Kris Vasudevan

The big picture: A lithospheric cross section of the North American continent

A lithospheric cross section constructed within a 6000-kmlong corridor across southern Canada and its margins at 45– 55°N illuminates the assembly of the North American continent at an unprecedented scale. Based on coordinated, multidisciplinary research, the profile emphasizes lithospheric-scale relationships between orogens—plate collisions and accretions have sequentially stacked orogen upon orogen such that the older crust forms basement to the next younger. This largescale perspective highlights the similarities among crustal structures produced by orogenic processes despite the broad range of age from the Mesoarchean to the present. Heterogeneities in the lithospheric mantle suggest that, in certain situations, relict subducted or delaminated lithosphere can remain intact beneath, and eventually within, cratonic lithospheric mantle. In contrast, the dominantly subhorizontal Moho appears to be reequilibrated through mechanical and/or thermal processes; few crustal roots beneath orogens are preserved. INTRODUCTION A unique cross section of the North American continent represents a synthesis of more than two decades of coordinated research conducted by Lithoprobe, Canada’s national geoscience project. Based on existing interpretations within eight study regions, or transects, that are linked directly or by projection along strike, we have constructed a transcontinental lithospheric profile (Fig. 1 and poster insert). From west to east, this 6000-km profile crosses the Juan de Fuca oceanic plate, the active Cascadia subduction zone, the southern Cordillera (0.19 Ga–present), the Alberta and Trans-Hudson orogens (1.92–1.8 Ga), the Superior Province (3.82–2.60 Ga), the Mid-Continent Rift System (1.1–1.0 Ga), the Grenville orogen (1.19–0.99 Ga), the Newfoundland Appalachian orogen (0.47–0.28 Ga), the Grand Banks continental shelf, and the Atlantic passive margin (0.2 Ga). The diversity of tectonic history and ages included in the section facilitates direct comparison of the secular and spatial variation of orogenic processes. Data and interpretations are based on coordinated multidisciplinary research combined with a strong, steadily improving base of regional geotectonic knowledge. The structures displayed are primarily based on active-source seismic (reflection and refraction) data. However, the regional geometry and interpretations of the structure and tectonic processes utilize the full array of geological, geochemical, and geophysical data available for that region. Appendix 1 (see GSA’s supplemental data repository) summarizes how the cross section was constructed. A complete listing of references used to construct the cross section is provided in Appendix 2 (see footnote 2). In addition, Hammer et al. (2010) provide an in-depth description and two complementary lithospheric cross sections. The cross section is portrayed in terms of the “tectonic age” within the crust. We define this as the time since the most recent episode of significant tectonic deformation (Fig. 1 and insert [see footnote 1]). Tectonic age was chosen over more typical designations (e.g., geology or terranes/domains) because it simplifies the interpreted cross section to highlight comparative structures and to convey the sequence of orogenic development based on the current structural interpretations. In some areas, we chose to modify the tectonic age designations in order to convey key aspects of structure as well as the sequence of orogenic development based on current structural interpretations. For example, the Archean Sask, Hearne, and Superior continents were welded together in the Paleoproterozoic Trans-Hudson Orogen (1.92–1.80 Ga), yielding the core of the Laurentian craton. The largely unexposed Sask craton, discovered by Lithoprobe seismic studies (e.g., Lucas et al., 1993; Lewry et al., 1994; Hajnal et al., 2005), lies almost entirely beneath juvenile crustal imbricate structures. Although the Sask craton dates to 2.45–3.3 Ga, the lithospheric fragment was likely deformed by the Paleoproterozoic orogeny. However, to clarify its role in the assembly of Laurentia, we have chosen to label it with an Archean tectonic age but stippled to indicate Paleoproterozoic modification. Similar display procedures have been applied in other parts of the lithospheric cross section.

Application of the genetic algorithm to residual statics estimation

We present an application of a genetics-based optimization algorithm in an attempt to compute residual statics in seismic data processing. Genetic algorithms have a long history, but have only recently been applied to complex optimization problems. In this paper, optimization results are shown for a synthetic data set to test whether the genetic algorithm can obtain the correct qualitative and/or quantitative features.

Simulated annealing statics computation using an order-based energy function

The residual statics problem in seismic data analysis is treated by introducing an optimization function that emphasizes the coherence of neighboring common depth point (CDP) gathers within a nonlinear simulated annealing technique. This optimization criterion contrasts with stack power optimization which only considers the coherence between traces within a single CDP. Emphasizing coherence between CDPs removes many of the phase space degeneracies that result from stack‐power based optimization techniques. We have applied the method to both synthetic and real data sets, and initial results display significant improvement over the input data in the coherence of reflections, even in structurally complex areas.

Empirical mode skeletonization of deep crustal seismic data: Theory and applications

Seismic skeletonization is a pattern recognition technique used to decompose reflection seismic data into events or reflectors and their seismic attributes, and to store the results in an “event file” for an analysis toward seismic interpretation. The extraction of instantaneous frequency from the event file for any statistical analysis of the seismic attribute analysis is currently not possible, thus precluding the complete parameterization of the reflected seismic wave field in terms of its decomposition products. Empirical mode decomposition of reflection seismic data has uniquely addressed the question of computing the instantaneous frequency from its decomposition products using the Hubert transform. Although the decomposition products of the seismic skeletonization and the empirical mode decomposition are conceptually different, they share a common link to the primitive features of the seismic waveforms. In this paper, we introduce a new decomposition technique, empirical mode skeletonization, that combines the features of both seismic skeletonization and empirical mode decomposition. By this process, it is now possible to subject the reflection seismic data to an analysis that could include a parameterization of the reflected wave field. We apply the new technique to a segment of seismic line 1 of the Lithoprobe Slave Northern Cordillera Lithosphere Evolution (SNORCLE) transect to extract event-oriented instantaneous frequency attributes and also to analyze the decomposition products for any scaling behavior of the reflected wave field.

Residual statics estimation using the genetic algorithm

An optimization problem as complex as residual statics estimation in seismic image processing requires novel techniques. One interesting technique, the genetic algorithm, is based loosely on the optimization process forming the basis of biological evolution. The objective of this paper is to examine this algorithm’s applicability to residual statics estimation and present three new ingredients that help the algorithm successfully resolve residual statics. These three ingredients include (1) breaking the population into subpopulations with restricted breeding between the subpopulations, (2) localizing the search, to varying degrees, about the uncorrected input stack, and (3) modifying the optimization function to take account of CDP‐dependent structural features. Introducing subpopulations has the effect of enhancing the search when the volume of phase space being searched is large and limited information is given about where the algorithm should concentrate its efforts. Subpopulations work well initially,...

Seismic skeletonization: A new approach to interpretation of seismic reflection data

Automatic identification and isolation of events on seismic reflection data provides the basis for a new approach to seismic interpretation. The approach uses a syntactic pattern recognition method in which an objective function accommodates near-neighbor and non-neighbor trace cycle correlation and in which correlations between three traces (triplets), two traces (doublets), and single traces (singlets) are all included in configurations. The method yields a file that includes a listing of all identified events and their characteristics such as length (number of traces), dip, frequency, and amplitude. Thus, for the first time, these data can be statistically analyzed according to the relational attributes of seismic events.

Skeletonization of aeromagnetic data

Skeletonization is a syntactic pattern-recognition method that is applied to gridded data to produce an automatic line drawing, with an associated event catalog. Previous implementations of skeletonization have been tailored for seismic data. Here, we modify that technique to render it more suitable for other types of gridded data, with particular emphasis on aeromagnetic maps. A modification from previous schemes is the use of a two-pass approach, to reduce the effects of an otherwise problematic directional bias that discriminates against events oriented parallel to columns of the grid. The method can be used effectively for filtering aeromagnetic data on the basis of strike direction, event linearity, event amplitude, and polarity. It is based on the delineation of peak-trough pairs (cycles), which are traced throughout the grid to form contiguous events. Cycles and events are characterized by attributes that include amplitude, polarity, and pulse width. Events are further characterized by length, average strike direction, and linearity. The event attributes are stored in a catalog, thus enabling one to perform attribute-based analysis and data filtering. We illustrate our algorithm using two regional aeromagnetic examples from different parts of the Canadian Shield. The first, from the Great Slave Lake shear zone, is dominated by linear anomaly trends produced by faults and mafic dikes. The second, from the Manicouagan region of northeastern Quebec, contains abundant subcircular and arcuate anomaly patterns caused by large intrusive complexes and a meteorite impact structure.

Skeletons and fractals — a statistical approach to deep crustal seismic data processing and interpretation

Abstract Coherent seismic reflection events are identified using seismic skeletonization, an automatic pattern recognition technique in which waveforms are parameterized such that the attributes of each event are identified and stored in a relational database, or ‘event file’. This new approach to seismic interpretation allows statistical analyses of the attributes including estimation of scaling laws for reflectivity observed on deep seismic data, and development of new filtering techniques based on characteristics such as dip and reflection event length. Applications of the technique to deep crustal reflection data sets from the Alberta Basement Transect of the LITHOPROBE project assist interpretation in areas that lack surface outcrop or other means of relating reflections from the deep crust to the surface.

Identification and interpretation of azimuthally varying crustal reflectivity with an example from the southern Canadian Cordillera

A new approach to seismic interpretation, seismic skeletonization, is used to analyze crustal reflection data by identifying numbers of reflection events, their lengths, their energies, and their dips, with respect to azimuths of recorded profiles. Application of the method to crustal reflection data in the southern Canadian Cordillera demonstrates strong directional dependence of crustal reflectivity, with persistent trends at 025–040° (205–220°), 085–095° (265–275°), 115–125° (295–305°), and 140–160° (320–340°). These trends are visible in both the upper and lower crust and may be related to regional structural trends. Further applications of the technique, particularly in different tectonic regions, may allow categorization of seismic patterns and cataloguing of signatures for complex reflection data in a manner similar to seismic stratigraphy and structural patterns in stratified rocks.

A comparison of several cooling schedules for simulated annealing implemented on a residual statics problem

A study of several cooling schedules for the simulated annealing optimization algorithm, as applied to a residual statics problem in seismic data processing, is undertaken and discussed. A variety of schedules which depend on user-controlled parameters and an adaptive annealing schedule which depends on the system itself are considered. The algorithm, with each schedule implemented in turn, is tested on a residual statics estimation where the optimization function is a coherence function between adjacent common depth point (CDP) gathers. Results of this study suggest the usefulness of a hybrid cooling schedule to reduce the number of cycles. Constrained adaptive annealing schedule offers a viable alternative.