1 New Geologic Mapping, Tahoe/Donner Region  Arthur Gibbs Sylvester 1,  William S. Wise 1, Jordan Hastings 2  And many UCSB undergraduate geology students 1 UCSB Dept. of Geological Sciences 2 UCSB Dept. of Geography University of California, Santa Barbara

2 Purposes of this Talk n To present new geologic maps of all or parts of nine 7.5’ quadrangles in the Tahoe/Donner area. n To present the major geologic findings resulting from the new mapping. Index Map Yellow = UCSB 7.5’ mapping

3 Purposes of the Mapping n To investigate the relation of volcanic eruption centers to basement structure. n To determine the geologic history of the area, including Tertiary volcanism and faulting. n To complement the geologic data base of the greater Tahoe area. Tahoe City 7.5’ Quadrangle

4 Main Findings n Tahoe/Truckee graben is bounded on west by Donner deformation zone (Hudson, 1948). n Little evidence of recent fault displacement. Norden 7.5’ Quadrangle

5 Main Findings n Tahoe/Truckee graben is bounded on east by major fault along state line. n Much evidence of recent faulting on other faults (Schweickert et al., 2000). Kings Beach 7.5’ Quadrangle

6 Main Findings n Lake level was 75 m higher during glacial times than today (Birkeland, 1964). n Ancient shorelines, peperites. Kings Beach 7.5’ Quadrangle

7 Main Findings n Oligocene rhyolite tuff in Onion Creek, Emigrant Meadow, Beacon Peak, Boreal Ridge, and Castle Valley areas (Lindgren, 1897). n Nine separate tuffs. n Tuffs filled an Eocene paleocanyon cut in Cretaceous granite. Norden 7.5’ Quadrangle

8 Main Findings n Presence of a major volcanic debris avalanche in Norden quadrangle (Fosdick et al., 2004; Rotberg et al., 2004). n Derived between Ma ago from major volcano to SE (Tinker Knob? Squaw Peak?). Norden 7.5’ Quadrangle

9 Main Findings n Graben filled with Tertiary volcanic rocks of Oligocene, Miocene, Pliocene, and Recent age. n Seven major volcanoes: l Four along Donner deformation zone: Tinker Knob, Squaw Peak, KT22, Twin Peaks. l Three in the graben: Mt. Pluto, Mt. Watson, Martis Peak. Martis Peak 7.5’ Quadrangle

10 Main Findings n Young tuff ring at Lake Forest. Kings Beach 7.5’ Quadrangle

11 Main Findings n Young tuff ring at Lake Forest not far from seismic activity in Carnelian Bay in u Many small earthquakes (< M.3) u Deep earthquakes (~ 30 km) u Indicative of volcanic source. Kings Beach 7.5’ Quadrangle 2003 earthquake swarm

12 Status of the Research n Field mapping in summer 2004 directed to: u detailed work in basalt fields in Truckee and Martis Peak quadrangles. u details of glacial deposits u Map stateline fault n Collect specific rocks for radioisotopic dating Truckee 7.5’ Quadrangle

13 Status of the Maps n All field maps now compiled at scale of 1:24,000 in hand colored, analog sheets of paper. n These maps now being digitized using ArcMap 8.3 in GIS format. n Kings Beach quadrangle is finished. n Others are raster images posted at: Homewood 7.5’ Quadrangle

14 Ultimate Intention n Put all maps on Web as Dynamic Digital GIS documents that can be queried for: u age, type of rocks, chemical analyses u faults, geologic structures u outcrop and vista images Wentworth Springs 7.5’ Quadrangle

15 Ultimate Objective  To complement the collection of all research in the greater Tahoe region available to regional planners and other users.