Browsing by Author "Johnson, Eneanwan Ekpo"

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    Geophysical constraints on basement faulting in west-central Alberta: Implications for induced seismicity and post-collisional modification of western Laurentia
    (2020-06-29) Johnson, Eneanwan Ekpo; Eaton, David W.; Gilbert, Hersh; Nair, Rajeev; Lawton, Don Caleb; Unsworth, Martyn J.
    Tectonic elements that make up the core of the Canadian Shield were formed during collisional assembly of the craton, Laurentia, between 2.0 Ga to 1.75 Ga. In west-central Alberta, the Precambrian crystalline crust that underlies the Western Canada Sedimentary Basin (WCSB) is dominated by curvilinear arcuate domains that were welded onto the edge of the Archean Rae Province ca. 2.0 -1.9 Ga. Evidence from various sources, including geophysical data acquired as part of Canada’s LITHOPROBE program, indicates that faults in the Precambrian crystalline basement exerted a profound influence on the Phanerozoic development of the sedimentary cover. In the upper crust, however, these faults may be (near) vertical, rendering them difficult to image using conventional seismic methods. The importance of identifying and delineating basement faults has recently come into sharper focus due to the localized onset, about one decade ago, of induced seismicity from unconventional resource development. This long-standing problem is revisited in this thesis using a multidisciplinary approach that incorporates modelling of potential-field data, investigation of duplex waves in LITHOPROBE seismic shot records, and petrographic analysis of drillcores. In one area that is prone to induced seismicity near Fox Creek, Alberta, aeromagnetic anomaly patterns reveal conspicuously rectilinear boundaries along the northern and southern margins of the southern Chinchaga domain (SCD). These margins have near-perfect (jigsaw-like) conjugate geometry and appear to bound a regionally extensive mid-crustal sill complex known as the Winagami Reflection Sequence (WRS) on seismic reflection profiles. To explain these features, I propose that the SCD may have formed as a pull-apart basin in a back-arc setting. Modern back-arc extension analogs include the Omineca belt in the Canadian Cordillera, the Basin and Range in southwest US and the Japan Sea. Restoration of putative extension implied by this model aligns two otherwise dismembered continental arcs, suggesting a simpler model for tectonic evolution for northern Alberta basement than previously hypothesized models that focus primarily on collisional assembly. Examination of sparse drillcore from the SCD indicates that basement composition contains undeformed mafic igneous rocks, consistent with a back-arc extensional setting. A north-south linear magnetic high that underlies the region of recent occurrence of induced seismicity, herein named the Simonette Anomaly (SA), is interpreted as the expression of a strike-slip fault that was active as a transform fault during back-arc extension. This feature is inferred to be the basement root to shallower fault segments that have been reactivated by hydraulic fracturing. A regional framework for this interpretation has been developed through modelling and analysis of regional gravity and magnetic data. In addition, I assess the feasibility to delineate near-vertical basement faults using duplex waves, which are waves that reflect from a vertical boundary as well as a deeper horizontal reflector. Probable duplex waves observed in a LITHOPROBE seismic shot record, where the WRS reflections furnish the deeper horizontal reflection, provide the first documented case where a near-vertical fault in the crystalline basement has been seismically imaged. Taken together with the proposed back-arc rifting model, this thesis thus provides new evidence that is significant for understanding post-collisional modification of western Laurentia.