B. Milkereit

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).

Asymmetric deep crustal structure across the Juan de Fuca Ridge

A multichannel line was shot across the Endeavour segment of the Juan de Fuca Ridge. Careful processing of 25 km of the line centered about the rise axis shows that young crustal structure is asymmetric. An intermittent reflector from the top 1 km of crust follows topography on both the Pacific and the Juan de Fuca plates. It could be either the pillow basalt-diabase dike contact or a metamorphic front within the pillow basalts. A reflection exists 2-3 km below the rift-valley floor; diffractions from its edges indicate that the rift valley9s velocity structure is markedly different from the structure of the flanks. The deep crustal structure is highly asymmetric. Moho exists within 4 km of the rift valley on the Pacific plate, in contrast to the Juan de Fuca plate, which shows a reflector at intermediate depths dipping away from the rise to reach Moho depth 12 km from the rift valley. This reflector9s origin is enigmatic; we speculate that the thermal history and structure of the two plates differ because of local differences in mantle structure caused by either ridge migration or a melting anomaly.

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.

Paleoproterozoic collisional orogen beneath the western Canada sedimentary basin imaged by Lithoprobe crustal seismic-reflection data

Exceptionally clear images of crustal structure of the Canadian Shield that underlies the western Canada sedimentary basin beneath 3.5–2.2 km of Phanerozoic sedimentary strata have been obtained on a seismic-reflection profile acquired by Lithoprobe. The profile crosses tectonic domains of central Alberta and delineates a major buried orogenic belt of Paleoproterozoic (∼1.8 Ga) age associated with crustal scale thrust imbrication and deflections in the crust-mantle boundary. Available geochronologic data suggest that crustal imbrication observed in the Alberta basement was coeval with that documented in the Trans-Hudson orogen to the east (1.80–1.83 Ga) and implies that a large region of continental crust, extending >1000 km from the western Superior province to the Snowbird tectonic zone, underwent considerable shortening during assembly of this part of the Canadian Shield.

A three dimensional perspective on the evolution of Archaean crust: LITHOPROBE seismic reflection images in the southwestern Superior province☆

In 1990–1991 the LITHOPROBE project completed 450 km of seismic reflection profiles across the late Archaean crust of the southwestern Superior province. The results define a broad three-fold division of crust: upper crust in the Abitibi greenstone belt is non-reflective and is a 6–8 km veneer of volcanic and plutonic supracrustal rocks, whereas, in the sediment-gneiss dominated Pontiac subprovince, upper crust comprises shallow northwest-dipping turbidite sequences; mid-crust, in both the Abitibi and the Pontiac subprovinces, is interpreted as imbricate sequences of metasedimentary and metaplutonic rocks; lower crust in both subprovinces has a horizontal layer parallel strycture which may represent interleaved mafic-intermediate gneisses. The seismic signature of the northern Abitibi greenstone belt may be represented in an exposed 25 km crustal section in the Kapuskasing stuctural zone. Preliminary tectonic models based on the seismic data are consistent with a plate-tectonic scenario involving oblique subduction and imbrication of sedimentary, plutonic and volcanic sequences. The northern Abitibi supracrustal sequences either represent an allochthon, or overlie an allochthonous underthrust metasedimentary and plutonic sequence which may be equivalent to a metasedimentary subprovince such as the Pontiac or Quetico. Seismic velocities have yet to be defined. However, crustal thicknesses are relatively constant at 35–40 km. The thinnest crust is adjacent to the Grenville Front where Moho is very well defined.

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.

Integrated seismic and borehole geophysical study of the Sudbury Igneous Complex

Reflection seismic and borehole geophysical data place important constraints on the subsurface geometry of the Sudbury Structure, which is the site of the worlds largest Ni-Cu camp. Seismic reflections can be traced from outcrop within the Sudbury North Range to about 4.5 km depth beneath the center of the Sudbury Basin, where the layer thickens abruptly from 1 to 3 km. Further south the North Range norite can be followed to about 10 km depth beneath the South Range. Borehole studies show systematic variations of p- and s-wave velocity, Poissons ratio and density within the Igneous Complex. Quartz-rich granophyre is distinguished from the norite and footwall rocks by relatively low Poissons ratios (0.20–0.23 versus 0.23–0.25). These changes in physical rock properties define an important subdivision of the Igneous Complex, compatible with a simple model involving differentiation of melted crustal rock into dominantly felsic and mafic components. This study documents the importance of interlayering to the seismic reflection response of the crystalline crust.

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.

Towards an improved seismic imaging technique for crustal structures : the Lithoprobe Sudbury experiment

Under the Canadian Lithoprobe program, conventional low- (10–55 Hz) and high-frequency (30–140 Hz) vibroseis reflection surveys were conducted across the Sudbury Structure. The main objective of the study was to evaluate the performance of seismic exploration techniques in a complex, mainly crystalline geological setting. The Sudbury experiment demonstrated that high input frequencies can be preserved through appropriate acquisition and processing procedures. Consequently, the high-frequency seismic data proved capable of imaging complex shallow crustal structures not ‘seen’ by the conventional low-frequency seismic survey. In crystalline environments, improved high-frequency seismic exploration technology will provide a better link between seismic images and surface geology.