Shear stresses on megathrusts: Implications for mountain building behind subduction zones
Shear stresses on megathrusts: Implications for mountain building behind subduction zones
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DOI：
10.1029/2005jb003916
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发表时间：
2006-07-04
影响因子：
3.9
通讯作者：
Lamb, Simon
中科院分区：
文献类型：
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作者：
Lamb, Simon
[ 1]俯冲巨型逆冲断层上的剪应力τ在确定俯冲带附近造山作用的作用力方面起着重要作用。本文测定了环太平洋地区（Hikurangi，Tonga，Izu-Ogasawara，Western Nankai，Northeast Japan，Aleutians，Western Alaska，Cascadia，北方智利，Southern Chile）和东南亚（北方苏门答腊）11个俯冲带中巨型逆冲断层的温度和剪切应力。主要的限制因素是俯冲带后面高地下面的垂直正应力几乎等于水平正应力，在海沟或弧法向剖面的平面内。对于典型的脆性和韧性巨型逆冲断层流变学，对于深度z，摩擦剪切应力τ = μ rho gz，在温度T下，韧性剪切应力τ = A exp（B/RT），其中μ、A、B是被视为常数的流变学参数。所有巨型逆冲断层共有的流变常数（m地壳，m地幔，B）是通过使用单纯形最小化算法同时求解上覆楔和巨型逆冲断层热结构中的力平衡来确定的，考虑到在深度处诱发的地幔角流（65 +/- 15 km（2 sigma））和恒定的辐射加热（0.65 +/- 0.3 mu W m（-3）（2 sigma））。假设板间滑动地震的最大深度标志着脆韧性转变，每个俯冲带单独求解A常数。最佳拟合解显示了两组巨型逆冲断层，其中大多数俯冲带的平均剪应力较低，范围为7 - 15 MPa（mu（地壳）= 0.032 +/- 0.006，mu（地幔）= 0.019 +/- 0.004），并且无法支持海拔> 2.5 km。对于一个典型的摩擦滑动系数类似于0.5，低的有效摩擦系数表明高孔隙流体压力在类似于95%的岩石静压力。汤加和北方智利需要较高的剪切应力，mu（地壳）= 0.095 +/- 0.024，mu（地幔）= 0.026 +/- 0.007，表明孔隙流体压力略低，接近81%的岩石静力。地壳中的韧性剪切很难分辨，但在地幔中似乎表现出很强的幂律依赖性，B = 36 +/- 18 kJ mol（-1）。A（地幔）值对B的精确值敏感，但在1 - 20 kPa范围内。地幔流的幂律指数n约束很差，但可能很大（n > 4）。地壳的脆-韧性转变发生在370摄氏度至512摄氏度的温度范围内，通常接近地壳底部，而地幔的温度则低得多（180摄氏度至300摄氏度），这可能反映了孔隙流体压力或准韧性和亚摩擦性质的显著变化。在俯冲带中，俯冲板片的年龄超过50 Ma，大逆冲断层上的综合剪切力的很大一部分是在它切割地幔的地方产生的，在高安第斯山脉的温度为4 km。然而，在沉积物匮乏和润滑不良的俯冲带中，地壳较薄，如汤加，平均剪切应力仍将较低。沉积物可以润滑容纳陆壳下冲作用的巨型逆冲断层，如喜马拉雅山或安第斯山脉中东部，这些断层的平均剪应力很低，约为15 MPa。
[ 1] Shear stresses tau 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 tau = mu rho gz, for depth z, and ductile shear stress tau = A exp (B/RT) at temperature T, where mu, 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 (2 sigma)) and constant radiogenic heating (0.65 +/- 0.3 mu W m(-3) (2 sigma)) 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 (mu(crust) = 0.032 +/- 0.006, mu(mantle) = 0.019 +/- 0.004) and unable to support elevations > 2.5 km. For a typical frictional sliding coefficient similar to 0.5, the low effective coefficients of friction suggest high pore fluid pressures at similar to 95% lithostatic pressure. Tonga and northern Chile require higher shear stresses with mu(crust) = 0.095 +/- 0.024, mu(mantle) = 0.026 +/- 0.007, suggesting slightly lower pore fluid pressures, at similar to 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(-1). 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 degrees C - 512 degrees C, usually close to the base of the crust and in the mantle at much lower temperatures ( 180 degrees C - 300 degrees 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 4 km in the high Andes.However, where the crust i thin in sediment-starved and poorly lubricated subduction zones, such as Tonga, the mean shear stress will still be low. Sediment may lubricate megathrusts accommodating underthrusting of continental crust, such as in the Himalayas or eastern central Andes, which have a low mean shear stress similar to 15 MPa.