MECHANICAL BEHAVIOR OF MAJOR PLATE BOUNDARY FAULT SYSTEMS: INSIGHTS INTO THE STRESS STATE OF THE NANKAI SUBDUCTION-ACCRETION COMPLEX OFFSHORE SW JAPAN AND SLIP STABILITY OF THE ALPINE FAULT ZONE IN NEW ZEALAND
Open Access
- Author:
- Valdez, Robert Dennis
- Graduate Program:
- Geosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 28, 2018
- Committee Members:
- Demian Saffer, Dissertation Advisor/Co-Advisor
Demian Saffer, Committee Chair/Co-Chair
Chris J Marone, Committee Member
Donald Myron Fisher, Committee Member
Jonathan P Mathews, Outside Member - Keywords:
- Consolidation state
Nankai Trough
In situ stress state
Friction
Slip stability
Scientific drilling
Alpine Fault
Accretionary prism
Rate and state friction - Abstract:
- Understanding the mechanics of seismogenesis along major plate boundary fault zones requires not only quantification of physical properties along the plate interface but also the characterization of consolidation properties and stress state in the surrounding material. In subduction margins, fault behavior is influenced by the interplay of accretionary wedge and active décollement strength and stress state. Similarly, in other tectonic settings, such as major crustal fault zones, the onset of seismogenesis may be due to the evolving frictional or mechanical behavior along the plate interface and immediately adjacent wall rock material. Despite their importance, there are few studies that determine the stress regime from changes in consolidation state of prism sediments and characterize the frictional strength and stability of natural fault gouges at elevated temperatures. Laboratory experimentation is essential in characterizing these properties because it allows for a detailed investigation of deformation along numerous loading paths and frictional behavior at in situ geothermal conditions. In order to examine the variations in consolidation and stress state within the Nankai Trough offshore southwest Japan and to characterize the frictional behavior along the Alpine Fault zone in New Zealand, I evaluated uniaxial consolidation and triaxial deformation experiments on natural prism and fault zone materials. These experiments are designed to 1) provide the first comprehensive assessment of consolidation state across an accretionary complex, 2) explore how the consolidation state can be used as a proxy for past loading history and in situ stress state in the vicinity of a major fault system in the Nankai Trough, and 3) evaluate the importance of temperature on the slip stability of the Alpine Fault zone. Results of uniaxial consolidation experiments on Nankai prism materials, which were collected during numerous Integrated Ocean Drilling Program expeditions, indicate that there is an across-trench variation in consolidation state within the Nankai accretionary complex. The outer accretionary prism sediments near the deformation front (Sites C0006 and C0007) and megasplay fault zone (Sites C0001 and C0004) exhibit the most severe apparent over-consolidation, as documented by large over-consolidation ratios (OCR) ranging from 2.98-4.17 and 0.82-4.45, respectively. Farther landward, the forearc Kumano Basin (Site C0002) and underlying accretionary prism (Sites C0002 and C0009) have lower OCR values that range from 2.26-3.04 and 1.23-1.93, respectively. These results, in combination with shipboard data, suggests the presence of erosive events near the megasplay fault zone, minor cementation in the forearc basin, and an increase in horizontal stress associated with a complex tectonic loading history within the outer and inner accretionary prism. All of these processes will lead to an apparent over-consolidation (i.e. OCR >1) in the one-dimensional consolidation experiments. To further investigate the effects of complex loading on the consolidation state and deformation behavior of Nankai prism materials, I conducted additional uniaxial consolidation and triaxial deformation experiments targeting slope basin sediments near the megasplay fault zone. These samples were collected from a laterally continuous slope apron section that is progressively incorporated into the shear zone of a major out-of-sequence thrust. These results indicate that the slope basin sediments become increasingly over-consolidated with proximity to the megasplay fault zone, with OCR values of 0.39-1.84 at Site C0008 to 2.31-3.79 and 0.94-3.99 at Sites C0022 and C0004, respectively. The high OCR values at Sites C0022 and C0004 could be explained by the onset horizontal tectonic loading and compression as the megasplay fault zone accommodates tectonic stress in response to plate convergence. To explore this hypothesis, I conducted triaxial experiments on Site C0022 samples to probe the yield envelope along different loading paths. The yield stresses are consistent with a model in which the maximum horizontal stress increases while the effective vertical stress remains constant as the megasplay fault is approached. The change in stress state can explain the gradual increase in the OCR values from normally consolidated sediment at Site C0008 to highly over-consolidated sediment at Sites C0022 and C0004. This is consistent with independent observations of stress state near the megasplay fault zone, which show a strike-slip to thrust faulting stress regime, and suggests that this is a novel approach for interpreting the complex in situ stress state within tectonically loaded sediments. In Chapter 3, I characterize the frictional properties of the Alpine Fault zone, which is a major transpressional fault on the South Island of New Zealand, and explore their implications for the onset of earthquake nucleation and unstable slip at depth. I conducted high-temperature and -pressure shearing experiments of Alpine Fault principal slip zone (PSZ-1) and wall rock material from depths of 111.5-142.9 m obtained by the International Continental Scientific Drilling Program (ICDP) Deep Fault Drilling Project (DFDP). At temperatures ≤ 180°C, principal slip zone (PSZ-1) is frictionally weaker than the surrounding wall rock material, with friction coefficient (μ) values decreasing from μ = 0.46 at 23°C to 0.35-0.40 at 180°C. Following this, the slip zone drastically strengthens to values comparable to that of the wall rock material, with μ = 0.87-0.90 and μ = 0.87 at 500°C for the PSZ-1 and wall rock material, respectively. This contrast in frictional strength between the fault-core and wall rock material suggests shear localization within the upper ~3-4 km of the Alpine Fault zone. The overall increase in friction strength is accompanied by a transition from stable to unstable behavior at temperatures ≥180°C, as documented by a decrease in the friction rate parameter (a-b) for all material. The frictional results, in combination with constraints on the rheologic critical stiffness (kc), suggests that the highest potential for earthquake nucleation correspond to depths ≥ 4 km along the Alpine Fault zone. Collectively, the results of the experiments in this dissertation provide insight into the consolidation and stress state of actively accreting subduction margins and new constraints on the updip limit of seismicity within crustal fault zones. I demonstrate that there is an across-trench variation in consolidation state within the Nankai accretionary prism. This variation can be used as a proxy for the stress state and history of sediments within the accretionary prism, with implications for sediment cementation, erosive events, or enhanced lateral tectonic loading. This work also provides a new technique for interpreting the in situ stress state of tectonically loaded sediment through meticulous analysis of sediment yield behavior and consolidation state. This novel technique can be implemented for future investigations of stress state in other tectonic regions and subduction margins. Additionally, I show that an increase in temperature will result in a concurrent increase in frictional strength and transition to potentially unstable behavior within the Alpine Fault zone. By analyzing the frictional results in the context of a simple stiffness stability criterion, I am able to define the updip limit of seismicity along the Alpine Fault zone. This type of stiffness analysis can be incorporated into future frictional stability studies to provide additional constraints of seismogenesis along other plate boundary fault zones.