Role of Silica Redistribution in the Evolution of Subduction Megathrusts, Shimanto Belt Japan

Project: Research project

Project Details

Description

The last decade has seen two of the most destructive earthquakes ever recorded, with nearly 300,000 lives lost due to ground shaking and inundation of coastal areas by tsunami. In the case of the 2011 Tohoku earthquake and several other large recent earthquakes, the seismic event was preceded by, and therefore potentially triggered by, a type of fault behavior known as 'slow slip' -- a behavior that was unknown two decades ago. Therefore, the old paradigm that faults along plate boundaries either 'creep' or experience earthquakes due to 'stick-slip' behavior does not explain the full range of fault behavior along active subduction zones-- behavior that includes periodic 'slow earthquakes' and microseismicity that coincides with tremor. Recognition of this diversity of slip behavior demands a new paradigm that explains the role that these various behaviors may play in triggering earthquakes. The central tenet of this study is that information about heterogeneity in plate boundary slip behavior is not only recorded by seismicity, but also in the distributions and textures of veins, or mineralized cracks, that we can observe today in exhumed ancient subduction boundaries. This project is specifically designed to investigate the roles of natural hydrofracking and local redistribution of calcium carbonate and silica for the evolution of slip instabilities associated with microseismicity, slow slip, and the buildup and release of elastic strain in earthquakes. Vein systems will be examined in ancient plate boundary fault zones in Japan that record a range of conditions that reflect the depths and temperatures at which earthquakes are generated. The principal investigator and his colleagues have developed an over-arching hypothesis that relates silica redistribution in fault zones - as exemplified in vein textures and mineralogies - to earthquake dynamics. If the hypothesis is correct, models of subduction zone behavior must consider not just the frictional behavior of the fault but also the role of footwall hydrofracturing (i.e., a predictable function of fluid sources and permeability) and silica redistribution (i.e., a thermally activated process) as an explanation for the heterogeneity in plate boundary behavior in subduction zones. In addition to the research goals of the project, this award provides support for the training of an Hispanic female graduate student at Penn State thus contributing to broadening of underrepresented groups in a Science, Technologoy, Engineering and Math (STEM) discipline, as well as providing opportunities for the participation of an undergraduate student who will complete research-heavy independent senior thesis during year two of the project. Students will be trained in the use of lower temperature models for crack sealing that have been used along passive margins with application to the oil industry while bridging these models toward application to higher temperature rocks from subduction plate boundaries. The graduate student will also be involved in an international collaboration with Japanese scientists, and this study of exposed ancient rocks will complement the ongoing NantroSEIZE offshore drilling experiment designed to evaluate in situ the processes that characterize the plate boundary at depths where earthquakes are generated. Because subduction zones have significant potential for large magnitude earthquakes, studies of fault zone behavior in these settings have potentially significant implications for the health and economic well being of society.

This study is designed to investigate the hydrofracturing and subsequent healing recorded by mineralized veins and fabrics that develop in the footwall of subduction interfaces. It is hypothesized that the range of textures observed in veins is a manifestation of the range of heterogeneous plate boundary slip behavior that is observed along convergent margins. The healing of the fractures within the underthrusting sediments adjacent to the plate interface can occur in times that overlap with earthquake recurrence intervals at the temperatures of the seismogenic zone. Open fluid-filled cracks could impact the effective stress and the strength/elastic properties of the rocks that store and release elastic strain energy, so the healing of cracks could be fundamental to the locking behavior of the seismogenic zone. Vein systems and related scaly fabrics from six regionally extensive shear zones within the Shimanto Belt in Japan that formed during subduction and contain pervasive quartz veins representing the full range of temperatures within the seismogenic zone will be studies. The principal investigators will: 1) characterize the vein systematics on the outcrop both as a function of lithology and position relative to major faults, 2) evaluate vein microstructures petrographically and with cathodoluminescence, and 3) develop elemental maps of potential silica sources adjacent to veins, including scaly fabrics. To evaluate the role of local diffusion of silica, we will map major element concentrations in areas that define potential silica sources (e.g., scaly fabrics and wall rock with cleavage adjacent to fractures). They will also determine the aperture of cracks and the degree to which fractures are sealed based on microstructures. Microstructural information will be used in conjunction with scanline surveys of vein spacings, thicknesses, and lengths to put constraints on the spacings of open fractures and the times needed to seal fractures. This study will enable us to address fundamental questions about quartz veins adjacent to the subduction interface: What are the sources of silica within veins? Can we identify silica depletion zones typical of local silica redistribution? How does crack aperture and crack spacing vary as a function of rock type and temperature? Is there a systematic variation in vein textures with increasing temperature and depth within the seismogenic zone? The Shimanto belt of Japan is ideal for this study as it exposes regional fault zones that are interpreted as paleo-decollements (active plate boundary damage zones in the underthrusting footwall) and for comparison, examples of out-of sequence splays that accommodated seismic slip and juxtaposed rocks of the existing accretionary prism.

StatusFinished
Effective start/end date7/1/156/30/21

Funding

  • National Science Foundation: $324,925.00

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