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

来自加州大学戴维斯分校、俄勒冈大学和西华盛顿大学的一个研究小组正在对加州南部菲什克里克-瓦莱西托盆地保存的上新世晚期到现代古侵蚀速率的非常完整和暴露良好的档案进行多学科研究。物源的侵蚀速率是根据碎屑石英中的宇宙成因核素浓度确定的，并对盆地沉积物快速埋藏和折返过程中最小的沉积后核素生长进行了校正。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，评估了源区的地貌生产。沉积年龄和沉积速率用高精度的磁化反转和古强度地层学标定。这些数据产生了冲积沉积物中宇宙成因核素生长的高分辨率时间序列，记录了流域平均侵蚀速率随时间的重要变化。将源区侵蚀速率的时间变化与盆地相结构和沉积积累速率变化的地层记录进行比较，从而能够评估将侵蚀强迫与地层结果联系起来的过程-响应函数。利用这些方法，该小组正在检验以下三个假说：(1)大约250万至300万年前北半球冰川开始时，气候条件转变为更凉爽和更具时变性的气候条件，导致侵蚀增加，当地衍生的沉积物产生；(2)构造驱动的隆起增加，导致地形起伏和侵蚀加剧；(3)构造沉降率的降低推动了当地衍生的沉积物的进积，而不考虑源区侵蚀速率的变化。在一年的先导研究中收集的数据表明，自2.8 Ma以来，侵蚀率下降了40%。这一初步结果与气候或下沉速率的进积驱动因素最为一致，尽管目前不能排除隆升驱动的模式。这项新的研究将进一步分析这一记录，并从第二个沉积物来源地区复制这一信号。众所周知，基岩侵蚀和泥沙产生的速率与气候强迫和地形起伏的构造构造密切相关，尽管这些控制的确切性质和反馈动力学仍不完全清楚。现代水系沉积物中碎屑的宇宙成因核素浓度被广泛用于量化短期集水区侵蚀速率，并用于评估构造作用力、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集流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，在连续沉积层序中的应用更是罕见。这项研究正在通过将这些方法应用于较老的沉积物来绘制新的领域。如果成功，地质学家将拥有新的工具来区分气候和构造在景观演变中的作用。

A research team from the University of California at Davis, 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 three hypotheses for tectonic - versus climate driven changes in erosion of the bedrock source and progradation of sediment derived from this source into the adjacent basin: (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 production of locally derived sediment; (2) an increase in tectonically driven uplift led to relief production and enhanced erosion; and (3) a reduction in tectonic subsidence rate drove progradation of locally derived sediments irrespective of changes in source-area erosion rate. Data collected during a 1-year pilot study show that erosion rates decreased by 40% since 2.8 Ma. This preliminary result is most consistent with either a climatic or subsidence-rate driver of progradation, though an uplift-driven model cannot be ruled out at this time. This new study will further analyze this record and replicate this signal from a second sediment source-area.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.