31 Geomorphology and Quaternary Geology

Saturday, 30 April 2005
1:20 PM - 5:00 PM

COMPARISON OF BEDROCK GEOLOGY WITH FRACTURE AND STREAM ORIENTATIONS IN YOSEMITE NATIONAL PARK, CALIFORNIA

Paper number: 31-4
Program: Technical Sessions
Time: 2:20 PM – 2:40 PM

Authors

Abstract

Fracture patterns in the granitic bedrock of Yosemite National Park provide a window into the thermal and regional stresses that followed plutonism from 105-86 Ma. Fractures may originate as joints from thermal stresses during cooling, or as faults from regional tectonic stresses. Bedrock fractures are well developed in Yosemite and control much of the Park's rugged terrain and scenery as well as drainage, slope and vegetation patterns. They also contribute to numerous, sometimes deadly, rock falls.

495 fractures ranging from 90 meters to 10 km in length were mapped over a 530 km2 region on 1-m digital orthophotoquads (DOQs) using ArcGIS. Fracture patterns for the study area are strongly bimodal with the primary set oriented 039 and a conjugate set roughly orthogonal. Stream data clusters in much the same fashion with a weighted mean of 027. The two largest plutons (El Capitan and Sentinel Granodiorites) exhibit a trellis network of two to three primary stream orientations similar to that of the joints. In addition, the regional slope of the large plateau north of Yosemite Valley trends 233 and seems to correspond well with the primary fracture orientation.

Several important observations concerning the jointing patterns are: 1) Strong bimodality of fractures in the 103 Ma El Capitan pluton is more apparent in the southeastern half and indicates a reorientation of the thermal stress field. 2) Some fractures cross contacts into surrounding younger units and are likely faults; the longest of these is a 10 km Yosemite Creek lineament cutting through 95 Ma Sentinel Granodiorite. 3) Fractures in the 93 Ma Kuna Crest Granodiorite clearly are not orthogonal with respect to the unit's boundaries and may indicate significant disruption of the stress field during cooling. 4) Mapped fracture density is significantly lower in the 93-89 Ma Half Dome Granodiorite, perhaps owing to a combination of lower quality DOQs, thicker vegetation, and the protracted period of plutonism that may have spanned up to 4 m.y., preventing a strong thermal stress field from generating large fracture sets.