UCLA

Fracturing is a fundamental mechanical weathering process that affects geology from global tectonics to local outcrops. Despite vast differences in size, fractures are formed through similar mechanisms and have interdependent relationships, where small fractures can contribute to the formation of larger ones and vice versa. Chapter 1 of this dissertation introduces the study of fractures and their wide-ranging effects on plate tectonics and landscape weathering. The subsequent chapters describe three research projects focused on the impacts of fractures on tectonic and geomorphological processes at two locations on the west coast of North America. These studies utilize a diverse array of methods from various fields, including geomorphology, structural geology, geochronology, hydrology, and geophysics. Together, this dissertation summarizes how fractures influence Earth’s history in fault systems, weathered profiles, and landslides.

Chapter 2 describes how faults on microcontinents record the dynamic evolution of plate boundaries. However, most microcontinents are submarine and difficult to study. Here, I show that the southern part of the Isla �ngel de la Guarda (IAG) microcontinent, in the northern Gulf of California rift, is densely faulted by a late Quaternary-active normal fault zone. To characterize the onshore kinematics of this Almeja fault zone, I integrate remote fault mapping using high-resolution satellite- and drone-based topography with neotectonic field mapping. I then analyzed 13 luminescence ages from sediment deposits offset or impounded by faults to constrain the timing of fault offsets. I found that north-striking normal faults in the Almeja fault zone continue offshore to the south and likely into the nascent North Salsipuedes basin southwest of IAG. Late Pleistocene and Holocene luminescence ages indicate that the most recent onshore fault activity occurred in the last ∼50 kyr. These observations suggest that the North Salsipuedes basin is kinematically linked with and continues onshore as the active Almeja fault zone. I suggest that fragmentation of the evolving IAG microcontinent may not yet be complete and that the Pacific-North America plate boundary is either not fully localized onto the Ballenas transform fault and Lower Delfin pull-apart basin or is in the initial stage of a plate boundary reorganization.

In Chapter 3, I explain how diverse mechanical and chemical processes contribute to the breakdown of fresh bedrock and generate the geologic critical zone (CZ) consisting of soil, saprolite, and weathered bedrock. The deep CZ can extend from 1 – 100’s m below ground. However, the spatial and depth distribution of weathered bedrock is difficult to determine from Earth's surface. In this study, I investigate the deep CZ structures at a well-studied, steep, and forested site near Coos Bay, Oregon, USA, using a combination of geophysical methods and process-based modeling. P-wave seismic refraction surveys show a sharp velocity transition at a surface-parallel, ~5 m-deep, and relatively slow velocity contour of 1220 m/s. I also show another transition near contours of 2200 m/s with spatially varying and undulating patterns. These boundaries may represent pervasively oxidized bedrock and fractured bedrock layers observed in a nearby, deep borehole. Then, comparison with Schmidt hammer and ground penetrating radar data reveal areas of highly weathered, porous, and heterogeneous soil and saprolite. I compare these datasets with two process-based models that predict weathered bedrock structures: one based on bedrock drainage of reactive water and the other on topographic stress-induced fracturing. Although these models can reproduce some first-order similarities in my surveys, the mismatch between simulated and inferred weathering may suggest other factors, such as pre-existing fractures, lithological heterogeneity, or landscape evolution, may contribute to variations in the deep CZ. Together, this study underscores the importance of site-specific field observations to evaluate bedrock weathering processes in natural landscapes.

Then, Chapter 4 investigates CZ controls on the occurrence of shallow soil landslides at the study site of Chapter 3, where mechanical and chemical processes may have generated weathered bedrock with increased porosities and hydraulic conductivities underneath soil. Field studies suggest that exfiltrating groundwater from weathered bedrock may be an important driver of initiating shallow landslides. However, variations of deep CZ structure are typically not considered in slope stability models due to a lack of information about the subsurface. Here, I conduct numerical experiments coupling process-based models of 1) deep critical zone development, 2) three-dimensional transient hydrology, and 3) multidimensional slope stability. I show that spatial variations of weathered bedrock thickness control the location of infiltrating and exfiltrating groundwater seepage, induce pore pressure variations at the soil-bedrock boundary, and impact shallow landslide occurrence, size, and location. This work suggests that characterizing the deep CZ is critical for effectively simulating groundwater seepage and assessing the likelihood, magnitude, and timing of shallow landslides.