LONG VALLEY CALDERA LAKE AND REINCISION OF OWENS RIVER GORGE

Owens Gorge was cut in granodiorite in the Neogene and then filled by a basaltic shield at 3.3 Ma. The basalt, 200 m thick, blocked a 5-km reach of the gorge, diverting Owens River into Rock Creek where it cut a second 200-m-deep gorge in the same granodiorite. During MIS 22, a piedmont glacier buried the diversion and deposited Sherwin Till across the basalt-filled reach on both sides of Owens Gorge. The Bishop Tuff blanketed the landscape with welded ignimbrite at 767 ka, covering the till, basalt, and granodiorite and filling all reaches of both gorges. Ignimbrite was nowhere inset against basalt, till, or granodiorite, showing that the basalt-blocked reach had still not been reexcavated. Caldera collapse left a depression >700 m deeper than the precaldera floor of Owens Gorge, which was beheaded, reversing its drainage, back into Long Valley. It took 600,000 years of sedimentation in the 26-km-long caldera lake to raise lake level to its spillpoint. Littoral facies define the highstand of the 320-km² paleolake, within which a central siltstone facies is >700 m thick and rich in rhyolite ash, ostracods, and diatoms. The east-side highstand shoreline has been tectonically tilted ~0.9º S (toward the Round Valley fault), lowering the basin spillpoint by 185 m relative to highstand elevation at Glass Mountain 14 km north. West-side shorelines were raised and tilted by caldera resurgence. Lake longevity is constrained by Ar-dated lava flows with or without lake features; by U-series ages for tufa; and by ages of many tephra layers and a geomagnetic excursion high in the lake silt sections. Regression began early in MIS 6, and the lake drained fully, through Owens Gorge, soon after 150 ka. Siltation of the basin, downwarping of what became the outlet, 50–80 m erosion of the lake-facing ignimbrite surface, and intensification of Sierran runoff during MIS 6 all contributed to eventual overflow. Reincision of the gorge took place, not by headward erosion upstream from Owens Valley, but along a compactional swale where ignimbrite had filled the Neogene gorge, promoting reverse headward erosion, which advanced eastward along a narrow arm of the lake, reincising the Neogene drainage through the Bishop Tuff and ultimately impinging on the 3.3-Ma basaltic blockage.