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Crustal block-controlled contrasts in deformation, uplift, and exhumation in the Santa Cruz Mountains, California, USA, imaged through apatite (U-Th)/He thermochronology and 3-D geological modeling 通过磷灰石（U-Th）/He 热年代学和三维地质建模了解美国加利福尼亚州圣克鲁斯山脉由地壳块体控制的变形、隆起和掘起对比情况

GSA Bulletin

Pub Date : 2024-07-01 DOI: 10.1130/b36528.1

Curtis W. Baden, David L. Shuster, Jeremy H. Hourigan, Jared T. Gooley, Melanie R. Cahill, George E. Hilley

Deformation along strike-slip plate margins often accumulates within structurally partitioned and rheologically heterogeneous crustal blocks within the plate boundary. In these cases, contrasts in the physical properties and state of juxtaposed crustal blocks may play an important role in accommodation of deformation. Near the San Francisco Bay Area, California, USA, the Pacific–North American plate-bounding San Andreas fault bisects the Santa Cruz Mountains (SCM), which host numerous distinct, fault-bounded lithotectonic blocks that surround the San Andreas fault zone. In the SCM, a restraining bend in the San Andreas fault (the SCM bend) caused recent uplift of the mountain range since ca. 4 Ma. To understand how rheologic heterogeneity within a complex fault zone might influence deformation, we quantified plausible bounds on deformation and uplift across two adjacent SCM lithotectonic blocks on the Pacific Plate whose stratigraphic and tectonic histories differ. This was accomplished by combining 31 new apatite (U-Th)/He ages with existing thermochronological datasets to constrain exhumation of these two blocks. Additionally, surface exposures of the latest Miocene to late Pliocene Purisima Formation interpreted in 18 structural cross sections spanning the SCM allowed construction and restoration of Pliocene deformation in a three-dimensional geologic model. We found that rock uplift and deformation concentrated within individual Pacific Plate lithotectonic blocks in the SCM. Since 4 Ma, maximum principal strain computed for the more deformed block adjacent to the fault exceeded that computed for the less deformed block by at least 375%, and cumulative uplift has been more spatially extensive and higher in magnitude. We attribute the difference in uplift and deformation between the two blocks primarily to contrasts in lithotectonic structure, which resulted from diverging geologic histories along the evolving plate boundary.Restraining bends in large, plate-bounding strike-slip faults produce deformation and create topography in the surrounding crust even in the absence of significant fault-normal plate motion. In these settings, local convergence of strike-slip plate motion across the restraining bend produces contraction and uplift (Segall and Pollard, 1980; Bilham and King, 1989; McClay and Bonora, 2001; Cunningham and Mann, 2007; Cooke and Dair, 2011). Over geologic time scales, strike-slip motion along the plate margin accumulates synchronously with bend-induced uplift, such that crust may advect into, through, and out of the zone of bend-induced deformation and uplift as it translates along the plate margin. This general kinematic model for long term uplift surrounding restraining bends in strike-slip faulting regimes has been used to describe the formation of numerous mountain ranges, and deformational patterns therein, from around the world (Anderson, 1990; Cowgill et al., 2004; Cunningham and Mann, 2007; Mann, 2007; Gudmundsdottir et

在每个 AHe 样品的时间-温度历史中，有几个不确定因素需要确认。首先，磷灰石中 He 扩散的温度敏感性因分析磷灰石晶体中的有效铀含量（eU）而异（Shuster 等人，2006 年；Flowers 等人，2009 年；Shuster 和 Farley，2009 年；Willett 等人，2017 年）。这种温度敏感性（HePRZ）的范围从 30 ℃（低 eU 晶体中完全的 He 保留）到 90 ℃（高 eU 晶体中完全的 He 扩散），典型范围约为 40-80 ℃（Willett 等人，2017 年）。(因此，磷灰石中的（U-Th）/He 测量为时间-温度预测提供了极少的信息，这些信息位于可信的 HePRZ 之外。例如，最小厚度方案中的 Purisima Formation 为 300 公里（Ross，1970 年）。La Honda 和 San Emigdio 两块地块上都有相关的软弱、延伸的岩浆基底岩石，变形在这两块地块上都有局部发生，这一事实可能表明，这些基底结构是沿南澳大利亚大陆岩石圈的明显薄弱环节。地壳异质性和由此产生的岩石圈尺度强度对比也可能决定了横断山脉西部变形的分布和规模，在那里，厚厚的沉积海盆沉积序列经过了广泛的推覆、隆起和掘起（Atwater，1998；Townsend 等，2021）。例如，在圣贝纳迪诺山脉尤卡帕山脊区块内采样的 AHe 年龄远远小于从周围区块采样的年龄，这意味着在这一毗邻 SAF 的狭窄地壳薄片内的掘起率远远高于周围地区（Spotila 等人，1998 年，2001 年）。Spotila 等人（1998 年、2001 年）将尤凯帕海脊区块的大范围掘起归因于板块边缘演化过程中断层活动和几何形状的变化。然而，区域地质图和构造解释（Matti 等人，2003 年；Cromwell 和 Matti，2022 年）中所报告的基底岩性差异表明，相邻地壳块体之间的流变对比在尤凯帕岭块体内部应变局部化方面所起的作用可能比以前所认识到的更为重要。具体来说，尤凯帕岭区块内的叶状片岩以及高度断裂和剪切的碎屑花岗岩（圣伯纳迪诺型基底）不同于邻近地壳区块内的相干花岗岩（半岛型和莫哈韦型基底），而且很可能比后者弱得多。我们的研究结果表明，这种推断出的强度对比可能有助于解释尤凯帕岭地块与邻近地块在所观测到的AHe年龄和相关的掘起速率上的明显差异。重要的是，断层带附近岩石的材料特性及其演变可能并不是相互独立的。在断层带附近地质薄弱的岩石中产生的巨大变形可能会改变断层本身的几何形状，从而改变变形边界沿线的应力和变形、沿太平洋-北美板块边界的广泛地形和隆起趋势被归因于穿越南澳大利亚板块的大尺度转位（Spotila 等人，2007a），而其他研究表明，近场隆起可能受到局部结构效应和物质各向异性的控制（Buscher 和 Spotila，2007 年；Spotila 等人，2007b）。然而，我们的研究结果以及之前的研究（Niemi 等人，2013 年；Townsend 等人，2021 年）都表明，流变驱动的应变局部化强烈影响了地壳块体之间沿 SAF 的变形分区。我们认为，流变驱动的应变局部化也会在更大的空间尺度上产生影响，并可能有助于解释整个大本德尺度上的地形起伏、断层密度和地震活动的对比。在大本德内的南澳大利亚大陆架以西，圣盖博山和横向山脉西部由地质异质的地壳块体和相对旋转和移位的地壳碎片组成（Atwater，1998 年）。这些地壳块中有许多承载着沉积岩厚层，沉积于以前的海洋盆地（如横断山脉西部；Hornafius 等人，1986 年；Atwater，1998 年；Townsend 等人，2021 年），还有一些承载着高度变形的结晶岩（如尤凯帕地块；Matti 等人，2003 年；Cromwell 和 Matti，2022 年）。该地区地形起伏较大，断层广泛，在某些地方还出现了高度隆起。相比之下，由晶质花岗岩组成的莫哈韦区块（SAF）以东的地形起伏和推断断层则微不足道（Hauksson，2011年）。诚然，这一假设概括了这一地区地壳和岩石圈的复杂构造，而这一构造在大弯曲的尺度上显然是复杂的（Plesch 等人，2007 年；Shaw 等人，2015 年）。此外，SAF 以西的断层活动、地形生成和掘起往往早于大拐弯周围 5 Ma 的约束性弯曲变形（Crowell，1979 年），因此圣盖博山和横断山脉西部的地形和掘起信号可能是从以前的构造构造中继承下来的（Spotila 等人，1998 年；Blythe 等人，2000 年，2002 年；Townsend 等人，2021 年）。尽管存在这些复杂性，但圣盖博山和横断山脉西部的断层外地震活动相对于在南澳大利亚大陆以东观测到的地震活动（Allen 等，1965 年；Hauksson，2000 年，2011 年）有所增加，这支持了以下假设：在当前的构造构造中，南澳大利亚大陆以西薄弱的断层地壳可能主要容纳大弯周围的断层外变形。Ten Brink 等人（2018 年）在美国阿拉斯加州太平洋-北美板块边界沿线的研究表明，与南澳大利亚断层不同，夏洛特女王断层沿线的绝大部分断层滑动和相关变形都集中在板块边缘的一个狭窄区域内。Ten Brink 等人（2018 年）将局部断层带归因于该地区海洋地壳与大陆地壳之间强烈的流变学对比。在阿拉斯加冰点附近，费尔韦瑟-夏洛特女王断层（Lease 等人，2021 年）约束性弯曲两侧的 AHe 冷却年龄对比也可以用地壳强度对比来解释。在这一地区，费尔韦瑟-夏洛特女王断层西南部的年轻 AHe 年龄位于褶皱和断层沉积岩中，而断层东北部的较老年龄则位于变质岩和柱状岩中，这些岩石可能更坚固。沿着阿拉斯加山脉的迪纳利断层，Fitzgerald 等人（2014 年）援引构造地层之间岩石圈强度的差异来解释变形的局部性，以及由此产生的沿麦金利山和海斯山约束弯的北美最高山峰。Benowitz等人（2022年）从Fitzgerald等人（2014年）划定的阿拉斯加山脉薄弱缝合区内采集的磷灰石裂变轨迹和锆石（U-Th）/He年龄表明，这块薄弱地壳的剥蚀率也非常高。沿着新西兰南阿尔卑斯山的阿尔卑斯断层，隆起和掘起的模式在以断层为界的地壳区块之间并不存在差异，而似乎与断层滑动速率的差异和岩石圈地幔流变学差异所驱动的深部断层几何形状有关（Little 等人，2005 年；Eberhart-Phillips 等人，2022 年）、在 SCM 中，Ben Lomond 和 La Honda 区块（图 12 和 13）内 4 Ma 后岩石隆起的模型差异所体现的应变分区，并没有在几十年来利用卫星大地测量所测量的整个地区的地震间运动中得到明确表达（d'Alessio 等人，2005 年；Bürgmann 等人，2006 年）。Baden 等人（2022 年）发现，震间运动很可能是由地壳的弹性弯曲主导的，而微妙的震间塑性应变累积似乎产生了在单片机弯曲内观测到的地质变形。在本案例中，两个地块之间观察到的长期变形的巨大差异似乎与地震间地表运动无关，这一事实表明，这些短期运动主要是由刚度相似的地壳块体的弹性弯曲引起的，而长期变形则揭示了地壳块体综合屈服强度的对

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