Gary J. Huftile

Present-day principal horizontal stress orientations in the Kumano forearc basin of the southwest Japan subduction zone determined from IODP NanTroSEIZE drilling Site C0009

A 1.6 km riser borehole was drilled at site C0009 of the NanTroSEIZE, in the center of the Kumano forearc basin, as a landward extension of previous drilling in the southwest Japan Nankai subduction zone. We determined principal horizontal stress orientations from analyses of borehole breakouts and drilling-induced tensile fractures by using wireline logging formation microresistivity images and caliper data. The maximum horizontal stress orientation at C0009 is approximately parallel to the convergence vector between the Philippine Sea plate and Japan, showing a slight difference with the stress orientation which is perpendicular to the plate boundary at previous NanTroSEIZE sites C0001, C0004 and C0006 but orthogonal to the stress orientation at site C0002, which is also in the Kumano forearc basin. These data show that horizontal stress orientations are not uniform in the forearc basin within the surveyed depth range and suggest that oblique plate motion is being partitioned into strike-slip and thrusting. In addition, the stress orientations at site C0009 rotate clockwise from basin sediments into the underlying accretionary prism.

Late Cenozoic Tectonics of the East Ventura Basin, Transverse Ranges, California

The east Ventura basin originated in the middle Miocene as a rift system bounded on one side by the Oak Ridge-Simi Hills structural shelf and on the other side by a granitic ridge parallel to the San Gabriel fault. This fault began accumulating right slip 10-12 m.y. ago at a rate of 4.5-9 mm/yr (depending on whether total slip is 45 or 60 km), slowing to about 1 mm/yr in the Quaternary. North of the Santa Clara River, rifting ended prior to deposition of the uppermost Miocene-lower Pliocene Towsley Formation. South of the Santa Clara River, the rift axis shifted southwest toward the Oak Ridge-Simi Hills shelf as the Towsley Formation accumulated against a normal-fault ancestor of the Santa Susana fault. A change to contractile tectonics occurred in the Pliocene with depos tion of the Fernando Formation, when the Newhall-Potrero anticline developed as a monocline above a blind reverse fault; the Pico anticline to the southeast and the Temescal and Hopper Ranch-Modelo anticlines to the northwest may have a similar origin. Tectonic inversion and displacement on the southwest-verging Santa Susana fault began about 0.5 Ma based on appearance of locally-derived clasts in the upper Saugus Formation and its equivalents, and continues today, along with the southwest-verging San Cayetano fault farther west. Also active are northeast-verging backthrusts occurring in the east Ventura basin fold belt, and a segment of the San Gabriel fault which now acts as a northeast-dipping oblique-slip reverse fault. Northeast-trending discontinuities and structures divide the present deformation zone into four segments. In the Hopper Canyon segment at the west end of the area in the west Ventura basin, the San Cayetano fault places Miocene Modelo Formation over Pliocene-Pleistocene strata more than 5 km thick, largely at maximum burial. To the southeast, the Newhall-Potrero segment is characterized by north-vergent backthrusts within the east Ventura fold belt and by southward thrusting of the basin sequence (tectonic inversion) over the structural shelf on the Santa Susana fault. Farther southeast, the Placerita segment is marked by reverse faulting on both the Santa Susana fault and the San Gabriel fault. Southeast of the basin in the San Fernando Valley, the Sylmar segment contains a thick Pli cene-Pleistocene sequence overridden by basement rocks of the San Gabriel Mountains as well as the south-verging Mission Hills-Granada Hills and Northridge Hills fault zones.

Late Cenozoic tectonics of the northern Los Angeles fault system, California

The northern Los Angeles fault system along the southern range front of the Santa Monica Mountains includes potentially seismogenic faults directly beneath the Los Angeles metropolitan area. For a better assessment of seismic hazards, we mapped late Cenozoic faults and folds in the northern Los Angeles basin using an extensive set of oil-well and surface geologic data. The northern Los Angeles fault system developed through early to late Miocene transrotational and transtensional regimes and a Pliocene and Quaternary transpressional regime. The Santa Monica, San Vicente, and Las Cienegas faults are early to late Miocene normal faults that were later reactivated as reverse faults, suggesting that the orientation of reverse faults is largely controlled by Miocene extensional tectonics rather than by the post-Miocene stress field. Tectonic inversion occurred at the beginning of Pliocene time with the reactivation of Miocene normal faults and initiation of reverse faults. Many Pliocene contractile structures became inactive by the middle Pleistocene, and younger deformation is taken up by new active structures, including the West Beverly Hills lineament and an active strand of the Santa Monica fault. The West Beverly Hills lineament is the northernmost segment of the Newport- Inglewood fault zone, which may have propagated northward to the Santa Monica Mountains in Quaternary time. The lineament acts as a segment boundary for the active left-lateral Santa Monica−Hollywood fault system and bounds the Hollywood basin to the west. Uplift of an oxygen- isotope substage 5e marine terrace north of the city of Santa Monica and an assumed dip of >45° for the Santa Monica Mountains thrust fault underlying and uplifting the Santa Monica Mountains suggest that an average dip-slip rate for the fault is <1.3 mm/yr. Crustal shortening across the northern Los Angeles fault system accounts for less than a third of the current rate of shortening between the San Gabriel Mountains and Palos Verdes Hills based on global positioning system observations.

Growth of borehole breakouts with time after drilling: Implications for state of stress, NanTroSEIZE transect, SW Japan

Resistivity at the bit tools typically provide images of wellbore breakouts only a few minutes after the hole is drilled. In certain cases images are taken tens of minutes to days after drilling of the borehole. The sonic caliper can also image borehole geometry. We present four examples comparing imaging a few minutes after drilling to imaging from about 30 min to 3 days after drilling. In all cases the borehole breakouts widen with time. The tendency to widen with time is most pronounced within a few hundred meters below the seafloor (mbsf), but may occur at depths greater than 600 mbsf. In one example the widening may be due to reduced borehole fluid pressure that would enhance borehole failure. In the three other cases, significant decreases in fluid pressure during temporal evolution of breakouts are unlikely. The latter examples may be explained by time-dependent failure of porous sediments that are in an overconsolidated state due to drilling of the borehole. This time-dependent failure could be a consequence of dilational deformation, decrease of pore fluid pressure, and maintenance of sediment strength until migrating pore fluids weaken shear surfaces and allow spallation into the borehole. Breakout orientations, and thus estimates of stress orientations, remain consistent during widening in all four cases. In vertical boreholes, breakouts wider than those initially estimated by resistivity imaging would result in higher estimates of horizontal stress magnitudes. Because the vertical overburden stress is fixed, higher estimated horizontal stresses would favor strike-slip or thrust faulting over normal faulting.

Measuring vertical tectonic motion at the intersection of the Santa Cruz-Catalina Ridge and Northern Channel Islands platform, California Continental Borderland, using submerged paleoshorelines

We used submerged paleoshorelines as strain markers to investigate Holocene and late Pleistocene vertical tectonic movement at the intersection of the offshore Santa Cruz–Catalina Ridge with the southern boundary of the Western Transverse Ranges, within the California Continental Borderland. Past submerged shoreline positions were identified using high-resolution multibeam bathymetry, side-scan sonar, submersible observations, and the presence of intertidal and subtidal invertebrate fossils. Numerous accelerator mass spectrometry (AMS) 14 C ages of shells from these paleoshorelines were found to be between ~27,000 yr radiocarbon (RC) and 11,500 yr before present, indicative of shoreline colonization during and following the Last Glacial Maximum (LGM), establishing these paleoshorelines as a usable datum for measuring vertical change since this time. Removal of the nontectonic component of vertical change using an ice-volume-equivalent eustatic sea-level compilation indicates between 20 and 45 m of uplift of the eastern part of the Northern Channel Islands block since the LGM lowstand, resulting in an uplift rate of 1.50 ± 0.59 mm/yr over the last 23 k.y. This rate closely matches uplift predicted by published slip rates for the Channel Islands thrust, which underlies the Northern Channel Islands platform. Results from post-LGM shoreline features on Pilgrim Banks are somewhat more ambiguous. Submarine paleoshoreline uplift, together with the extensive upper-crustal fold-thrust style of deformation, illustrates the transpressional interaction of the Borderland and the Western Transverse Ranges blocks where the Santa Cruz–Catalina Ridge and northern Channel Islands intersect.