摘要:

AbstractThe Precambrian sequences of the Avalon Zone in Canada (southeastern margin of the Appalachian Orogen) are interpreted as a Pan‐African orogenic belt incorporated into the Appalachian Orogen during Palaeozoic times as its southeastern margin. The Precambrian evolution of the Avalon Zone was genetically unrelated to subsequent Palaeozoic evolution. The Avalon Zone shows marked similarities in age, tectonic history, and facies development to the Pan‐African belts adjacent to the West African Craton.Precambrian evolution of the zone began with circa 800 Ma rifting of a sialic gneissic basement and deposition of a Middle Proterozoic(?) carbonate‐clastic cover sequence. Early crustal rifting was associated with localized partial melting and metamorphism. Limited crustal separation led to the restricted development of circa 760 Ma oceanic volcanics. Continued rifting and subsequent closure of these narrow ocean basins led to the eruption of widespread subaerial volcanic suites, block faulting, granite plutonism, and local, late Proterozoic sedimentary basin formation. Precambrian evolution of the zone terminated with the Avalonian Orogeny (circa 650‐600 Ma), a deformational event, the affects of which are most evident locally along the northwestern margin of the zone.The controlling features of the Proterozoic evolution of the Avalon Zone are a series of linear intracratonic troughs and small ocean basins that formed during thinning and separation of the crust by ductile spreading, rupture, and delamination (cf.Martin and Porada 1977). The variation in degree of crustal separation led to subsequent variation in orogenesis during late Proterozoic compression.The zone marks the original westward limit of Pan‐African activity and displays no apparent genetic link with the Appalachian Orogen in Canada until Devon

ISSN:0072-1050    

DOI:10.1002/gj.3350180302

出版商:John Wiley&Sons Ltd    

年代:1983

数据来源: WILEY

摘要:

AbstractIn the northeastern Appalachians crystalline, basement outcrops within a y‐shaped belt extending southwest from Bonavista Bay, Newfoundland, to Port aux Basques, and thence via the Cape Breton Highland to near Saint John, New Brunswick. Locally a Precambrian platformal sequence lies on the basement. Stratigraphic and isotopic evidence demonstrate that in the Saint John and northern Cape Breton regions, both basement and cover were remobilized about 600 million years ago, the basement probably for the second time. After deposition of a Cambro‐Ordovician cover, these rocks were again mobilized and intruded in mid‐Ordovician and Devonian time. Evidence for basement reactivation in Newfoundland, although fragmentary, is compatible with a similar history. Much of the basement gneiss is presently exposed in uplifted fault blocks.The observations can be explained in plate tectonic terms if the sedimentary accretion prism collapses during plate convergence, and is pushed onto the craton, which is itself thickened due to compressive stress. Because the edge of the cration is heated, due to thickening of both basement and sedimentary cover, the basement rises due to lowering of density by thermal expansion, and ultimately by partial melting. Depending on relative rates of heating, buoyant rise, and erosion, the process can lead either to a characteristic pattern of linear belts of reactivated basement which display positive gravity and magnetic anomalies, or to a chain of elongate gneiss domes exhibiting gravity lows. Application of the model to the northeastern Appalachians suggests that several continental fragments are present, and that basement rocks have been involved in repeated continental collision and br

ISSN:0072-1050    

DOI:10.1002/gj.3350180303

出版商:John Wiley&Sons Ltd    

年代:1983

数据来源: WILEY

摘要:

AbstractExaminations of Grenville massifs in the Blue Ridge Geologic Province of Virginia and North Carolina indicate that the country rocks (∼ 1100–1450 Ma) are layered gneisses that were metamorphosed during Grenville orogenesis (∼ 1000–1100 Ma) to amphibolite to granulite facies and intruded by plutonic suites. Subsequently, the Grenville terrane was intruded by a suite of peralkaline granitic plutons (∼ 700 Ma) and progressively overlapped westward by Upper Precambrian to Cambrian sedimentary and volcanic rocks.Following deposition of Upper Precambrian and Palaeozoic rocks, the Blue Ridge Geologic Province was subjected to Taconic metamorphism (∼ 450–480 Ma) which generally increased in intensity southeastward from greenschist (chlorite grade) to upper amphibolite (sillimanite grade) facies. Large‐scale late Devonian thrusting (∼ 350 Ma) along the Fries fault system and the Brevard zone‐Yadkin fault system produced the present day distribution of juxtaposed Grenville massifs and Palaeozoic metamorphic zones in the Blue Ridge Geologic Province.Palinspastic restoration of the Taconic metamorphic zones to their pre‐late Devonian relative positions yields an ∼ 50 km displacement on the Fries fault system near the Grandfather Mountain window and and an ∼ 80 km displacement on the Smith River allochthon farther east.Restoration of the Grenville massifs to this same palinspastic base shows that Grenville metamorphic grade decreased southeastward from the deeper granulite facies (opx + gar) to the shallower granulite facies (opx ±

ISSN:0072-1050    

DOI:10.1002/gj.3350180304

出版商:John Wiley&Sons Ltd    

年代:1983

数据来源: WILEY

摘要:

AbstractBasement is constituted of rocks which belong to a previous orogenic cycle which have been reactivated and incorporated into a younger cycle. Basement massifs may be classified according to their relative position in an orogen as external or internal massifs. They may also be categorized according to their role in deformation, as thrust‐related, fold‐related and composite massifs.All Appalachian external massifs were transported following their removal from the overridden edge of the ancient North American continental margin. Most of the internal massifs are also probably transported, but several (Pine Mountain and Sauratown Mountains) may be present as windows exposing parautochthonous basement beneath the main thrust sheet. The latter reside immediately west of the low (west) to high (east) gravity gradient which probably outlines the old edge of Grenvillian crust. Reactivated crustal material generated during early Palaeozoic orogeny plays the same mechanical role in reactivation as basement from the previous Grenville cycle. The domes of the Bronson Hill anticlinorium cored with Ordovician or older gneisses illustrate this behaviour.Basement (Grenville) massifs are distributed throughout the Appalachians as a belt of external massifs (Blue Ridge, Reading Prong, Hudson and Berkshire Highlands, Green Mountains, and Long Range Mountains) along the western edge of the crystalline metamorphic core. Additionally, internal massifs are also present (Pine Mountain belt, Tallulah Falls and Toxaway domes, Sauratown Mountains anticlinorium, State Farm gneiss dome, Baltimore Gneiss domes, Mine Ridge anticline, and Chain Lakes massif).Basement internal massifs probably served to localize thrusts by causing them to ramp over and around the massifs. Their antiformal shape may in part be as much related to thrust mechanics as to fold

ISSN:0072-1050    

DOI:10.1002/gj.3350180305

出版商:John Wiley&Sons Ltd    

年代:1983

数据来源: WILEY

摘要:

AbstractThe external massifs along the Appalachian orogen include Precambrian basement rocks with attached cover. To the northwest (cratonward), in the Appalachian foreland fold and thrust belt, Palaeozoic sedimentary rocks, but no basement rocks, are exposed; that belt was the subject of the classic debate about thin‐skinned (deformed cover rocks detached from undeformed basement) and thick‐skinned (basement deformed with attached cover) structural styles. Presently available data indicate detached cover rocks and thin‐skinned style in the fold and thrust belt: large‐scale thrusting occurred late in the orogenic history. In the external basement massifs, late Precambrian graben‐fill sedimentary and volcanic rocks indicate early basement faults; and within the craton, steep basement faults bound graben blocks of Cambrian age. Distribution of known basement faults suggests that basement rocks beneath the fold and thrust belt may also be faulted. Local episodic synsedimentary structural movement through much of the Palaeozoic is documented by stratigraphy in the fold and thrust belt. Axes of early synsedimentary structures are approximately coincident with axes of late folds and thrust fault ramps, but stratigraphic data show that magnitude of the early structures was much less than that of the late structures. These relations suggest the interpretation that early low‐magnitude structures formed in cover rocks over basement faults and that the early structures, or the basement faults, significantly influenced the geometry of later detachment structures during large‐scale horizonta

ISSN:0072-1050    

DOI:10.1002/gj.3350180306

出版商:John Wiley&Sons Ltd    

年代:1983

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350180308

出版商:John Wiley&Sons Ltd    

年代:1983

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350180309

出版商:John Wiley&Sons Ltd    

年代:1983

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350180312

出版商:John Wiley&Sons Ltd    

年代:1983

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350180313

出版商:John Wiley&Sons Ltd    

年代:1983

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350180314

出版商:John Wiley&Sons Ltd    

年代:1983

数据来源: WILEY