ASSESSING THE STATISTICAL SIGNIFICANCE OF MASS AND VOLUME CHANGES IN THE DEVELOPMENT OF SAPROCK FROM CORESTONE NEAR THE ELSINORE FAULT ZONE, SAN DIEGO COUNTY, CALIFORNIA: POTENTIAL IMPLICATIONS FOR GROUND SHAKING EVENTS

Spheroidal weathering resulting in the development of saprock from corestone has been observed in gabbro and granodiorite near the Elsinore fault zone, San Diego County, California. Thin section observations reveal that plagioclase and biotite (gabbro and granodiorite), and amphibole and pyroxene (gabbro only), are altered in various proportions to various clay minerals during the development of saprock. Additionally, the transport function τ, indicates an elemental mass change of -5% ±1 Ca, -2% ±1 Na, -3% ±1 Mg, +11% ±3 Mn, -5% ±2 Zn, -5% ±3 V, -27% ±4 K, and -39% ±18 Ba, during the formation of saprock in the gabbro. In the granodiorite, τ indicates an elemental mass change of +7% ±4 Al, -41% ±4 Ca, -38% ±4 Na, -12% ±5 K, -24 ±7 Mn, and +25% ±9 Ti. The development of saprock from corestone is also accompanied by a positive volumetric strain (ε) of 12% ±2% in the gabbro and 35% ±11% in the granodiorite. The change in bulk mass (Τ) was determined to be -0.2% ±0.8 in the gabbro and -0.6% ±2% in the granodiorite. Porosity in the gabbro is 12% ±2% and 26% ±4% in the granodiorite. Statistical analysis indicates that these results are significant at the 95% confidence level. These observations and data are inconsistent with the commonly observed isovolumetric development of saprock from corestone in non-seismogenic areas. The proximity of the study areas to the Elsinore fault zone, and the many ground shaking events from earthquakes since inception of the fault, have likely contributed to the observed positive volumetric strain. Ground shaking events have weakened and broken intercrystalline bonds, and opened migration pathways through rotation of grains during dilation. Such a process provides pathways for fluids and therefore enhances breakdown and weathering of the corestone.