various earthquake parameters and likely plays at least some part in controlling seismogenic behavior. Other studies have used observations of present-day topography to constrain the effective coefficient of friction on subduction thrusts to be less than 0.2 [Cattin et al, 1997] or 0.03 to 0.09 [Lamb, 2006].…”

A high-resolution, time-variable afterslip model for the 2010 Maule Mw = 8.8, Chile megathrust earthquake

“…The displacement gradient is calculated from analytical solutions of dislocations in an elastic halfspace (Okada, 1992) using the same elastic parameters for the halfspace as used in the inversion models. We chose a homogeneous effective coefficient of friction 0.1 (Lamb, 2006). We tested the sensitivity of the calculations to changes in friction coefficient and found no qualitative impact.…”

Continental margin deformation along the Andean subduction zone: Thermo-mechanical models

“…Interplate friction in these contexts is considered as an effective value that includes the "pore 182 pressure" effect of lubricating sediments entering the trench on lowering the "basal" shear stress (e.g. Davis et al 1983; 183 Lamb, 2006).…”