摘要:

AbstractUnderstanding the evolution and destruction of past oceans not only leads to a better understanding of earth history, but permits comparison with extant ocean basins and tectonic processes. This paper reviews the history of the Early Paleozoic circum‐Atlantic oceans by analogy with the Pacific Ocean and Mesozoic Tethys. Rifting and continental separation from 620 to 570 Ma led to the development of passive margins along parts of the northern margin of Gondwana (the western coast of South America); eastern Laurentia (eastern North America, NW Scotland and East Greenland), and western Baltica (western Scandinavia). Meagre paleomagnetic data suggest that western South America and eastern North America could have been joined together to form facing margins after breakup. Although western Baltica is an apparently obvious candidate for the margin facing NW Scotland and East Greenland, the paleomagnetic uncertainties are so large that other fragments could have been positioned there instead. The Iapetus Ocean off northeastern Gondwana was probably a relatively wide Pacific‐type ocean with, during the late Precambrian to early Ordovician, the northern margin of Gondwana as a site of continentward‐dipping subduction zone(s). The 650‐500 Ma arc‐related igneous activity here and the associated deformation gave rise to the Cadomian, ‘Grampian’, Penobscotian, and Famantinian igneous and orogenic events. By 490‐470 Ma, marginal basins had formed along the eastern Laurentian margin as far as NE Scotland, along parts of the northern margin of Gondwana, and off western Baltica, but none are known from the East Greenland margin. These basins closed and parts were emplaced as ophiolites shortly after their formation by processes that, at least in some cases, closely resemble the emplacement of the late Cretaceous Semail ophiolite of Oman. This orogenic phase seems to have involved collision and attempted subduction of the continental margin of Laurentia, Gondwana and Baltica. In Baltica it gave rise to some eclogite facies metamorphism. Marginal basin development may have been preceded by arc formation as early asca510 Ma. A double arc system evolved outboard from the eastern Laurentian and western Baltica margins, analogous to some of the arc systems in the present‐day western Pacific. At 480‐470 Ma, there was a second phase of breakup of Gondwana, affecting the active Gondwanan margin. Eastern and Western Avalonia, the Carolina Slate Belt, Piedmont, and other North American exotic continental blocks rifted away from Gondwana. Farther east, Armorica, Aquitainia, Iberia and several European exotic continental blocks also rifted away, though it is unlikely that they all rifted at the same time. Between 460‐430 Ma, peaking atca450 Ma, orogenic events involved continuing arc‐continent collision(s). From 435‐400 Ma the remaining parts of the Eastern Iapetus Ocean were destroyed and the collision of Baltica with Laurentia caused the 430‐400 Ma Scandian orogeny, followed by suturing of these continents during the Siluro‐Devonian Acadian orogeny or Late Caledonian orogeny to 380 Ma, leaving a smaller but new ocean south of the fragments that had collided with the Laurentian margin farther south. The Ligerian orogeny 390‐370 Ma collision of Gondwana‐derived Aquitaine‐Cantabrian blocks with Eastern Avalonia‐Baltica and removed the part of the Iapetus south of Baltica.Prior to any orogenic events, the Eastern Iapetus Ocean between Baltica and Laurentia may have resembled the present‐day central Atlantic Ocean between Africa and North America. The ocean appears to have closed asymmetrically, with arcs forming first outboard of the western margin of Baltica while the East Greenland margin was unaffected. The Western Iapetus Ocean between Laurentia and Gondwana also closed asymmetrically with a dual arc system developing off Laurentia and an arc system forming off the northern margin of Gondwana. Like the Pacific Ocean today, the Eastern Iapetus Ocean had a longer and more complex history than the Western Iapetus Ocean: it was already in existence at 560 Ma, probably developed over at least 400 million years, by mid‐Cambrian time was many thousands of kilometres wide at maximum extent, and was associated with a<30 million year phase of marginal basin formation. In contrast, the Western Iapetus Ocean appears to have been much narrower, shorter lived (probably<100 million years), and associated with the rifting to form two opposing passive carbonate margins, analogous to the Mesozoi

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1995.tb00132.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThe Waipapa Terrane in northern North Island, New Zealand, is a Mesozoic accretion‐ary complex formed along the Gondwana margin. It contains abundant melange rocks with distinctive characteristics. Precise analyses of their mesoscopic fabrics in Waiheke Island near Auckland have revealed the following sequence of deformation. The earliest phase of deformation of the sandstone/mudstone association, which is the main constituent of this terrane, originated by chaotic mixing of sand and mud due to liquidization of water‐saturated, poorly consolidated sediments. The second phase was characterized by hydrofracturing and subsequent forceful injection of ductile mud into rather brittle sand. Local intrusions of sand forming dykes and sills followed these events, as well as intrusions of pelagic/hemipelagic green argillite originally underlying the sandstone/mudstone association. An abundant occurrence of these mixing and multi‐stage injection/intrusion fabrics strongly suggest that the Waipapa Terrane around the study area was a site of high pore‐fluid pressure. Scaly‐foliated melange fabrics with monoclinic symmetry, originating from layer‐parallel shearing, were then locally superimposed on the pre‐existing melange fabrics. Similar scaly‐foliated fabrics also developed in the chert beds originally intercalated between the green argillite and the uppermost part of the oceanic crust. These scaly fabrics might have been related to the regional stacking and juxtaposition of the accreted sediments. The sequence and variation in style of deformation forming the melange fabrics presumably reflected changes in porosity and state of compaction of accreted sediments in a shallow

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1995.tb00133.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY