LATE QUATERNARY DEFORMATION ALONG THE MONO LAKE, TRENCH CANYON AND OTHER OFFSHORE FAULTS MONO LAKE BASIN, CALIFORNIA

Mono Basin lies within a releasing bend in the tectonically active part of the Walker Lane belt. Structural and magmatic features include: active ENE-trending, left-lateral, transpressive faults east of Paoha Island; and late Pleistocene to Recent basaltic to high-silica rhyolitic magmatism along the west margin of the basin. The ENE-trending fault zones imaged by high-resolution seismic reflection collected between 2009 and 2011 show syntectonic folding along strike of the faults. The faults deform post-last glacial age deposits and locally appear to offset mid-late Holocene horizons.

The NNW-trending Mono Lake fault zone, a segment of the Sierra Nevada frontal fault system, was imaged on profiles that show coherent reflectors to about 30 m depth north of the Lee Vining delta. The fault zone can be traced on 8 lines, including 4 lines ~normal to the range front. Two main dip-slip offsets of ~0.7 to 1.7 + 0.3 m and ~3 to 3.5 + 0.5 m are inferred to be ~2.4 + 0.6 ka and ~7.0 + 1.0 ­ ka. Ages of reflection horizons are estimated using published Holocene and late Pleistocene sedimentation rates from cores in conjunction with correlating core depths to seismic horizons using cores proximal to seismic lines. The Holocene fault slip rate based on the most recent event and one or more deeper events suggest about 0.36 to 0.59 m/ka using sedimentation rate of ~0.75 m/ka.

The ENE-striking Trench Canyon fault appears from surficial deposits to offset nearshore lake deposits in the northeast corner of the basin and has been imaged offshore trending towards Negit Island. Faulting associated with folding offshore appears to be active, potentially early Holocene, with offsets along the axis of a laterally continuous anticline. Sparker imaging shows the deformation zone was exposed during low stands of the lake, developing an unconformable erosion surface subsequently overlain by Holocene horizons with minor discontinuities that may be offsets.

A third third zone of ENE-trending faulting was imaged east of Paoha Island. It is characterized by two fault strands associated with a broad pop-up or flower structure that displaces late Quaternary horizons. The pop-up structure shows meter-scale offset of pre-Holocene horizons as well as minor offset plus discordant on-lapping of early and mid Holocene horizons.