Collaborative Research: Timing and Controls on Plio-Pleistocene Erosion in the Eastern Peninsular Ranges, Southern California

来自北卡罗来纳州大学、俄勒冈州大学和西华盛顿大学的一个研究小组正在对保存在加州南部鱼溪-瓦列西托盆地的一个非常完整和暴露良好的上新世晚期至现代古侵蚀率档案进行多学科检查。侵蚀速率的来源确定宇宙成因的核素浓度在碎屑石英，最小的沉积后核素在盆地沉积物的快速掩埋和折返过程中的增长与校正。通过比较现代基岩暴露于盆地沉积物中碎屑矿物的（U-Th）/He冷却年龄，评估了源区的救灾生产。沉积年龄和沉积速率用高精度的磁反转和古强度地层学标定。这些数据产生了一个高分辨率的时间序列的宇宙成因核素在冲积沉积物的生长，记录了重要的变化，流域平均侵蚀速率随时间的推移。源区侵蚀速率的时间变化进行比较，在盆地的相结构和沉积物积累速率的变化的地层记录，允许评估的过程响应函数，连接侵蚀迫使地层结果。利用这些方法，该小组正在测试基岩源侵蚀中的构造与气候驱动变化的假设：（1）大约250万至300万年前北方冰川开始时，气候条件转变为更冷和更随时间变化的气候条件，导致当地沉积物的侵蚀加剧和明显的侵蚀;（2）大约100 - 130万年前，上地幔驱动的显著隆升导致半岛山脉的地貌产生和侵蚀增强。有充分的证据表明，基岩侵蚀和沉积物产生的速率与气候强迫和地形地貌的构造建设密切相关，尽管这些控制的确切性质和反馈动力学仍然不完全理解。现代河流沉积物中碎屑宇宙成因核素浓度被广泛用于量化短期流域侵蚀速率，并用于评估构造强迫、岩性和侵蚀过程对沉积物产生速率的作用。虽然这种方法在现代沉积物集水系统的研究中很常见，但它在古代沉积物中的应用相对较少，而在连续沉积序列中的应用则更少。这项研究通过将这些方法应用于更古老的沉积物来绘制新的地面。如果成功的话，地质学家将有新的工具来区分气候与构造在景观演化中的作用。

A research team from the University of North Carolina, University of Oregon, and Western Washington University is conducting a multi-disciplinary examination of a remarkably complete and well exposed archive of late Pliocene to modern paleo-erosion rates preserved in the Fish Creek - Vallecito basin in southern California. Erosion rates in the source are determined from cosmogenic nuclide concentrations in detrital quartz, with corrections for minimal post-depositional nuclide in-growth during rapid burial and exhumation of basinal sediments. Relief production in the source areas are assessed by comparison of (U-Th)/He cooling ages in modern bedrock exposures to detrital minerals in basinal sediments. Depositional ages and sedimentation rates are calibrated with high-precision magnetic reversal and paleointensity stratigraphy. These data yield a high resolution time series of cosmogenic nuclide in-growth of alluvial sediments, which record important changes in catchment-averaged erosion rates through time. Temporal changes in source-area erosion rate are compared to the stratigraphic record of changes in facies architecture and sediment accumulation rates in the basin, allowing assessment of the process-response functions that link erosional forcing to stratigraphic outcomes. Using these approaches, the team is testing hypotheses for tectonic - versus climate driven changes in erosion of the bedrock source: (1) a shift into cooler and more temporally variable climate conditions at the onset of northern hemisphere glaciation approximately 2.5 to 3.0 million years ago caused increased erosion and pronounced progradation of locally derived sediment; and (2) significant, upper mantle-driven uplift lead to relief production and enhanced erosion in the Peninsular Ranges beginning at about 1.0 to 1.3 million years ago.It is well documented that rates of bedrock erosion and sediment production are closely linked to climate forcing and tectonic construction of topographic relief, though the exact nature and feedback dynamics of those controls remain incompletely understood. Detrital cosmogenic nuclide concentrations in modern stream sediments are widely used for quantifying short-term catchment erosion rates, and for assessing the role of tectonic forcing, lithology, and erosion processes on sediment production rates. While this method is common in studies of modern sediment-catchment systems, its application to ancient sediments is relatively rare, and its use in continuous sedimentary sequences is rarer still. This study is charting new ground through the application of these methods to older sediments. If successful, geologists will have new tools to distinguish the role of climate versus tectonics in landscape evolution.