PRINCIPAL AXES OF STRESS AND STRAIN IN THE KUMANO FOREARC BASIN FROM INVERSION OF A NORMAL FAULT POPULATION MAPPED IN A 3D SEISMIC VOLUME, NANKAI TROUGH, SW JAPAN

Open Access
Author:
Sacks, Alison F
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
May 30, 2011
Committee Members:
  • Demian Saffer, Thesis Advisor
  • Donald Myron Fisher, Thesis Advisor
Keywords:
  • Nankai
  • normal faults
  • stress
  • strain
  • subduction zones
Abstract:
In subduction zones, strain and stress in the upper plate reflect the mechanical properties and conditions along the plate boundary fault below, the material properties within the over-riding and down-going plates, and subduction zone geometry [Davis et al., 1983; Wang and Hu, 2006]. Observations of stress and strain are usually limited to the expression of deformation at the seafloor, and to regions accessible to drilling. Drilling can constrain instantaneous stress orientations and magnitudes at the borehole location, but provides limited information about how stress varies in the rock volume around the borehole, or about variations in stress state over time. A high-resolution 3D seismic survey spanning the accretionary wedge and forearc basin of the Nankai Trough offshore SW Japan was used to map a population of normal faults within the forearc basin, in order to constrain the progression of stress states and deformation, and extending observations of the instantaneous, local stress conditions from measurements at boreholes in the basin. IODP drilling at the Nankai Trough off Kii Peninsula documented an abrupt change in the orientation of maximum horizontal stress across a major out-of-sequence-thrust fault (OOST). This fault extends >120 km along strike and separates the outer accretionary wedge from a large forearc basin. Borehole breakouts indicate that the orientation of maximum horizontal stress changes from 150°/330° in the outer prism to 044°/224° in the forearc basin across the OOST, just 10 km landward. A borehole ~15 km further landward in the forearc basin indicates an orientation of 148°/328°. A population of normal faults within recent (0-3.8 Ma) Kumano forearc basin strata defines current and historical strains and stresses. The normal faults are restricted to the gently dipping cover sequence that unconformably overlies the older (late Miocene) accretionary prism. Dips for all faults range from 45° to 75°, with an average dip of 55°. Strikes of faults cluster about NW-SE and NE-SW orientations. The NW-SE striking faults fall into two groups. One group is planar with an average strike of 155°/335°, and the other contains arcuate faults with a range of strikes from 085°/265° to 165°/345°. Within the NE-SW striking group, strikes of planar faults cluster around three average strikes of 045°/225°, 065°/245°, and 080°/260°. Arcuate geometry defines a fourth subset of this NE-SW striking population that spans a 60° range of strikes from 060°/240° to 120°/300°, with an average orientation of 085°/265°. Faulting has occurred in three phases. Phases 1 and 2 consist of NW-SE striking faults, which are always cross-cut. Stratigraphic relationships distinguish Phase 1 faults as the oldest group. The NE-SW striking faults are classified as Phase 3. Phase 1 faulting ended between 1.6 and 2.1 Ma. Phase 2 and 3 faults are much more recent, typically penetrating to within a few reflectors of the seafloor, and in some cases cutting to the seafloor. More than 60% of Phase 3 faults exhibit scarps at the seafloor. Inversion for maximum extension direction [Molnar, 1983] shows NE-SW extension, ranging from 027°/207° to 073°/253° for Phases 1 and 2. Phase 3 extension ranges from 153°/333° to 165°/345°, nearly parallel to the direction of plate convergence and the shortening direction inferred for the outer accretionary wedge near the trench [Chang et al., 2010; Lin et al., 2010]. Inversions for stress were performed using T-TECTO 3.0 software. Assuming that motion along faults is purely dip-slip, inversion [Angelier, 1984] yields a vertical σ1 and sub-horizontal σ2 and σ3, consistent with a normal faulting regime, and generally consistent with the orientation of maximum horizontal stress inferred from two IODP boreholes in the basin. All phases yield a minimum to maximum stress ratio (σ3/σ1) of 0.27. Most Phase 1 and 2 subgroups indicate relative stress magnitudes (σ1:σ2:σ3) of 1.0:0.35:0.27. All other phases indicate relative stress magnitudes of 1.0:0.27:0.27, with equal magnitudes of σ2 and σ3. The principal axes of strain and stress determined from inversion of Phase 3 faults are generally consistent with stresses inferred from borehole breakouts at the two boreholes in the basin. Extension may be a response to gravitational stresses within the upper part of the hanging wall of the megasplay fault, driven by uplift of the seaward edge of the basin associated with coseismic slip. The wide range of fault strikes observed throughout the basin may indicate equal relative magnitudes of stresses in the horizontal plane (σ2 = σ3), or that the orientations of nearly equal σ2 and σ3 alternate throughout the seismic cycle in response to fluctuations in the magnitude of stress parallel to convergence.