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Workshop Review GEOSCIENCE CANADA Volume 31 Number 1 March 2004 23 WORKSHOP REVIEW PanLITHOPROBE synthesize information from their Participants were provided with a respective areas of expertise, and to list of parameters considered to be Workshop IV: Intra- compare orogenic evolution over large significant in describing orogenic Orogen Correlations and tracts of ground and a wide swath of structure and history, and were asked to Comparative Orogenic geological time. compile characteristics (in bold below) Anatomy J.A. Percival1, W. Bleeker1, F.A. Cook2, T. Rivers3, G. Ross4, C. van Staal1 1Geological Survey of Canada 601 Booth St., Ottawa ON K1A 0E8 2Dept. of Geology and Geophysics University of Calgary, Calgary, AB T2N 1N4 3Dept. of Earth Sciences Memorial University of Newfoundland St. John’s, NF A1B 3X5 4Geological Survey of Canada 3303 33rd St. N.W., Calgary, AB T2L 2A7 present address: Box 458, Kula HI 96790 INTRODUCTION Canada’s diverse continental landmass has long been recognized as a collage of distinct orogenic provinces (e.g. Price and Douglas 1972). Work over the past twenty years during the tenure of the LITHOPROBE program has revealed intriguing similarities and differences in orogens formed during the four billion year record of geological time. Armed with common geological, geophysical, geochemical and geochronological observations from each orogen, Earth scientists can now focus on key orogenic parameters to explore how crust- and mantle-forming processes may have Figure 1 Precambrian tectonic domains for North America classified using U-Pb ages and Nd changed over time. At a recent signature. The Sri (initial 87Sr/86Sr) line in the Cordillera marks the western edge of thick PanLITHOPROBE workshop, Precambrian crust (from Armstrong, 1988). EAO: East Alberta orogen; MRV: Minnesota River representatives working on the major valley gneiss; Mak-Ket: Makkovik-Ketilidean. Modified from Hoffman (1989). Detailed orogenic provinces of Canada legend for Paleoproterozoic units is given in Figure 8; cross pattern indicates Archean crust. investigated by LITHOPROBE LITHOPROBE transects: AB: Alberta basement; AG: Abitibi-Grenville; ECSOOT: Eastern (Cordillera, Appalachians, Grenville, Canadian Shield onshore-offshore transect; KSZ: Kapuskasing structural zone; LE: Lithoprobe Paleoproterozoic, Superior, Slave east; SNORCLE: Slave-Northern Cordillera Lithospheric evolution; SC: Southern Cordillera; orogens; Fig. 1) met to distill and WS: Western Superior; THO: Trans-Hudson orogen. 24 of their orogen of interest within this 50 discussion topics. These were as regional late extension has not been common template. Beginning with assembled and digested by a panel recognized there and the regional physical parameters, the list also (Chris Beaumont, Jeremy Hall, Karl metamorphic grades of surface rocks do included important aspects of orogenic Karlstrom, Ray Price), who then led a not indicate similar large uplifts. and postorogenic evolution. Crustal discussion on comparative orogenic Estimates of lithospheric thickness range thickness and its variation describe the anatomy on the final day of the from 50 to 60 km in the Intermontane end result of major crust-forming workshop. Belt, where high heat flow is interpreted events, as well as evidence of “scars” to represent high temperatures at depth, produced during the process. Crustal CORDILLERA to >100 km in the Foreland Belt and structure, composition, seismic velocity F.A. Cook (on behalf of the Cordillera east of the Cordillera. and reflectivity define many first-order working group: S. Carr, R. Clowes, The oldest rocks in the characteristics of the orogenic crust. D. Francis, L. Hollister, R. Hyndman, Cordillera (~2.1 Ga) are present in Mantle lithospheric structure and A. Jones, R. Price, G. Ross, D. uplifts that were exposed as a result of composition incorporate information Symons, D. Thorkelson) post-contractional extension and from geophysical and xenolith studies The Canadian portion of the provide a temporal link with the sub- constraining the nature of the shallow Cordillera in western North America is Phanerozoic rocks of the western orogenic mantle. The nature of orogen one of the classic orogens of the world. Canadian Shield. These are generally margins (e.g. continents) is important in It is bounded on the east by a stable interpreted as remnants of the cratonic assessing whether orogens had “rigid craton that was extended during crust that was rifted during backstops” and conversely, the degree of Proterozoic and Paleozoic rifting events Mesoproterozoic extension, the eastern orogenic reworking. The age and style of and on the west by the actively limit of which may be reflected in a rifting describe the nature of the deforming convergent/transform margin prominent series of west-facing crustal- continental breakup phase preceding of the eastern Pacific Ocean (Gabrielse scale homoclines beneath the Foreland orogeny. The nature and age of accreted and Yorath, 1991). The Cordillera is Belt. The nature and style of the earliest terranes provide insight into processes thus the youngest orogen in which rifting is preserved in thick strata of the and paleogeography preceding terrane LITHOPROBE has worked and Wernecke Supergroup (ca.1.8–1.7 Ga) accretion. The nature and age of portions of two major transects have in the north and the Belt-Purcell continental magmatic arcs give been devoted to studying it (Southern Supergroup (ca. 1.5–1.4 Ga) in the information on the character and Canadian Cordillera Transect, 1984– south, and may have been diachronous longevity of subduction preceding 1995; Cook, 1995, and Slave from north to south. Whether these collision. The location and nature of NORthern Cordillera Lithospheric rocks were deposited in deep, paleo-suture zones point to zones where Evolution, or SNORCLE, transect, intracratonic basins, or whether they oceanic crust has been consumed. The 1995–2003; Fig. 1), thus providing a were open to large oceans is still age and style of accretion, including basis for comparisons along strike. debated. There is evidence for periods collisional geometry, explore the three- Geomorphologic belts are used for of pre-Cordilleran contractional dimensional form (thin-skinned vs thick- simplicity in referencing the deformation within these and other skinned), kinematics and duration of descriptions (Fig. 2). Precambrian strata throughout the collisional processes. Current or In the southern Cordillera Cordillera, but the regional extent and paleotopography and evolution of P-T (Price, 1981), the crustal thickness paleogeographic significance of such conditions through time describe vertical varies from >40 km in the Foreland Belt deformation are uncertain. crustal movements during orogenesis as to 33 to 35 km beneath the Omineca Neoproterozoic–Cambrian rifting well as the movement of heat through Belt. The transition occurs in the initiated the development of the the crust. The age and style of post- vicinity of the southern Rocky westward-thickening Cordilleran collisional processes provide insight into Mountain Trench (Fig. 2), west of miogeocline. the final processes that have shaped which the thickness remains uniform to Continental basement and cover orogenic crust. Variation in the the Coast Belt, where it thickens again rocks are traced in the crust beneath development and/or preservation of by a few kilometres. The northern much of the Cordillera. In the north, these parameters explains many of the Cordillera exhibits the same gross the SNORCLE reflection data have differences between Canada’s orogenic crustal structure, except that the been interpreted as having images of the provinces. thickest crust occurs ~100 to 150 km thick supracrustal strata and their Each working group compiled east of the deformation front. The 33 basement projecting as far west as the background material in advance of the to 35 km crustal thickness of the British Columbia–Alaska border (Cook meeting, then spent one to two days interior southern Cordillera is et al., unpublished work). In the south, preparing a short synthesis (Intra-orogen commonly explained as a consequence some basement rocks that were correlations) presented in a plenary of Paleocene–Eocene extension and metamorphosed to amphibolite grade session. The six presentations and associated uplift of mid-crustal rocks. are now exposed as a result of post- accompanying discussions occupied The explanation for the thin crust in the contractional extension (e.g., Parrish et most of a day and led to a list of about northern Cordillera is more problematic al., 1988). Seismic reflection data from GEOSCIENCE CANADA Volume 31 Number 1 March 2004 25 Cordillera since the Paleoproterozoic. Evidence of early magmatic episodes is a key tool for regional correlations of Proterozoic stratigraphic sequences because both the igneous rocks and their detritus provide age constraints. Paleozoic magmatism was sparse along the North American margin, but many of the accreted terranes have distinct magmatic signatures. Regionally extensive magmatic activity accompanied two major accretionary episodes, in the Jurassic–Early Cretaceous and the Late Cretaceous– Paleocene, during which widespread arc magmas and granites intruded through both accreted rocks and underlying North American rocks (e.g., Guichon Creek, Cassiar, etc.). Post-contraction regional extension in the south during the Eocene was accompanied by the last phase of orogen-scale magmatism. Distinct differences in geological evolution between the northern Cordillera
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