Date of Award
Master of Science (MS)
Karl J. Mueller
Anne F. Sheehan
Greg E. Tucker
The release of seismic moment represented by slip rates on faults is characterized by increasing stability over progressively longer time periods. Yet the transition between patterns of complex moment release at short, paleoseismic timescales and rates of stable slip on faults at longer timescales is largely unknown. We use a record of syntectonic, shallow marine sediments tied directly to the marine eustatic record to map a precise history of uplift across the southern-half of the Osaka-Wan blind thrust fault in central Japan. Results indicate that the blind thrust lengthens rapidly after the onset of slip, and may be weak based on restorations of the upwardly propagating tip of the fault from trishear kinematics. The average rate of slip is more rapid in the middle of the fault, consistent with the distribution of seismic slip in earthquakes and total displacement profiles on other thrust faults. Fine details of the time-displacement length history, however, reveal that uplift rates vary over 60% at scales of 100 ka and 5-10 km along the thrust. In addition, no measureable uplift occurs along much of the fault for a period of 50-100 Ka and may be taken up by a nearby thrust. These results confirm analogue studies of complex fault slip behavior and suggest that the Osaka-Wan thrust may accelerate and decelerate, or even shut off entirely at intermediate timescales.
Inverse trishear modeling undertaken to constrain the geometry of the thrust suggests that the thrust formed as a steeply dipping fault from shallow levels in the crust and propagated upward at rates similar to other basement cored thrusts. Mapping of 14 through-going seismic reflectors tied directly to highstands in the global eustatic record suggests that the southern half of the fold has undergone a complex growth history. This analysis indicates that the fold: 1) began growing shortly after 1500 ka at rates of 0.19 to 1.13 mm/yr along nearly its entire length; 2) grew laterally outward to its present endpoint almost immediately; 3) shut off for 50 - 100 Ka; and 4) underwent differential uplift that ranged from 0.75 mm/yr in its center to 0.21 mm/yr at its endpoint. Results thus yield a hitherto unavailable high-resolution record of thrust fault and fold growth at short, 100 Ka timescales.
The record of displacement along Osaka-wan suggests that it grew by a mix of scaling laws. The total along-strike displacement profile most closely matches a linear-taper model, yet its early history is consistent with an elliptical-taper slip model of fault scaling. Comparison of possible influences on fold growth suggests that sedimentation rate at the location of each seismic profile in the basin is nearly constant and unlikely to be related to transient changes in slip rate, at least to the variation seen in the data. Evidence for reactivation of an older fault is not apparent based on offset of growth strata in comparison to that across the underlying basement-cover contact. However, analysis of fault tip propagation based on trishear kinematics, places the fault tip no deeper than 4 km at its onset, too shallow to have originated as a new fault from deep within the crust. We speculate that a larger subparallel thrust located in the western part of the basin (the Kariya fault) may have accommodated shortening at the expense of the Osaka-wan thrust at different periods in its history, although we do not as yet have the seismic reflection data required to test this hypothesis.
Grothe, Pamela Ruth, "A precise time-displacement-length growth history of the Osaka-wan blind thrust: evidence for long-term unstable slip" (2012). Geological Sciences Graduate Theses & Dissertations. 47.