Tectonic response of the central Chilean margin (35–38°S) to the collision and subduction of heterogeneous oceanic crust: a thermochronological study

Along-strike geological segmentation in the Andean chain has been recognized at various scales and is usually attributed to changes in plate motion vectors, as well as the upper-plate expression of differing subducted slab age, strength and composition. We present new multi-phase 40Ar/39Ar, apatite fission-track and zircon and apatite (U–Th)/He data from a north–south-oriented traverse between 35 and 38°S along the Principal Andean Cordillera of Chile that reveal (1) rapid cooling at 18 and 15 Ma, which can be attributed to both thermal relaxation following magmatic intrusion and regional-scale exhumation, and (2) along-strike differences in the extent of exhumation since 7.5 Ma that may be a consequence of the subduction of the Juan Fernandez Ridge above the flat-slab segment at 32°, since 10 Ma. A comparison of the response of the South American Plate to the collision of the Carnegie, Nazca and Juan Fernandez ridges suggests that slab flattening is not the dominant driving force that exhumes the upper plate in these settings. Rather, the extent of exhumation is controlled by pre-existing structural weaknesses, the time duration of the dynamically supported topography, and climate.安第斯山脉（Andean chain）沿走向的地质分段现象已在多种尺度下被识别，其成因通常被归因于板块运动矢量的变化，以及俯冲板片的年龄、强度与成分差异在上覆板块的表现。本文报道了沿智利安第斯主科迪勒拉山系（Principal Andean Cordillera）、位于南纬35°至38°区间的南北向剖面获取的新多阶段测试数据，涵盖40Ar/39Ar测年、磷灰石裂变径迹（apatite fission-track）测年，以及锆石与磷灰石（U–Th）/He测年数据。这些数据揭示了两项关键结果：其一，18 Ma与15 Ma时期发生了快速冷却，该现象可同时归因于岩浆侵入后的热松弛作用与区域尺度的地壳剥露作用；其二，7.5 Ma以来，剥露作用的范围存在沿走向的差异，这一差异可能源于10 Ma以来胡安·费尔南德斯海岭（Juan Fernandez Ridge）在32°纬度处的平板俯冲段（flat-slab segment）上方的俯冲活动。对比南美板块（South American Plate）对卡内基海岭（Carnegie Ridge）、纳斯卡海岭（Nazca Ridge）与胡安·费尔南德斯海岭碰撞的响应可知，在这类地质背景中，平板俯冲并非驱动上覆板块发生剥露的主导驱动力。相反，剥露作用的范围受控于先存构造薄弱带、动力支撑地形的持续时长，以及气候条件。