Thermal models of core-complex evolution in Arizona and New Guinea: Implications for ancient cooling paths and present-day heat flow
Thermal models of core-complex evolution in Arizona and New Guinea: Implications for ancient cooling paths and present-day heat flow
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亚利桑那州和新几内亚核心复合体演化的热模型：对古代冷却路径和当今热流的影响
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发表时间：
1996
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通讯作者：
R. Ketcham
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作者：
R. Ketcham
一个演化的变质核杂岩的二维传导热模型为同伸展期和伸展后热历史提供了见解。这些模型假定具有一个低角度（20° - 30°）拆离断层的旋转枢纽几何形状，该断层在12 - 15千米深度变平。模拟揭示了在伸展过程中底盘岩石所经历的温度 - 时间路径的细节，这对热年代学数据的解释具有重要意义。演化的等温线在拆离断层的地表出露点附近大致水平，在深部随着接近斜坡 - 平面界面而朝着拆离斜坡角度旋转。这导致沿拆离断层面出现高度弯曲的表观地热梯度，地表附近的局部梯度高达深部的3倍。随着岩石接近地表，冷却加速，并且速率在剥露的底盘地块内的不同位置以及所涉及的温度区间内变化可达一个数量级。随着伸展的进行，等温线沿着拆离断层面向上推进。因此，通过绘制40Ar/39Ar或裂变径迹年龄与沿拆离滑动方向的距离的关系图来计算伸展速率，可能会低估真实速率达40%。对拆离倾角的估计也可能同样受到沿断层同伸展期热梯度的非线性的影响。对巴布亚新几内亚当特尔卡斯托群岛核杂岩冷却路径的模拟显示，模型预测与从P - T - t数据确定的冷却路径之间具有显著的一致性。这些模型还有助于研究与亚利桑那州和南加利福尼亚州的核杂岩带平行的20 - 40%的热流低值背后的可能机制。当前的热流模式和低起伏的莫霍面与一种平衡的几何形状最为匹配，在这种几何形状中，核杂岩的剥露由远离核杂岩中心的纯剪切伸展来补偿，而核杂岩本身是一个几乎没有内部剪切的巨型布丁构造。这种几何形状也可能类似于下地壳流动补偿构造剥蚀的情况。然而，基于来自圣卡塔利娜山详细的生热数据的模型表明，核杂岩的形成可能不是该地区低热流的原因。此外，基于亚利桑那州核杂岩的面积平衡地壳剖面中的净伸展量似乎不足以解释整个南盆地和山脉所具有的高热流，这表明可能需要除区域伸展所引入的热量之外的额外热源来解释区域热异常。
Two-dimensional conductive thermal models of an evolving metamorphic core complex provide insights into synextensional and postextensional thermal history. The models assume a rotating-hinge geometry with a low-angle (20°–30°) detachment fault that flattens out at a depth of 12–15 km. Simulations reveal details of the temperature-time paths experienced by footwall rocks during extension that have significant implications for interpretation of thermochronological data. The evolving isotherms are roughly horizontal near the surface breakout of the detachment fault and at depth are rotated toward the detachment ramp angle as the ramp-flat interface is approached. This leads to highly curved apparent geothermal gradients along the detachment fault surface, with local gradients near the surface being up to 3 times those at depth. Cooling accelerates as rocks approach the surface, and rates vary by up to an order of magnitude with position within the unroofing footwall block and the temperature interval in question. Isotherms advance upward along the detachment fault surface as extension progresses. As a result, calculations of extension rate made by plotting 40Ar/39Ar or fission-track dates against distance along the detachment slip direction can underestimate the true rate by up to 40%. Estimates of detachment dip can be similarly affected by the nonlinearity of the along-fault synextensional thermal gradient. Simulation of cooling paths in the D'Entrecasteaux Islands core complex in Papua New Guinea reveals a remarkable congruence between model predictions and the cooling path determined from P-T-t data. The models also serve to examine possible mechanisms behind the heat flow low of 20–40% that parallels the band of core complexes in Arizona and southern California. The present-day heat-flow patterns and low-relief Moho are most closely matched by a balanced geometry in which unroofing of the core complex is compensated by pure shear extension off center from the complex, with the core itself being a megaboudin of little or no internal shear. This geometry may also be analogous to a scenario in which lower crustal flow compensates for tectonic denudation. However, models based on detailed heat-production data from the Santa Catalina Mountains suggest that core-complex formation may not be responsible for the low heat flow in that area. Furthermore, the net amount of extension in area-balanced crustal cross sections based on Arizona core complexes does not appear to be sufficient to explain the high heat flow that characterizes the overall southern Basin and Range, indicating that additional sources of heat over that introduced by regional extension may be required to explain the regional thermal anomaly.