Analysis and interpretation of magnetic anomalies observed in north-central California Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/wd375z39v

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  • To assist in the assessment of the geothermal potential of north-central California and to aid in defining the geologic transitions between the physiographic provinces of the Klamath Range, the Cascade Range, the Modoc Plateau, the Great Valley, and the Sierra Nevada Range, personnel from the Geophysics Group in the College of Oceanography at Oregon State University conducted a detailed aeromagnetic survey extending from 40°15' to 42°00'N latitude and from 120°45' to l22°45'W longitude. Two forms of spectral analysis, the energy spectrum and the exponential methods, were used to make source-top and source-bottom depth calculations. The magnetic source-bottom depths were interpreted as Curie-point isotherm depths. Based on the energy spectrum analysis, several regions with elevated Curie-point isotherm depths were mapped: (1) the Secret Spring Mountain-National Lava Beds Monument area, (2) the Mount Shasta area, (3) the Big Valley Mountains area, and (4) an area northeast of Lassen Peak. The elevated Curie-point isotherm depths within these areas, as shallow as 4 to 7 km below sea level (BSL) in the Secret Spring Mountain-National Lava Beds Monument area, the Mount Shasta area, and the area northeast of Lassen Peak, and 4 to 6 km BSL in the Big Valley Mountains area, imply vertical temperature gradients in excess of 70°C/km and heat flow greater than 100 mW/m² when assuming a Curie-point temperature of 580°C. Shallow source-bottom depths of 4 to 5 km BSL were mapped in the Eddys Mountain area and interpreted to be the depth of a lithologic contact. Source-top depths show that the magnetic basement varies from about 3.5 km BSL, beneath the sedimentary assemblages of the Great Valley and the eastern Klamath Range, to near sea level in the Cascade Range. The exponential approximation method yielded source-bottom depths which agreed, in general, with depths determined by the energy spectrum method. However, this method appears less reliable and its depth estimates less accurate compared to the energy spectrum method. A broad negative anomaly, observed on the total field magnetic intensity map and low-pass filtered anomaly maps, suggests the sedimentary rocks of the Klamath Complex underlie Mount Shasta and the Medicine Lake Highlands. Magnetic lineations are oriented mainly NW.
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