Paleomagnetism: A Geophysical Tool for Bedrock Mapping and the Assessment of Fault Kinematics in Volcanic Terrains, Experience from Burney Spring Mountain, NE California

摘要

Bedrock mapping in volcanic terrains is a challenge, and generally requires extensive field mapping and petrographic and geochemical analysis. Paleomagnetism, when used in conjunction with field, geochemical and petrographic data offers a complimentary geophysical tool to field mapping, assisting in the correlation of lava flows across faults and aiding in determination of fault kinematics. Secular variation of the Earth's magnetic field imprints individually distinguishable magnetic directions in igneous rocks emplaced >100 years apart, resulting in magnetic fingerprints that can be used to correlate lava flows across eroded areas, or that have been displaced by faulting or modified by weathering. A successful paleomagnetic study requires establishment of a well constrained magnetic direction for individual lava flows, against which structural corrections can be made for sample sites in rotated blocks. The resulting structural corrections provide insight into the mode and degree of movement along the fault since emplacement of the lava flows. This methodology is currently being applied to mapping a tectonically modified Pliocene-Pleistocene volcanic edifice, Burney Spring Mountain, within the Hat Creek Graben of northeastern California. The establishment of a general range of paleomagnetic directions for Burney Spring Mountain serves to distinguish between lava flows sourced from Burney Spring Mountain and those that overlap the edifice from surrounding volcanic vents. Paleomagnetic results have thus assisted in delineating the areal extent of Burney Spring Mountain and have furthermore revealed reactivation of what are observed in the field to be normal faults cross-cutting the mountain, as right-lateral strike-slip faults.