Graduate Thesis Or Dissertation

 

Structure of the crust beneath Lake Superior from forward modeling of large aperture seismic data Public Deposited

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  • Both travel times and amplitudes of large offset refracted and reflected arrivals observed during GLIMPCE (Great Lakes International Multidisciplinary Program on Crustal Evolution) along line A in Lake Superior have been modeled using two-dimensional ray tracing techniques. Forward modeling was used to iteratively refine an initial velocity model that was constructed from results of tau-sum analysis of the travel time data at each station combined with information from the coincident common depth point (CDP) reflection profile. When converted to time the resulting model agrees quite well with the CDP reflection profile. A 50-100 ms time advance anomaly associated with the Isle Royal Fault is observed at every station. This anomaly has been modeled as shallow, high velocity blocks located directly beneath the fault. The blocks correlate well with the walls of a steep-sided bathymetric trough and are believed to represent highly indurated upper Keweenawan sediments which may have resulted from hydrothermal alteration. Approximately 2 km of sedimentary rock (2.8-4.6 km/sec) overlie an 8 km thick sequence of volcanics and interflow sediments (5.0-6.5 km/sec) within the rift graben observed on the reflection data. Beneath this sequence is a 6-8 km thick sequence of 6.6-7.0 km/sec material that is interpreted to represent metamorphosed volcanics. The velocity of the material at the base of the rift graben is not well constrained (approximately 7.0 -7.2 km/sec), but probably comprises an additional 10-12 kilometers of meta-volcanic rocks and intrusions that extend to the base of the graben as imaged on the CDP reflection profile. Boundaries between these sequences are indicated by reflections observed at several of the wide-aperture stations. A marked decrease in the apparent velocity and amplitude of the first arrivals is observed on reversed sections at ranges exceeding 100 km. This decrease in apparent velocity has been modeled as lower velocity continental crustal rocks (approximately 6.5 km/sec) at a depth of about 15-20 km adjacent to the 7.0 km/sec material in the graben. Calculation of the gravity response of the seismic model demonstrates that the gravity high centered over the rift can be entirely attributed to high density rocks occupying the central half-graben imaged on the CDP profile. Wide angle reflections from about 15-30 km depth beneath the flanks of the graben indicate the presence of velocity discontinuities that may represent rift related detachment surfaces and/or pre-rift structures. Modeling of wide angle reflections indicate a high degree of structural relief preserved within the lower crust. The high velocities modeled for this region, coupled with information from the CDP profile, suggest that the lower crust represents Archean crust that has been either heavily intruded or underplated by mafic magma. The style and volume of volcanic emplacement is similar to that of Phanerozoic rifted continental margins and flood basalt provinces. By analogy, the volcanism within the midcontinent rift appears to have resulted from decompression melting during lithospheric extension above a broad, asthenospheric thermal anomaly recently referred to in the literature as the "Keweenawan hot spot".
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