Double seismic zone of the Nazca plate in northern Chile: High-resolution velocity structure, petrological implications, and thermomechanical modeling
Double seismic zone of the Nazca plate in northern Chile: High-resolution velocity structure, petrological implications, and thermomechanical modeling
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DOI：
10.1029/2008gc002020
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发表时间：
2008-07-08
影响因子：
3.5
通讯作者：
Guiraud, Michel
中科院分区：
文献类型：
--
作者：
Dorbath, Catherine;Gerbault, Muriel;Guiraud, Michel
本文介绍了智利北部双地震带的跨学科研究。首先，用TomoDD双差层析成像方法获得了俯冲纳斯卡板块的高分辨率速度结构。双地震带深度在80~140公里之间，两个地震面相距20公里。在此基础上，利用现有的热-岩石学-地震学模型推导了与该岩石圈有关的大洋岩石圈的化学和岩石学特征，并与数值热力学模型提供的压力-温度条件进行了比较。我们的结果与普遍的假设一致，即上下两个平面的地震活动都与变质脱水反应相关的流体释放有关。在位于上地壳内部的地震上平面，这些反应将影响玄武岩(MORB)组成的物质，并记录在高P(>；2.4 Gpa)和低T(130 Km)、劳钠闪石-角闪岩榴辉岩条件下发生的不同变质反应。位于大洋地幔的下层可能与蛇纹岩的脱水反应有关。两个平面之间区域的Vp和VS特征与部分(类似于25-30体积%的闪锌矿，类似于0-10%的水镁石，类似于4-10体积%的绿泥石)水化的方辉橄榄岩相一致。差异持续存在，我们将其归因于不同结构组成所固有的复杂性。虽然各种地球物理指标表明纳斯卡板块下降和大陆前弧都特别寒冷，但热力学模型表明，这两个地震面都在400摄氏度(+/-50摄氏度)等温线附近划定了板块内的挤压带。据预测，较低平面的地震将发生在板块弯曲的中性面，从周围的变质反应中释放的流体可能会在那里聚集并触发地震活动。从下面的张力带向上迁移的流体在上升过程中可能会被该平面上方的挤压带阻挡，从而产生一层自由流体，或蛇纹岩层。因此，地震既可能呈现下倾挤压特征，也可能呈现下倾拉伸特征。数值试验表明，板的热结构并不是控制板内受压的唯一因素。(1)弱韧性俯冲通道和(2)冷地幔前弧都有利于板块内挤压，促进了大陆岩石圈向板块内的压应力传递。(3)板的曲率半径减小会使板内受压深度变宽，而(4)上板收敛程度的减小会降低板内受压的强度。所有这些因素表明，如果开发区真的等高板内板压缩，它们不仅与板的弯曲有关，而且还与高压应力从上板向板的传递有关。
This paper presents an interdisciplinary study of the northern Chile double seismic zone. First, a high-resolution velocity structure of the subducting Nazca plate has been obtained by the tomoDD doubledifference tomography method. The double seismic zone (DSZ) is observed between 80 and 140 km depth, and the two seismic planes is 20 km apart. Then, the chemical and petrologic characteristics of the oceanic lithosphere associated with this DSZ are deduced by using current thermal-petrological-seismological models and are compared to pressure-temperature conditions provided by a numerical thermomechanical model. Our results agree with the common hypothesis that seismicity in both upper and lower planes is related to fluid releases associated with metamorphic dehydration reactions. In the seismic upper plane located within the upper crust, these reactions would affect material of basaltic (MORB) composition and document different metamorphic reactions occurring within high-P (>2.4 GPa) and low-T (130 km), lawsonite-amphibole eclogite conditions. The lower plane lying in the oceanic mantle can be associated with serpentinite dehydration reactions. The Vp and Vs characteristics of the region in between both planes are consistent with a partially (similar to 25-30 vol % antigorite, similar to 0-10% vol % brucite, and similar to 4-10 vol % chlorite) hydrated harzburgitic material. Discrepancies persist that we attribute to complexities inherent to heterogeneous structural compositions. While various geophysical indicators evidence particularly cold conditions in both the descending Nazca plate and the continental fore arc, thermomechanical models indicate that both seismic planes delimit the inner slab compressional zone around the 400 degrees C (+/-50 degrees C) isotherm. Lower plane earthquakes are predicted to occur in the slab's flexural neutral plane, where fluids released from surrounding metamorphic reactions could accumulate and trigger seismicity. Fluids migrating upward from the tensile zone below could be blocked in their ascension by the compressive zone above this plane, thus producing a sheeted layer of free fluids, or a serpentinized layer. Therefore earthquakes may present either downdip compression and downdip tensile characteristics. Numerical tests indicate that the slab's thermal structure is not the only factor that controls the occurrence of inner slab compression. (1) A weak ductile subduction channel and (2) a cold mantle fore arc both favor inner slab compression by facilitating transmission of compressional stresses from the continental lithosphere into the slab. (3) Decreasing the radius of curvature of the slab broadens the depth of inner slab compression, whereas (4) decreasing upper plate convergence diminishes its intensity. All these factors indicate that if DSZs indeed contour inner slab compression, they cannot be linked only to slab unbending, but also to the transmission of high compressional stresses from the upper plate into the slab.