A.G. Green

Crustal structure of the Grenville front and adjacent terranes

Under the auspices of the Great Lakes International Multidisciplinary Program on Crustal Evolution, approximately 320 km of deep seismic reflection data were collected in Lake Huron along a profile that extends east from the Manitoulin terrane across the Grenville front to the interior of the Grenville orogen. The Manitoulin terrane is characterized by a series of gently east-dipping reflections at about 20 km depth that separate a highly reflective lower crustal layer from a markedly less reflective upper layer. Imaged by strong reflections at the western end of a spectacular band of moderately east-dipping reflections, the Grenville front clearly truncates Manitoulin terrane structures to the west. These data are interpreted in terms of a speculative two-stage model involving (1) creation of a major decollement during northward collision of an allochthonous terrane with the southern Superior cratonic margin (1.83-1.89 Ga; Penokean orogeny) and (2) northwest-directed stacking of microterranes at the southeastern margin of the craton, followed by crust-penetrating ductile imbrication under high-pressure-high-temperature conditions leading to the ramping of deeply buried rocks to the near surface (1.0-1.3 Ga; Grenvillian orogeny).

Deep geometry of the Sudbury structure from seismic reflection profiling

Seismic reflection data show the deep geometry of the Sudbury structure to be markedly asymmetric. The Sudbury North Range comprises shallowly south dipping sedimentary strata overlying a thick unit of heterolithic breccia and melt bodies, layered rocks of the Sudbury igneous complex, and footwall gneiss. Reflections from the upper layers are interrupted by faults near the center of the Sudbury basin, whereas the basal mafic units and footwall gneiss continue dipping southward and appear to be tightly folded or truncated near the Creighton fault. In contrast, the seismic image of the South Range is dominated by a distinctive series of moderately south dipping reflections interpreted as thrust faults or shear zones on which severe telescoping and imbrication of lithological units and considerable northwest-southeast shortening of the Sudbury structure have occurred. Such shortening explains the noncircular shape of the structure, and thus removes a critical objection to the Sudbury impact crater hypothesis. The new structural model is shown to be consistent with regional magnetic and gravity fields; a large hidden ultramfic-mafic mass is no longer required to explain the potential field data.

Crustal structure of the Midcontinent rift system: Results from GLIMPCE deep seismic reflection profiles

Interpretation of Great Lakes International Multidisciplinary Program on Crustal Evolution (GLIMPCE) seismic reflection profiles indicates that the Midcontinent (Keweenawan,1100 Ma) rift system of volcanic rocks and postvolcanic and interbedded sedimentary rocks extends to depths as great as 32 km (about 10.5-s reflection time) along profiles crossing western, central, and eastern Lake Superior and the northern end of Lake Michigan. This area may overlie the greatest thickness of intracratonic rift deposits on Earth. Times to Moho reflections vary along strike from 11.5 to 14 s (about 37-46 km depth) in the west, to 17 s (about 55 km) in the center, and 13 to 15 s (about 42-49 km) in the eastern end of Lake Superior. The prerift crust, however, was thinned 25-30 km beneath the central rift (compared with its flanks), providing evidence for crustal extension by factors of about 3-4. The Midcontinent rift system differs from Phanerozoic rifts in having total crustal thicknesses equal to or greater than the surrounding (presumably unextended) regions.

GLIMPCE Seismic reflection evidence of deep-crustal and upper-mantle intrusions and magmatic underplating associated with the Midcontinent Rift system of North America

Abstract Deep-crustal and Moho reflections, recorded on vertical incidence and wide angle ocean bottom Seismometer (OBS) data in the 1986 GLIMPCE (Great Lakes International Multidisciplinary Program on Crustal Evolution) experiment, provide evidence for magmatic underplating and intrusions within the lower crust and upper mantle contemporaneous with crustal extension in the Midcontinent Rift system at 1100 Ma. The rift fill consists of 20–30 km (7–10 s) of basalt flows, secondary syn-rift volcaniclastic and post-basalt sedimentary rock. Moho reflections recorded in Lake Superior over the Midcontinent Rift system have times from 14–18 s (about 46 km to as great as 58 km) in contrast to times of about 11–13 s (about 36–42 km crustal thickness) beneath the surrounding Great Lakes. The Seismically complex deep-crust to mantle transition zone (30–60 km) in north-central Lake Superior, which is 100 km wider than the rift half-graben, reflects the complicated products of tectonic and magmatic interaction of lower-crustal and mantle components during evolution or shutdown of the aborted Midcontinent Rift. In effect, mantle was changed into crust by lowering Seismic velocity (through intrusion of lower density magmatic rocks) and increasing Moho (about 8.1 km s−1 depth.

Origin of deep crystal reflections: seismic profiling across high-grade metamorphic terranes in Canada☆

Abstract In an attempt to better understand the origin of deep crustal reflections LITHOPROBE has sponsored or co-sponsored Seismic reflection surveys across tracts of high-grade metamorphic rock in the Archean Superior craton, the Proterozoic Grenville orogen and the Phanerozoic Cordilleran orogen. Common to these three diverse terranes are near-surface zones of prominent Seismic reflectivity that are typically associated with velocity discontinuities at highly strained contacts between gneissic rocks of varying lithology. At some locations the reflective layering resulted from transposition and rearrangement of previously layered rocks (stratified assemblages, sills, etc.), whereas in other regions it was generated by extreme attenuation, stretching and ductile flow of weakly layered or irregularly organized rocks. It seems likely that compositionally layered gneissic rock is a common source of reflections in the deep crust, with reflections originating at lithological boundaries and zones of mylonite.

Geophysical investigations and crustal structure of the North American Midcontinent Rift system

Abstract Geophysical investigations have had a prominent role in studying the 1100 Ma Midcontinent Rift system because of limited surface exposures and deep drilling of the rift rocks. Gravity and magnetic anomaly data delineate the rift system—its central volcanic-filled graben and overlying and adjacent post-rift sedimentary basins—and the major faults of the system over its 2000 km length in central North America. Crustal seismic studies indicate a high-velocity crust that is thickened to as much as 55 km along the rift. Deeply-penetrating seismic reflection data obtained across the rift show that the crust was thinned to less than one-half of its original thickness during rifting and the present crust has been thickened by addition of up to a total of 30 km of mantle-derived volcanic rocks within grabens and clastic sediments deposited in overlying sag basins as well as by intrusions into the crust and crustal underplating. These data also confirm the occurrence of a post-rift regional compressional event that locally caused broad folding of the rift rocks and reactivation of the graben normal faults as high-angle reverse faults with vertical throws of several kilometers. Geophysical data indicate that the rifting event had a profound effect on the entire vertical range of the crust and that structures, and thus the tectonic evolution, are similar along the entire length of the rift.

Evolution of Proterozoic terrains beneath the Williston Basin.

A new tectonic map of the Precambrian basement beneath the Phanerozoic Williston Basin of central North America shows the Archean Superior craton separated from the Archean Churchill and Wyoming cratons by a broad region of mostly Proterozoic origin. Included in the Proterozoic terrain are the vestiges of two or more volcanic island arcs and associated fore-arc and back-arc basins. The North American Central Plains electrical conductor and related seismic low-velocity zones are explained by the presence of hydrated oceanic-type crustal material within the remnant back-arc basins and adjoining terrains. Using geological, geophysical, and geochronological data from the Williston Basin and exposed Canadian Shield, we derive a plausible plate-tectonic model for the Proterozoic evolution of this region.

Thin thrust sheet formation of the Kapuskasing structural zone revealed by Lithoprobe seismic reflection data

Regional and high-resolution seismic reflection data across the Kapuskasing structural zone in Ontario, Canada, image at least three significant thrust faults that are low angle, merge into a flat detachment on the west, and together were responsible for the uplift of amphibolite and granulite facies rocks. Their geometry resembles a ramp-and-flat style of deformation that results in a thin upper plate above the 10-12 km (about 4.0 s) detachment. Northwest-southeast horizontal shortening is estimated to be at least 55 km. This large amount of shortening implies that much of the Superior province was detached during the formation of the Kapuskasing structural zone.

Deep crustal structure of the Precambrian basement beneath northern Lake Michigan, midcontinent North America

A deep seismic-reflection profile in northern Lake Michigan, midcontinent North America, provides a cross section of the crust across the 1850 Ma Penokean orogen, in which an Early Proterozoic island-arc complex was deformed between two converging Archean continental masses. The island-arc crust is about 40 km thick and has a few kilometres of intensely reflective rocks near its base, above which it is variably reflective to transparent. The Archean terranes have thicker crust, as much as 50 km, the lower 20-25 km of which is strongly reflective. Abrupt offsets of Moho near terrane boundaries may have been preserved since accretion during the Penokean orogeny.

Pre- and poststack migration of GLIMPCE reflection data

Abstract GLIMPCE deep Seismic reflection profiles across the Midcontinent Rift System beneath Lake Superior reveal a central asymmetric rift with an enormous thickness of volcanic and sedimentary rocks. True amplitude cmp-processing, poststack and prestack migration and forward modelling are used to improve images of steeply dipping faults, unconformities and other discontinuities in the deep Seismic data. With prestack migration important steeply dipping structural features of the Lake Superior data set are revealed for the first time. Improved images of high-angle normal faults, later reactivated as reverse faults, provide key structural information for understanding the evolution of the rift system. Results illustrate that structural interpretations of complex deep reflection records, such as those recorded by GLIMPCE, should always be based on migrated data. Furthermore, depth-migrated sections provide useful starting models for forward velocity modelling of Seismic data.