3D GROWTH OF THE OSAKA-WAN THRUST AND FAULT-RELATED FOLD - A DETAILED 1250 KA RECORD OF RAPID OUTWARD LATERAL PROPAGATION FOLLOWED BY HIGHER RATES OF CENTRAL UPLIFT

We use high-resolution seismic reflection surveys tied to a 1.6 km-long core to constrain the 3D upward and lateral propagation of the Osaka-wan blind thrust and fault-related fold in Japan. Inverse trishear modeling undertaken to constrain the geometry of the thrust suggests: 1) a steeply dipping reverse fault formed along the transpressive length of the right-lateral Nojima fault system; 2) a broad trishear envelope; and 3) upward propagation to dip slip ratios of 2-3, broadly comparable to other fault propagation folds. Mapping of six high-amplitude, through-going seismic reflectors tied directly to the global eustatic record suggest the southern half of the fold has undergone a complex growth history. This analysis indicates the fold: 1) began growing at ca 1240-1260 Ka at a constant rate of 1 mm/yr along nearly its entire length; 2) grew laterally outward to about its present endpoint in less than ~200 Ka; 3) decreased in length inward towards its crest at least twice during its subsequent history until the present; and 4) after 1120 Ka 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 it grew by more than one scaling law. Its 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. The fold also exhibits differential (higher rates) of uplift towards its center over much of its subsequent growth history. Perhaps even more surprising, the fold shortened to about 80% of its maximum length for more than half its growth history. A complete inventory of possible influences on fold growth awaits further study. Sedimentation rates in the basin do not appear to vary drastically along the structure based on along-strike seismic reflection profiles. Little evidence exists for reactivation of an older fault, based on offset of growth strata in comparison to that across the underlying basement-cover contact. A smaller, subsidiary thrust located in the southern part of the basin may however have accommodated shortening at the expense of the primary Osaka-wan thrust.