Correlation of lava flows on Cascade volcanoes: Tool development and example from Burney Spring Mountain, California
O'Brien, Timothy Michael
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Bedrock mapping in volcanic terrains is a challenge, and generally requires extensive field work 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 orientations 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 orientation 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 was 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 orientations 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 the presence of local block rotations adjacent to the fault, clarifying the kinematics of the faults themselves. Supporting geochemical analyses were conducted to assist in the correlation of Burney Spring Mountain lava flows involving the use of an electron-dispersive x-ray spectrometer (EDS) outfitted scanning electron microscope (SEM) and a portable x-ray fluorescence (pXRF) device. Part of the geochemical approach was novel, using the SEM-EDS to target titanomagnetite crystals within studied lava flows; attempting to distinguish between the flows using the geochemistry of the crystals. This technique is typically applied to differentiating between tephra deposits. The combination of the paleomagnetic and geochemical data provided a means of correlating and differentiating the lava flows of ancestral Burney Spring Mountain volcano. The results, particularly from paleomagnetic data, strongly support a short growth period for ancestral Burney Spring Mountain volcano. This has interesting implications for understanding the smaller monogenetic and polygenetic volcanism that define the Cascades magmatic axis. Ancestral Burney Spring Mountain volcano was a short-lived polygenetic stratovolcano that failed to achieve the size and maturity of the major stratovolcanoes that dominate the Cascade's skyline.