Depleted lithosphere, cold, trapped asthenosphere, and frozen melt puddles above the flat slab in central Chile and Argentina
Depleted lithosphere, cold, trapped asthenosphere, and frozen melt puddles above the flat slab in central Chile and Argentina
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DOI：
10.1016/j.epsl.2006.02.014
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发表时间：
2006-05
影响因子：
5.3
通讯作者：
L. Wagner;S. Beck;G. Zandt;M. Ducea
中科院分区：
文献类型：
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作者：
L. Wagner;S. Beck;G. Zandt;M. Ducea
最近对智利中部和阿根廷平坦板块上方上地幔的研究表明，该地区的地震速度结构与俯冲带中具有“正常”、较陡板块几何形状的地震速度结构非常不同。水平剖面上的地幔具有低纵波速度、高横波速度和低Vp/Vs比值的特征。当板块开始向南过渡到一个更正常的倾角时，它上面的地幔也发生了变化。在这个“过渡带”之上，地幔的特征是高P波速度、高S波速度和通常高的Vp/Vs比值。在这个高速异常之上，在过渡带的南倾板片和正常俯冲带的东倾板片之间的拐角处，存在一个小的浅异常，其P波和S波速度非常低，Vp/Vs比值中等偏高。我们解释这些异常代表非常耗尽冷岩石圈，冷却困软流圈，冻结池熔体，分别。最近的研究表明，低Vp、高Vs和低Vp/Vs的特征表明岩石圈为超高压岩石圈。这表明，在平板之上的物质可能是古劳伦特岩石圈，或其他熔融贫化的干燥岩石圈物质。冻结的软流圈的存在下，南部的平板可能表明，大量的软流圈被取代提前向东推进的扁平板之间的10和2 Ma，迫使流量模式的变化，从沟槽正常SSW周围的东南角的扁平板。随着变平过程的进行，软流层被困在过渡带的上方。最后，温度下降到足以冻结最后剩下的口袋提取熔体已收集在下降板的角落。
Recent studies of the upper mantle above the flat slab in central Chile and Argentina indicate the seismic velocity structures in this area are very different from those found in subduction zones with “normal,” steeper slab geometries. The mantle above the horizontal section of the flat slab is characterized by low P-wave velocities, high S-wave velocities, and low Vp/Vsratios. As the slab begins to transition to a more normal dip to the south, the mantle above it changes as well. Above this “transition zone”, the mantle is characterized by high P-wave velocities, high S-wave velocities, and generally high Vp/Vsratios. Above this high velocity anomaly, in the corner between the south-dipping slab of the transition zone and the east-dipping slab of the normal subduction zone, lies a small shallow anomaly distinguished by its very low P- and S-wave velocities and its moderately high Vp/Vsratio. We interpret these anomalies to represent very depleted cold lithosphere, cooled trapped asthenosphere, and frozen pooled melt, respectively. Recent research looking at the elastic properties of cratonic xenoliths indicate that a low Vp, high Vs, and low Vp/Vssignature is indicative of cratonic lithosphere. This suggests that the material above the flat slab may be ancient Laurentian lithosphere, or other melt-depleted, dry lithospheric material. The presence of frozen asthenosphere to the south of the flat slab may indicate that large volumes of asthenosphere were displaced in advance of the eastward progression of the flattening slab between 10 and 2Ma, forcing a change in flow patterns from trench normal to SSW around the south eastern corner of the flattening slab. As flattening progressed this asthenosphere became trapped above the transition zone. Eventually, temperatures dropped sufficiently to freeze the last remaining pocket of extracted melt which had collected in the corner of the descending slab.