SAN JOAQUIN RIVER INTRACANYON TRACHYANDESITE LAVA FLOW 100 KM LONG, RECOGNITION OF ITS VENT NEAR DEVILS POSTPILE, AND 2 KM ROCK UPLIFT OF SIERRA NEVADA CREST SINCE 9.3 MA

Trachyandesite of Kennedy Table is a chain of 32 remnants of an intracanyon lava flow as thick as 60 m, preserved for 21.5 km from Squaw Leap to Little Dry Creek (LDC). Several remnants lie on fluvial gravel of the Miocene San Joaquin River. N. King Huber (1981) thought the lava covered its own vent, but in 2011 we identified the vent on the Middle Fork, 13.5 km S of Deadman Pass and 70 km NE of Kennedy Table. It intrudes granite, has 285 m relief, is a columnar intrusion that grades up into glassy lava flows. Composition and Ar age (9.3 Ma) are identical at vent and downstream. Basal elevations of all remnants were recorded. Present-day basal gradients, corrected for apparent dip and projected along a vertical plane at 220º (estimated tilt azimuth), range 1.08º–2.21º and are thus unreliable measures of tilt. Likewise, relief atop most table remnants renders modeling upper surfaces suspect. At LDC a distal chain of 8 remnants rests on fluvial gravel, trends 230º, and its smooth basal contact now dips 1.36º (corrected to 220º). Projection 89 km from the 207-m base of the SW remnant at LDC to the 2,760-m intrusion-to-lava transition defines present-day average basal gradient of 1.643º for the lava flow. The Miocene gradient is uncertain. Huber chose 1 m/km (0.057º) at LDC, and below Friant Dam today’s gradient diminishes from 0.04º to 0.02º. Stream concavity requires a steepening gradient upstream. Phillips et al. (Cordilleran GSA 2020) estimated 0.3º in the McKenzie Table reach, and Huber estimated 0.33º near Squaw Leap. Subtracting such Miocene values from 1.643º yields vent uplift of 2087 or 2040 m relative to LDC since 9.3 Ma. Similarly, relative to Huber’s inferred hingeline 4.4 km SW of LDC, vent uplift has been 2190 or 2140 m. Projection 4.86 km NE of vent to the Sierran divide falls 270 m short of the 3,170-m Mammoth Crest, suggesting that rangecrest topography steepened sharply NE of the Miocene Middle Fork. Huber’s analysis (USGS PP 1197) was outstanding.