Ductile deformation and mass loss in the Franciscan Subduction Complex: implications for exhumation processes in accretionary wedges
Ductile deformation and mass loss in the Franciscan Subduction Complex: implications for exhumation processes in accretionary wedges
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方济各俯冲复合体中的延性变形和质量损失：对增生楔中折返过程的影响
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发表时间：
1999
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通讯作者：
M. Brandon
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文献类型：
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作者：
U. Ring;M. Brandon
摘要本文利用从方济各杂岩东带三个研究区采集的64个砂岩样品的变形测量结果，评价了方济各楔体高压变质内部是如何被挖掘出来的。压力估计表明，方济各会建筑群的这一部分有25-30公里的挖掘。东部带的大部分地区都有一个半穿透的劈理，这是在岩石移动通过楔形体时由溶液质量转移（SMT）形成的。单个样品具有SX = 1.00-1.52、SY = 0.60-1.21和SZ = 0.33-0.81的绝对主拉伸。应变的大小和方向在局部尺度上变化很大。通过计算测量组的张量平均值来估计区域尺度的变形。这三个研究区域的间距为c。沿方济各会边缘沿着500 km的区域，给出了非常相似的平均值，这表明东部带的变形在区域尺度上是一致的。所有数据的张量平均值表明接近垂直的Z方向，SX = 0.96，SY = 0.92，SZ = 0.70。SX和SY接近1，因为在局部尺度上，X和Y方向在取向上变化很大，这意味着它们的拉伸贡献在区域尺度上被平均。这种不寻常的应变类型，包括平面应变和单轴缩短，结果的事实，即缩短Z平衡的普遍质量损失体积应变，平均约38%。定向纤维过度生长的几何形状被用来测量内部旋转。这些数据表明，在砂岩中，SMT变形几乎是同轴的（在区域尺度上平均运动学涡度数为0.05，而单个样品通常<0.4）。一个简单的一维稳态模型表明，韧性减薄只占c。占总挖掘量的10%。方济各楔的延展性缩短非常缓慢，速率<8 × 10−17 s−1（<0.3% Ma−1）。假设这种应变在宽度小于200 km的横向走向带中活动，我们估计水平韧性流将占整个方济各边缘总会聚的<0.25%。我们的结论是，SMT机制作为一个背景变形过程缓慢运作，位错滑移机制是完全不活跃的深度为25-30公里。因此，方济各楔的稳定性可能更好地定义库仑楔标准比粘性楔标准。在东部带内或附近没有发现确定的正断层。因此，我们推断楔锥的形成主要是由弧前高压的深吸积和侵蚀作用控制的。
Abstract Deformation measurements from 64 sandstone samples collected in three study areas from the Eastern Belt of the Franciscan Complex are used to evaluate how the high-pressure metamorphic interior of the Franciscan wedge was exhumed. Pressure estimates indicate 25–30 km of exhumation in this part of the Franciscan Complex. Much of the Eastern Belt has a semi-penetrative cleavage that formed by solution mass transfer (SMT) while the rocks were moving through the wedge. Individual samples have absolute principal stretches of SX = 1.00–1.52, SY = 0.60–1.21, and SZ = 0.33–0.81. Strain magnitudes and directions are quite variable at the local scale. The deformation at the regional scale is estimated by calculating tensor averages for groups of measurements. The three study areas, which are spaced over a distance of c. 500 km along the Franciscan margin, give remarkably similar averages, which indicates that the deformation of the Eastern Belt is consistent at the regional scale. The tensor average for all data indicates a nearly vertical Z direction with SX = 0.96, SY = 0.92, and SZ = 0.70. SX and SY are near one because at the local scale, the X and Y directions vary considerably in orientation, which means that their stretch contributions are averaged out at the regional scale. This unusual strain type, consisting of both plane strain and uniaxial shortening, results from the fact that shortening in Z was balanced by a pervasive mass-loss volume strain, averaging about 38%. The geometry of directed fibre overgrowths was used to measure internal rotations. These data indicate that in sandstones, SMT deformation was nearly coaxial (mean kinematic vorticity number is 0.05 at the regional scale and generally <0.4 for individual samples). A simple one-dimensional steady-state model indicates that ductile thinning accounted for only c. 10% of the overall exhumation. Ductile shortening across the Franciscan wedge was very slow, at rates <8 × 10−17 s−1 (<0.3% Ma−1). Assuming that this strain was active in an across-strike zone <200 km wide, we estimate that horizontal ductile flow would have accounted for <0.25% of the total convergence across the Franciscan margin. We conclude that the SMT mechanism operated slowly as a background deformation process, and that the dislocation glide mechanism was completely inactive down to depths of 25–30 km. Thus, the stability of the Franciscan wedge was probably better defined by the Coulomb wedge criterion than by a viscous wedge criterion. No definitive normal faults have been found in or adjacent to the Eastern Belt. Therefore, we infer that wedge taper was mainly controlled by deep accretion and erosion of an emergent forearc high.