Orogeny Can Be Very Short
Total Page：16
File Type：pdf, Size：1020Kb
Orogeny can be very short John F. Dewey* Department of Geology, University of California, Davis, CA 95616; and University College, Oxford OX1 4HH, United Kingdom This contribution is part of the special series of Inaugural Articles by members of the National Academy of Sciences elected on April 29, 1997. Contributed by John F. Dewey, July 12, 2005 In contrast to continent͞continent collision, arc–continent collision margin, common during the early stages of continental collision generates very short-lived orogeny because the buoyancy-driven when, driven by subduction roll-back, arcs bend and wrap impedance of the subduction of continental lithosphere, accom- themselves into remnant oceanic holes (7). Arc͞rifted margin panied by arc͞suprasubduction-zone ophiolite obduction, is re- collision involves the blocking of a subduction zone with buoy- lieved by subduction polarity reversal (flip). This tectonic principle ant, weak, continental crust, followed by subduction polarity is illustrated by the early Ordovician Grampian Orogeny in the reversal͞flip (8, 9). To achieve a flipped polarity, the preflip British and Irish Caledonides, in which a wealth of detailed sedi- subducted lithosphere must be removed by slab break-off to mentologic, heavy mineral, and geochronologic data pin the Orog- allow postflip subduction (10, 11). This process has occurred on eny to a very short Arenig͞Llanvirn event. The Orogeny, from the many active continental margins, including New Guinea during initial subduction of continental margin sediments to the end of the Miocene (9, 12), north Borneo (13), New Caledonia during postflip shortening, lasted Ϸ18 million years (my). The collisional the Eocene (14), the Sierra Nevada during the Jurassic (15), the shortening, prograde-metamorphic phase of the Orogeny lasted 8 Urals during the late Devonian (16, 17), the Appalachian͞ my, extensional collapse and exhumation of midcrustal rocks Caledonian System during the Ordovician (10, 18), and in the lasted 1.5 my, and postflip shortening lasted 4.5 my. Strain rates northern midcontinent USA during the Paleoproterozoic (19). ؊15 were a typical plate-boundary-zone 10 . Metamorphism, to the The subduction-flip mechanism generates a very short-lived second sillimanite isograd, with extensive partial melting, occurred orogeny because the resistive forces of collisional shortening are within a few my after initial collision, indicating that conductive relieved by reversed subduction. Thus, the collision of short-lived models for metamorphic heat transfer in Barrovian terrains are oceanic arcs with rifted continental margins, commonly involv- incorrect and must be replaced by advective models in which large ing suprasubduction zone (SSZ) ophiolite oduction, is followed volumes of mafic͞ultramafic magma are emplaced, syn-tectoni- by the longer subduction of a postflip ocean and the protracted cally, below and into evolving nappe stacks. Arc͞continent collision events leading to continental collision, with which ophiolite generates fast and very short orogeny, regional metamorphism, and exhumation. obduction is rarely associated (20, 21). The timing and duration of an orogeny or orogenic cycle is notoriously difficult to define, 8 million year orogeny ͉ continent͞arc collision ͉ orogenic mechanisms stratigraphically, by the youngest deformed and metamorphosed sediments, and the oldest sediments that rest unconformably on the denuded orogen, which yield, generally, long outside esti- rogeny is continental deformation resulting from plate- mates. Integrated geochronology, sedimentary facies package boundary-scale shortening, which usually generates moun- O analysis, and heavy mineral analysis enable much tighter and tain belts and occurs in several ways. First, continent͞continent finer definitions (10). collision may cause doubling of the thickness of the continental I report here on a short-lived early Ordovician orogeny (Gram- crust over a very wide (1,000 km) zone, with far-field continental shortening, and last for 50 million years (my). Such zones are pian; roughly equivalent to the Taconic Orogeny of the Appala- characterized by wide orogenic plateaux, with surrounding and chians) in the Caledonian Belt from Ireland to Scotland, in which internal basins bordered by thrust belts, such as the Himalayan͞ geologic data tightly constrain the orogenic sequence from the Tibetan͞Kunlun͞Tienshan͞Altai system. Second, there is the initial subduction of rifted margin sediments beneath a continent- ͞ spectrum from the progressive, mono-vergent, shortening col- ward-facing oceanic arc [478 mega-annum (Ma)], arc ophiolite lapse, over 40 my, of narrow ‘‘para-oceans’’ like the Alps and the obduction, through 8 my of dextral transpressional (22) collision, Dinarides, to the far-field intracratonic shortening and inversion crustal thickening and collisional tightening (475–467 Ma), sub- of rift complexes like the Amadeus Basin in central Australia, the duction polarity flip (467 Ma), extensional collapse (467–464.5 Gamburtsev Mountains of Antarctica (1), and the Ancestral Ma), renewed shortening and retrocharriage during postsubduction Rockies (2) during the Carboniferous, the La Salle deformation flip shortening (464.5–460 Ma) to postorogenic (443 Ma) uncon- belt of Illinois (3), the Yanshan Orogen in China (4), the formity (10, 23, 24). Kennedy (25) recognized that the Grampian Palmyride Belt of Syria (5), the late Paleozoic Cuvette Centrale Orogeny must have been an Ordovician (26) rather than a Pre- of Zaire (6), and, probably, the Proterozoic Knoydartian Orog- cambrian or pre-Ordovician event. Dewey and Mange (10) recog- eny in Scotland. Third, Andean-style orogeny is generated by nized that the Orogeny must have occurred during a very short time episodic shortening of the leading edge of a plate overriding a interval during the deposition of the conformable stratigraphic subduction zone, with short-lived ignimbrite ‘‘flare-ups’’ and sequence of the South Mayo Trough (SMT), supported by geo- shortening events punctuating longer periods of ‘‘stasis.’’ Fourth, chronology (27) and geology (28). Orogeny is, perhaps, best un- decollement at the Moho may allow the crust to detach and be derstood by studying the facies and detrital mineralogy of coeval fed through a thickening wedge into a continuously and rapidly sedimentary basins; the Grampian Orogen permits the integration eroded crustal monocline, as in the transpressive Southern Alps of these data with metamorphic͞geochronologic data. of New Zealand. These four orogenic mechanisms involve long-lived, continuous, or episodic events. Fifth, locking͞ restraining bends in large intracontinental transform faults, such Abbreviations: Ma, mega-annum; my, million years; CSB, Clifden Steep Belt; RBS, Renvyle͞ as the Transverse Ranges in California and the Anti-Lebanon Bofin Slide; SMT, South Mayo Trough; SSZ, suprasubduction zone. Mountains, generate zones of ephemeral crustal shortening. All See accompanying Profile on page 15283. these forms are distinct from the sixth style of orogeny, gener- *E-mail: [email protected]. ated by the collision of an island arc with a rifted continental © 2005 by The National Academy of Sciences of the USA 15286–15293 ͉ PNAS ͉ October 25, 2005 ͉ vol. 102 ͉ no. 43 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0505516102 Downloaded by guest on October 2, 2021 The Western Irish and Scottish Caledonides: ophiolite sheet with increasing amounts of detrital chromite (10) The Grampian Orogen and Cr͞Ni͞Mg and decreasing Fe͞Ti in the sediments (36). The The Grampian Orogen consists of the deformed and metamor- Glensaul Group is roughly correlative with the Derrymore͞ INAUGURAL ARTICLE phosed 15-km Dalradian sequence, exposed in the Caledonian Sheeffry and consists of a silicic arc sequence, indicating the orthotectonic zone (29) from Connemara, in western Ireland, to subduction of rifted continental margin sediments (37). Silicic the Grampian Highlands of Scotland (Fig. 1A). It continues into tuffs and lahars in the Sheeffry were derived from the Glensaul the Appalachian Taconic Orogen. Barrovian metamorphic as- arc to the south. The lower Derrylea is similar to the Sheeffry semblages, in fold and thrust nappe stacks, include garnet, with thicker turbidites; as in the Sheeffry, west- and south- staurolite, kyanite, sillimanite, andalusite, and chloritoid. In flowing, dark green turbidites contain abundant chrome spinel Scotland, the Grampian Zone has Siluro-Devonian sinistral and rounded purple zircon, whereas north-flowing, paler, tur- terrane boundaries, the Great Glen and Highland Boundary bidites contain silicic arc debris including euhedral colorless Faults, both with displacements equal, at least, to their lengths zircons. In the upper Derrylea, a major change occurs at a (30). In Scotland, a direct stratigraphic lower age limit to massive, thick, turbidite horizon in which staurolite, almandine, Grampian deformation is given by unconformable Lower De- and chloritoid appear with floods of large muscovite grains; the vonian strata (417 Ma), a certain upper age limit by the topmost zircon population shows a diminution of rounded purple in favor Lower Cambrian Dalradian Leny Limestone (525 Ma), and a of subhedral and euhedral. The Glenummera is pelitic with thin, probable upper age limit by the putatively early Ordovician fine-grained turbidites and a sparse heavy mineral population Dalradian McDuff Slates (485 Ma). In western Ireland (Fig. 2), similar to the upper Derrylea. The Mweelrea is, mainly, a coarse, a lower