Shear heating reconciles thermal models with the metamorphic rock record of subduction
Shear heating reconciles thermal models with the metamorphic rock record of subduction
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DOI：
10.1073/pnas.1809962115
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发表时间：
2018-10
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影响因子：
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通讯作者：
M. Kohn;Adrian E. Castro;Buchanan C. Kerswell;C. Ranero;F. Spear
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文献类型：
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作者：
M. Kohn;Adrian E. Castro;Buchanan C. Kerswell;C. Ranero;F. Spear
意义热结构控制俯冲带变质、流变和熔融的许多方面。许多热模型假设摩擦系数很小或可以忽略不计，并将俯冲带变质岩记录的压力-温度（P-T）条件低估数百摄氏度。在热模型中加入剪切加热，可以同时再现地表热流和折返变质岩的P-T条件。干热的岩石比湿冷的岩石密度更大，因此来自超高温俯冲系统的岩石密度更大，更难通过浮力挖掘出来。因此，变质记录可能低估了热-年轻俯冲作用，而高估了老-冷俯冲作用。目前俯冲带的一些普遍引用的热-力学模型表明，在30-80公里深度处的温度比从折返的变质岩中通过热压测定确定的压力-温度条件低100-500 °C。准确推断俯冲带热结构，无论是从模型还是岩石，对于预测变质反应和相关的流体释放、弧下熔融条件、流变学和断层滑动现象至关重要。在这里，我们汇编了来自全球俯冲带的地表热流数据，并表明数值高于通常假设用于建模的无摩擦俯冲界面所能解释的数值。一个额外的热源-可能剪切加热-需要解释这些电弧前热流值。摩擦系数至少为0.03，在某些情况下可能高达0.1，可以解释这些数据，我们建议使用0.05 ± 0.015的临时平均值进行建模。即使是很小的摩擦系数也能在30-80公里深处产生几百度的加热。将这种剪切应力加入热模型中，定量地再现了折返变质岩记录的压力-温度条件。相对较高的温度通常会促使岩石脱水和致密化，因此，在给定的深度，较热的岩石比较冷的岩石更致密，并且更难通过浮力机制挖掘出来。因此，与以前的建议相反，挖出的变质岩可能过度代表了古老的冷俯冲，其中板片界面处的岩石比超热俯冲带中的岩石更潮湿，浮力更大。
Significance Thermal structure controls numerous aspects of subduction zone metamorphism, rheology, and melting. Many thermal models assume small or negligible coefficients of friction and underpredict pressure–temperature (P–T) conditions recorded by subduction zone metamorphic rocks by hundreds of degrees Celsius. Adding shear heating to thermal models simultaneously reproduces surface heat flow and the P–T conditions of exhumed metamorphic rocks. Hot dry rocks are denser than cold wet rocks, so rocks from young-hot subduction systems are denser and harder to exhume through buoyancy. Thus, the metamorphic record may underrepresent hot-young subduction and overrepresent old-cold subduction. Some commonly referenced thermal-mechanical models of current subduction zones imply temperatures that are 100–500 °C colder at 30–80-km depth than pressure–temperature conditions determined thermobarometrically from exhumed metamorphic rocks. Accurately inferring subduction zone thermal structure, whether from models or rocks, is crucial for predicting metamorphic reactions and associated fluid release, subarc melting conditions, rheologies, and fault-slip phenomena. Here, we compile surface heat flow data from subduction zones worldwide and show that values are higher than can be explained for a frictionless subduction interface often assumed for modeling. An additional heat source––likely shear heating––is required to explain these forearc heat flow values. A friction coefficient of at least 0.03 and possibly as high as 0.1 in some cases explains these data, and we recommend a provisional average value of 0.05 ± 0.015 for modeling. Even small coefficients of friction can contribute several hundred degrees of heating at depths of 30–80 km. Adding such shear stresses to thermal models quantitatively reproduces the pressure–temperature conditions recorded by exhumed metamorphic rocks. Comparatively higher temperatures generally drive rock dehydration and densification, so, at a given depth, hotter rocks are denser than colder rocks, and harder to exhume through buoyancy mechanisms. Consequently––conversely to previous proposals––exhumed metamorphic rocks might overrepresent old-cold subduction where rocks at the slab interface are wetter and more buoyant than in young-hot subduction zones.