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How Do Passive Margins Become Active? Paul F Document generated on 09/30/2021 2:29 p.m. Geoscience Canada The Tooth of Time: How do passive margins become active? Paul F. Hoffman Volume 39, Number 2, 2012 URI: https://id.erudit.org/iderudit/geocan39_2col01 See table of contents Publisher(s) The Geological Association of Canada ISSN 0315-0941 (print) 1911-4850 (digital) Explore this journal Cite this document Hoffman, P. F. (2012). The Tooth of Time:: How do passive margins become active? Geoscience Canada, 39(2), 67–73. All rights reserved © The Geological Association of Canada, 2012 This document is protected by copyright law. Use of the services of Érudit (including reproduction) is subject to its terms and conditions, which can be viewed online. https://apropos.erudit.org/en/users/policy-on-use/ This article is disseminated and preserved by Érudit. Érudit is a non-profit inter-university consortium of the Université de Montréal, Université Laval, and the Université du Québec à Montréal. Its mission is to promote and disseminate research. https://www.erudit.org/en/ GEOSCIENCE CANADA Volume 39 2012 67 COLUMN The Tooth of Time: How do restrial rift-basin. The Paleozoic ‘geo- Carboniferous molasse, the so-called syncline’ began with an Eocambrian Catskill Delta. But what were ‘geosyn- passive margins become (Ediacaran) conglomerate-basalt rift clines’ (Kay 1951; Aubouin 1965), and active? assemblage, overstepped by a west- why did long-lived carbonate shelves ward-tapering wedge of lower Cambri- suddenly founder at the beginning of Paul F. Hoffman an clastics derived from the craton, a the orogenic stage? 1216 Montrose Ave., Victoria, BC middle Cambrian clastic-carbonate I had heard Harry Hess speak V8T 2K4 transition, and an upper Cambrian at McMaster on the impermanence of through lower Ordovician sequence of ocean basins (Hess 1962), but all I Orogenic belts should be studied from shallow-marine carbonate rocks, mem- remember of the lecture was his ability the outside inwards: that was drummed orably exposed in cattle pastures in the to smoke two cigarettes simultaneously, into me at McMaster University by Great Valley of Virginia. The eastern without tipping an ash, while pacing Vint Gwinn, a young American sedi- limit of shallow-water carbonate rocks back and forth, or writing on the mentary and structural geologist with lay just to the east of the Blue Ridge blackboard. Bob Dietz, who had con- oil industry experience. It was 1963 Anticlinorium. Pettijohn called this ceived of sea-floor spreading inde- and he was putting the finishing touch- Sauk Sequence (Sloss et al. 1949; Sloss pendently of Hess (Menard 1986) and es on a paper, still cited today, proving 1963) the ‘pre-orogenic orthoquartzite- who had postulated that North Ameri- carbonate stage’ of the geosyncline, ca was overriding the Pacific Ocean that thin-skinned thrusts underlie the noting that a lithologically similar stage floor, based on the truncation of sea- Central Appalachian Plateau and occurs in the middle Precambrian floor magnetic anomalies at the conti- northwestern Valley and Ridge, where (Paleoproterozoic) geosyncline of Min- nental margin (Dietz 1961), visited no fault breaks the surface (Gwinn nesota (Pettijohn 1957, p. 641). Com- Johns Hopkins in 1965. Appropriately, 1964). Gwinn was informal yet pas- ing from the Devonian of the Niagara he presented his actualistic interpreta- sionate, invoking sedimentary succes- Escarpment, nonbioturbated carbon- tion of geosynclines as Atlantic-type sions in orogenic forelands as the best ates for me were a revelation. continental shelves (‘miogeoclines’) and records of subsidence and sedimenta- Then something different rises (‘eugeoclines’), that undergo tion related to orogenesis, as controls occurred: shallow-water carbonate gave orogeny when the overburdened on the trajectories of faults and related way unexpectedly to black sulfidic oceanic lithosphere spontaneously rup- folds, and as horizons for tracking shale, locally associated with submarine tures and ‘slides’ (subducts) under the deformation. Gwinn’s course primed landslides (‘olistostromes’). This was continental margin (Dietz 1963). His me to appreciate graduate school in the overlain by a thick middle Ordovician talk was not well received, paleocurrent Central Appalachians. sequence of greywacke turbidites, fol- studies having shown that the two At Johns Hopkins University, lowed by upper Ordovician–lower Sil- Appalachian molassic sequences were structural geologist Ernst Cloos and urian crossbedded sandstones of east- derived from the east (Pelletier 1958; sedimentologist Francis Pettijohn led erly derivation. Adopting terminology Yaekel 1959), implausibly from the off- field trips every weekend during the from a younger orogen, the European shore in Dietz’s (1963) model. Grant- Fall and Spring semesters. The field Alps, Pettijohn (1957) referred to the ed, his model worked pretty well for trips were obligatory for incoming ‘euxinic’ (black shale), ‘flysch’ the pre-orogenic stage, with the eastern graduate students and they offered an (greywacke turbidite) and ‘molasse’ limit of shallow-water carbonate mark- Appalachian cross-section from the (crossbedded sandstone) stages of the ing the continental shelf-slope break. inner edge of the Allegheny Plateau to geosynclinal cycle, and he related them But the orogenic scenario was not in the Atlantic coastal plain, encompass- to the Taconic orogeny. The geosyncli- fact actualistic because nowhere is sub- ing the Valley and Ridge fold-and- nal cycle was then repeated with a Sil- duction being initiated at an Atlantic- thrust belt, the basement-cored Blue urian–lower Devonian pre-orogenic type margin at present, not even in the Ridge anticlinorium, the metamorphic orthoquartzite-carbonate stage overlain Bay of Bengal where subduction initia- Piedmont, and the post-orogenic by middle Devonian euxinic shale and tion is optimized by the enormous load Newark Basin—a Triassic-Jurassic ter- flysch, followed by upper Devonian– of sediment and strong in-plane com- 68 pressive stress, related to subduction at the Java Trench and the topographic head of the Tibetan Plateau. After teaching at Franklin and Marshall College in southeastern Penn- sylvania for a year, I joined GSC in May 1969 in a position vacated by Pre- Rt cambrian paleontologist Hans Hof- mann. Ending was the era of military- style reconnaissance operations north of 60°, when 12 or more 1:250,000- Er scale sheets were mapped in a single sub-Arctic summer. Topical studies logically followed. Within weeks I was standing at the edge of the Wopmay Subprovince (Stockwell 1961), the Paleoproterozoic (then Aphebian) oro- genic belt I had invoked in my PhD thesis to account for the paleocurrent reversal in the East Arm of Great Slave Lake (Hoffman 1969), 600 km to the south. Actually, I was standing on Figure 1. Abrupt drowning of the Rocknest Formation (Er) marks the abortive 1.5 meters of lake ice, 20 km from the subduction of the passive-margin carbonate shelf in Wopmay orogen at 1882 Ma Arctic Ocean, looking at 70% snow (Bowring and Grotzinger 1992). Here, 30 km east of the frontal thrust and 300 km cover. If my timing was impetuous, the (palinspastic) landward of the ancient shelf-slope break, the basal foredeep deposits location was deliberate: under the melt- consist of craton-derived glauconitic siltstone (Rt), which tapers westward to a few ing snow was the best-exposed meters of granular ironstone, overlain by graphitic-pyritic slate and felspathic- autochthonous sedimentary section at greywacke flysch. Abortive subduction of the passive margin caused a subduction the edge of the orogen. To the west zone flip, forming an active margin with subduction dipping beneath the continent. was a broad belt of tightly-folded sedi- S.B. Lucas (foreground) ponders the metamorphic temperature. Tree River fold belt mentary rocks and beyond that was the at 67°26’05”N, 112°22’38”E, Nunavut, Canada. volcanic, plutonic and metamorphic core of the orogen. Our guides to this provide us with more structural relief cycle by a factor of four in its age. The place were an 8-mile-to-the-inch recon- than is exposed on any mountain on parallel tectonic development of geo- naissance map (Fraser 1960), a vision- Earth (Hoffman et al. 1988). synclines over the last two billion years ary regional stratigraphy (Fraser and A day later we returned to the was empirically evident, but not yet Tremblay 1969) and the Canadian Arc- top of the allochthonous dolomite, their interpretation in terms of plate tic Expedition (1913-1918) report on marked by a long narrow valley and a tectonics (Pettjohn 1957; Hoffman et the geology along the coast (O’Neill structurally intact sedimentary al. 1970). 1924). The next day we dropped down sequence. Bioherms of sunlight-seek- What caused the carbonate stratigraphically to the Archean base- ing columnar stromatolites distinguish shelves to founder at the beginning of ment, finding a depositional unconfor- the final meters of dolomite the orogenic stage? The answer would mity at the base of a transgressive (Grotzinger 1986b). They are abruptly eventually come from the Appalachian- marine quartzite sequence (Odjick For- overlain by a few meters of glauconitic Caledonian orogen, informed by theo- mation), gradationally overlain by 0.8 quartz-siltstone (Tree River Forma- retical geophysics and modern exam- km of cyclic peritidal dolomite, the tion), several decameters of sulfidic- ples. In 1964, Hank Williams (1964) Rocknest Formation (Grotzinger graphitic slate (Fontano Formation), recognized the Newfoundland 1986a). The 1.6-km-thick ortho- and then a kilometer or more of Appalachians as an orogenic system in quartzite-carbonate sequence dips to immature, coarse-grained, feldspathic- which two different continental shelves the west and above it we found a greywacke turbidites (Asiak Forma- (Dietz’s ‘miogeoclines’) face each other duplicate sequence, which rides on a tion). The abruptness of the transition across an early Paleozoic oceanic tract. paper-thin thrust fault detached a few and the thickness of greywacke tur- Tuzo Wilson (1966) extended this meters above the basal unconformity bidites diminish eastward (Fig.
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