Evidence for Slab Rollback, Back-Arc Rifting and Arc Dismemberment During Assembly of Western Laurentia

<p>The crystalline crust that underlies the Western Canada Sedimentary Basin in northern Alberta is composed of tectonic domains that accreted to the margin of the Archean Rae province of western Laurentia, ca. 2.1-1.9 Ga. Geophysical data indicate that the basement crust in this region hosts a vast, mid-crustal reflection sequence (Winagami Reflection Sequence) interpreted as assemblage of mafic sills and an unusually wide domain of Paleoproterozoic magmatic arcs (Taltson Magmatic Zone). The latter are interpreted to have formed during Paleoproterozoic tectonic assembly through near-synchronous closure of small oceanic basins along subduction systems of opposing polarity. Here, we introduce a new tectonic model, which postulates that the Taltson Magmatic Zone represents collated fragments that formed within a single subduction system. Comparison with modern analogs suggest that observed temporal relationships and present-day configuration of Paleoproterozoic arcs can be explained by plate-margin processes of slab rollback and back-arc rifting. Our model is consistent with re-interpreted basement-drillcore petrology, provides a genetic link for the association between magmatic arcs and the Winagami sill complex, explains an extraordinary fit between aeromagnetically defined “conjugate margins” and provides a tectonic framework for the origin of the enigmatic low<sup>-δ18</sup>O magmatic zone (Kimiwan anomaly) along the southern Chinchaga domain.</p>

Time-frequency analysis of deep crustal reflection seismic data using Wigner-Ville distributions

One important component of deep crustal reflection seismic data in the absence of drill-hole data and surface-outcrop constraints is classifying and quantifying reflectivity patterns. One approach to this component uses a recently developed data-decomposition technique, seismic skeletonization. Skeletonized coherent events and their attributes are identified and stored in a relational database, allowing easy visualization and parameterization of the reflected wavefield. Because one useful attribute, the instantaneous frequency, is difficult to derive within the current framework of skeletonization, time–frequency analysis and a new method, empirical mode skeletonization, are used to derive it. Other attributes related to time–frequency analysis that can be derived from the methods can be used for shallow and deep reflection seismic interpretation and can supplement the seismic attributes accrued from seismic skeletonization. Bright reflections observed from below the sedimentary basin in the Southern Alberta Lithosphere Transect have recently been interpreted to be caused by highly reflective sills. Time–frequency analysis of one of these reflections shows the lateral variation of energy with instantaneous frequency for any given time and the lateral variation of energy with time for any instantaneous frequency. Results from empirical mode skeletonization for the same segment of data illustrate the differences in the instantaneous frequencies among the intrinsic modes of the data. Thus, time–frequency distribution of amplitude or energy for any signal may be a good indicator of compositional differences that can vary from one location to another.

Turning waves and crustal reflection profiling

In the past, steeply dipping features were often recognized on seismic reflection profiles only from indirect evidence such as vertical offsets of cross‐cutting structures. New imaging algorithms, as for example, turning wave migration have had dramatic success in delineating steep, even‐overturned reflectors in sedimentary environments. Evaluation of the applicability of this technology to deep seismic recordings indicates that steep‐dip and turning wave migration will have limited practicality, generally, in the imaging of basement features because of the weak velocity gradients involved and the corollary requirement for large recording offsets. A potential exception arises when the basement structures to be imaged lie beneath a significant thickness of relatively young (i.e., steep velocity gradient) sedimentary cover.