摘要:

The spatial and temporal evolution of eastern Mindanao from a zone of active arc‐arc collision to one of pure strike slip environment is addressed. This is made possible by tracing the Philippine Fault, confined to a well defined and narrow zone of deformation, and by differentiating from it earlier collision‐related compressional features, whose deformation is distributed over a broad zone. A diachronous unconformity marking the end of collision‐related deformation reflects the southward propagating nature of the collision and provides limits on the age of initiation of the Philippine Fault. This diachronism suggests that the initiation of the Philippine Fault is related to the completion of the collision. We show the rapidity at which formation and propagation of the fault follows the locking of the collision zone. We likewise demonstrate that the Philippine Fault reactivates pre‐existing collisional features in cutting across the southern Pacific Cor

ISSN:0278-7407    

DOI:10.1029/95TC00480

年代:1996

数据来源: WILEY

摘要:

The age relations, geochemistry, and sedimentology of the Rocky Ridge Formation of the Skeena Group are used to test competing tectonic reconstructions for the mid‐Cretaceous Canadian Cordillera as well as the timing and location of the accretion of the Insular Superterrane. Pollen and macrofossil assemblages indicate that these intrabasinal basalts were erupted along the southern margin of the Bowser basin in the Early Albian to Early Cenomanian. Single‐crystal fusion and step‐heating40Ar/39Ar dating of hornblendes in one basalt flow from the uppermost part of the formation yielded Middle Cenomanian ages of 94.3 ± 0.4, 95.6 ± 1.6, and 95.0 ± 1.6 Ma. Vesicular basalt flows interbedded with crystal‐rich tuff breccias contain evidence for hot emplacement as pyroclastic flows. Individual eruptive centers are identified by their proximal facies, paleoflow indicators within the lava flows, paleoflow indicators within interbedded volcaniclastic fluvial deposits, geochemical differences, and geographic isolation of volcanic deposits. Major and trace‐element geochemistry from 20 sampled lava flows indicates an alkali basalt composition for the volcanics. The basalts of the northern Rocky Ridge volcanic center show enrichment of light rare earth and large ion lithophile elements with strong negative Nb‐Ta anomalies whereas the basalts of the southern Tahtsa Lake volcanic center show depletion to slight enrichment of light rare earth elements, slight enrichment of large ion lithophile elements with minimal negative Nb‐Ta anomalies. The geochemistry combined with paleogeographic and regional tectonic reconstruction suggests a continental arc setting with intraarc extension. The presence of deeper marine facies to the west and the lack of a western sediment source in the Skeena Group indicate that the technically active Insular Superterrane was not west of the study area during mid‐Cretaceous time. Thus we reconsider the Omineca Belt as the main axis of a mid‐Cretaceous continental arc, placing the Intermontane Superterrane in the intraarc to forearc position with the Rocky Ridge volcanics erupted along the forearc side of the Omineca arc. Coeval regional strike‐slip faulting and reconstructed oblique plate convergence suggest a transtensional setting for Rocky Ridge intraarc extension. An electronic supplement of Tables A1–A2 may be obtained on a diskette or Anonymous FTP from KOSMOS.AGU.ORG (LOGIN to AGU's FTP account using ANONYMOUS as the username and GUEST as the password. Go to the right directory by typing CD APEND. Type LS to see what files are available. Type GET and the name of the file to get it. Finally, type EXIT to leave the system.) (Paper 95TC03496, Mid‐Cretaceous transtension in the Canadian Cordillera: Evidence from the Rocky Ridge volcanics of the Skeena Group, Kari N. Bassett and Karen L. Kleinspehn). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009; $15.00. Pa

ISSN:0278-7407    

DOI:10.1029/95TC03496

年代:1996

数据来源: WILEY

摘要:

The Bragança Massif, in particular its Continental Allochthonous Terrane (CAT), is a keystone in the comprehension of the pre‐Variscan and Variscan geodynamic evolution of the Eurasian plate, in particular of the high‐grade metamorphic evolution. The Bragança CAT comprises three types of granulites (recording quite distinct metamorphic histories) and paragneisses with enclosed eclogite boudins. It has undergone five characterizable, main deformation phases during two orogenic cycles, separated by the extensive intrusion of mafic/ultramafic layered bodies (types 2 and 3 granulites) into a pervasively deformed granulitic lower continental crust (type 1 granulites). Geochronological data obtained on high‐grade metamorphic rocks from Bragança and other similar rocks in the Variscan fold belt have been typically interpreted in two different ways: polycyclical evolution, Precambrian and Paleozoic (Variscan), or monocyclical evolution (Variscan). This second hypothesis is in clear contradiction with field evidence, with tectonometamorphic data, and even with some geochronological data, as shown in this article. Thus we suggest that the tectonometamorphic evolution of the Bragança CAT, and, by comparison, of other similar CAT in the Variscan fold belt, took place during two orogenic cycles, one Precambrian (Grenvillian? Pan‐African/Cadomian?) and another one Paleozoic

ISSN:0278-7407    

DOI:10.1029/95TC03852

年代:1996

数据来源: WILEY

摘要:

The eastern flank of the Central Andes in southern Bolivia and northern Argentina is underlain by a fold‐and‐thrust belt of Neogene age. It comprises an external thin‐skinned segment with basal detachment in Silurian shales (Subandean Ranges) and an internal segment where units as deep as Precambrian metasedimentary and igneous rocks are involved in thrusting (eastern Cordillera Oriental). Between the Cordillera Oriental and the Subandean Ranges an intermediate segment (Interandean Zone) intervenes north of the Argentinian‐Bolivian border (22° S). The structure of the Interandean Zone is characterized by folded and thrust‐faulted Cambrian to Triassic strata. Balanced cross sections across the western Subandean Ranges, the Interandean Zone, and the eastern Cordillera Oriental are based on previously available data and recent field work. They suggest that the geological structure can be interpreted in terms of two major basement‐involved thrusts, here referred to as Interandean and Subandean thrusts. The more internal Interandean thrust caused the development of broad basement anticlines in the eastern Cordillera Oriental. However, at shallower levels, its displacement was accommodated by thin‐skinned folding and thrusting in the Interandean Zone. In‐sequence activation of the Subandean thrust uplifted the Interandean Zone on top of an imbricate basement sheet and caused the development of the thinskinned Subandean Ranges farther east. Published radiometric data and subsidence history of the Subandean foreland basin suggest that the Interandean thrust was active between about 10 Ma and 5 Ma. The Subandean thrust is still active. Southward displacement transfer from the Subandean thrust onto the Interandean thrust and a southward dipping lateral ramp of the Interandean thrust result in the gradual transition from the Interandean Zone north of 22° S to the Cordillera Oriental of northern Argentina. A similar but larger‐scale transfer may be involved in the southern termination of the thin‐skinned Subandean Ranges of Argentina at 23° S. Shortening in the western part of the thrust belt is of the order of 80–90 km. Total shortening in the Eastern Andean thrust belt from the undeformed foreland to the eastern margin of the Cordiller

ISSN:0278-7407    

DOI:10.1029/95TC03868

年代:1996

数据来源: WILEY

摘要:

The western branch of the East African Rift system is commonly cited as a result of Phanerozoic reactivation of the Paleoproterozoic Ubendian belt in western Tanzania. Geological evidence is provided to show that prominent mechanical anisotropies successively appeared during Proterozoic evolution of the Precambrian basement and that their different reactivation behavior contributed to the Phanerozoic rift pattern. The Ubende belt (1950–1850 Ma) is a NW oriented, amphibolite facies ductile lateral shear belt in which older (2100–2025 Ma) and complex granulite facies terranes are included along trend. Retrograde multiphase sinistral strike‐slip mylonites developed along the NW oriented ductile shear belt. They reflect persistent Proterozoic wrench fault reactivation of the latter. Shallow level sedimentary basins upon and along the ductile shear belt display deformational structures attributable to the Proterozoic wrench fault reactivation. Neoproterozoic sinistral transpression produced the final geometrical pattern of the wrench fault zone, which appears as an elongate and NW trending positive flower structure, locally enhanced by late Proterozoic contraction. Phanerozoic rifting is demonstrated by others to occur in three distinct episodes, during which the complex rift segment formed upon the multiphase Proterozoic wrench fault zone. The evaluation of the relationship between multiphase rift and multiphase prerift fabrics is reconsidered. The Proterozoic prerift fabrics correspond with a dextral transpressional and ductile deformational pattern, which became selectively reactivated by sinistral transpressional ductile‐brittle mylonites. Proterozoic mylonites constitute shallow level mechanical anisotropies and define the general trend of the rift faults. According to the position of these mylonites in the center or in the external parts of their NW oriented Neoproterozoic transpression, they reactivate as complex and multiphase rift faults or as normal and recent faults, respectively. The Paleoproterozoic NW oriented and ductile lateral shear belt constitutes the deep level mechanical anisotropy. Its reactivation in Phanerozoic stress fields is likely dextral oblique transtension, considered as a leading mechanism of the pluriphase and NW oriented deep rift

ISSN:0278-7407    

DOI:10.1029/95TC03685

年代:1996

数据来源: WILEY

摘要:

Uplifted Pleistocene coral terraces and modern earthquakes show that several large normal faults are presently active in the southern Gulf of Suez rift basin. The footwall of one of these faults is exposed at Gebel el Zeit, where terraces at elevations of +10–18 and +42 m have been radiometrically dated as 125 ka and 426 ka, respectively. After correcting for eustatic sea‐level changes, this results in net tectonic uplift rates of about 0.1 m/kyr. Published interpretations and our own observations indicate that the average extension direction during the Miocene to Pliocene synrift history was 055°. Analysis of borehole breakouts and published earthquake fault plane solutions, however, suggests that the present‐day stress field in the southern Gulf has a 010°–020°Shminorientation. Detailed structural observations show that a change in extension direction occurred in the late Pleistocene, with rotation of the stress field beginning prior to formation of the 125 ka terraces but after formation of older Pleistocene terraces whose ages are less tightly constrained. Using a horizontal slip direction of 015° and our observed net footwall uplift rate, we calculate a separation velocity between Sinai and Africa of 0.8–1.2 m/kyr. The proposed Pleistocene change in extension direction in the Gulf of Suez corresponds closely with the post‐500 ka change in extension direction documented in the Kenyan rift system and a similar change in extension direction recorded in the central Red Sea. These regional similarities in tectonic history suggest that the underlying causes of these events may be a plate‐scale phenomenon affecting the entire Afro‐Arabian rift system, rather than local changes in the Quat

ISSN:0278-7407    

DOI:10.1029/95TC03851

年代:1996

数据来源: WILEY

摘要:

The continuation of the basement of the southwestern Baltic Shield toward the SW is of major interest for the Caledonian evolution in the southwestern Baltic Sea and northern Germany. A reinterpretation of various marine seismic data suggests a reconsideration of the Caledonian collision pattern. The Sorgenfrei Tornquist Zone forming the northwestern extension of the Tornquist Zone is regarded as the northeasterly boundary fault and the Elbe Lineament is regarded as the southwestern limit of an area which suffered from the docking of East Avalonia to Baltica. It shows many northwestsoutheast striking structures, anomalies, and fault patterns. From our studies, two alternative models emerge: one which considers the Caledonian Deformation Front as the northeastern limit of East Avalonia and a zone of major transpression, collision, and subduction of the Tornquist Sea. Another model considers the Elbe Lineament in Schleswig‐Holstein (NW Germany) as a major (dextral) strike‐slip fault which is interpreted as the northeasterly limit of East Avalonia and the southwest boundary of the Baltic Shield. Along the Elbe Lineament the Tornquist Sea plus rotating Avalonia were displaced to the northwest. A large part of the area to be studied might have developed into a passive margin toward the Upper Ordovician. A decision which of the two models is to be prefered has to be the major goal of future geophysical experime

ISSN:0278-7407    

DOI:10.1029/95TC03686

年代:1996

数据来源: WILEY

摘要:

A new structural map of the western Sea of Okhotsk, based on grids of multichannel seismic data, provides several new insights into the Tertiary deformation of Asia, especially in regard to the extension of “extrusion” tectonics, linked to the India‐Eurasia collision, into northeastern Asia. The sedimentary basins in this offshore region are the result of two regional shear systems. In the west, the north trending Sakhalin‐Hokkaido dextral fault system transects Sakhalin Island. Accessory structures closely related to its dextral shear include northeast trending normal faults as well as northwest trending en echelon folds and thrusts. The accessory normal faults are predominantly Eocene to lower Miocene and indicate transtensional shear, whereas the folds and thrusts are mostly younger and indicate late Miocene and Pliocene transpression. In the north, an east‐west trending sinistral shear zone enters the sea at its northwest corner; this sinistral shear system links farther west to extensional faults of the early Tertiary Baikal rift. Extending northeastward from the terminus of this sinistral shear zone are a series of long, predominantly northwest dipping listric normal faults that form a large “lazy‐S” shaped pull‐apart basin, the Shantar‐Liziansky basin (SLB). Normal faults in this 500 × 140 km basin accommodate 15 to 20% extension. Extension and related sinistral shear appear to be largely Eocene to Oligocene in age, with lesser later activity. To the northeast of SLB, a further extension of the sinistral shear zone appears to bend northeastward, transecting Pustorets and Penzhina basins and following the course of the older, Mesozoic Mongol‐Okhotsk‐Chukotsk active margin and suture; it possibly reaches as far as the Arctic. As suggested by previous workers, the Baikal rift and its associated sinistral shear zone may be interpreted as by‐products of the India‐Eurasia collision in Eocene time; mapping results included here show that this diffuse sinistral shear system extends much farther to the northeast. The sinistral shear zone is transected by the dextral Sakhalin‐Hokkaido fault zone in the Kashevarov region of central SLB, where a fan of north to northwest striking late Tertiary transpressional fault splays marks the northern termination of the dextral fault zone. Older SLB normal faults are locally deformed, and half grabens are partly inverted, by the slightly younger dextral splays. The two regional fault systems are essentially conjugate, and the pattern of intersection closely resembles that predicted by recent modelling studies of Ind

ISSN:0278-7407    

DOI:10.1029/95TC03684

年代:1996

数据来源: WILEY

摘要:

Middle Proterozoic rocks underlying the Andes in western Bolivia, western Argentina, and northern Chile and Early Proterozoic rocks of the Arequipa massif in southern Perú form the Arequipa‐Antofalla craton. These rocks are discontinuously exposed beneath Mesozoic and Cenozoic rocks, but abundant crystalline clasts in Tertiary sedimentary rocks in the western altiplano allow indirect samples of the craton. Near Berenguela, western Bolivia, the Oligocene and Miocene Mauri Formation contains boulders of granodiorite augen gneiss (1171±20 Ma and 1158±12 Ma; U‐Pb zircon), quartzose gneiss and granofels that are inferred to have arkosic protoliths (1100 Ma source region; U‐Pb zircon), quartzofeldspathic and mafic orthogneisses that have amphibolite‐ and granulite‐facies metamorphic mineral assemblages (∼1080 Ma metamorphism; U‐Pb zircon), and undeformed granitic rocks of Phanerozoic(?) age. The Middle Proterozoic crystalline rocks from Berenguela and elsewhere in western Bolivia and from the Middle Proterozoic Belén Schist in northern Chile generally have present‐day low206Pb/204Pb (18.5), a difference inferred to reflect Grenvillian granulite metamorphism. The Pb isotopic compositions for the various Proterozoic rocks lie on common Pb isotopic growth curves, implying that Pb incorporated in rocks composing the Arequipa‐Antofalla craton was extracted from a similar evolving Pb isotopic reservoir. Evidently, the craton has been a coherent terrane since the Middle Proterozoic. Moreover, the Pb isotopic compositions for the Arequipa‐Antofalla craton overlap those of the Amazon craton, thereby supporting a link between these cratons and seemingly precluding part of the Arequipa‐Antofalla craton from being a detached fragment of another craton such as eastern Laurentia, which has been characterized by a different U/Pb history. Pb isotopic compositions for the Arequipa‐Antofalla craton are, furthermore, distinct from those of the Proterozoic basement in the Precordillera terrane, western Argentina, indicating a Pb isotopic and presumably a tectonic boundary between them. The Pb isotopic compositions for the Precordillera basement are similar to those of eastern Laurentia, and support other data indicating that these rocks are a detached fragment of North America. Finally, the distinct Pb isotopic evolution history of the Arequipa‐Antofalla craton and eastern Laurentia require minor modification to tectonic models linking eastern North America‐Scotland to the or

ISSN:0278-7407    

DOI:10.1029/95TC03248

年代:1996

数据来源: WILEY

摘要:

The study of ancient, deeply eroded orogenic belts can provide information about tectonic processes in the deep crust that is unobtainable from upper crustal sections in younger belts. In this light, the Paleoproterozoic Torngat Orogen in northeastern Canada is ideal because it is well exposed and eroded to maximum depths of 28 km, revealing information about transpressional deformation mechanisms during continent‐continent collision between the Archean Nain and southeastern Rae provinces. Evidence from detailed structural mapping, metamorphic petrology, and geothermobarometry of a 115‐km transect of the orogen is combined with previously published geochronology to construct pressure‐temperature‐time‐deformation (P‐T‐t‐d) paths for rocks from the granulite‐facies core and amphibolite‐facies Foreland zone of the orogen. Results show that rocks followed a clockwiseP‐T‐tpath, but different parts of the orogen reached peak conditions at different times. The orogen evolved through three temporally and kinematically distinct events over ∼80 m.y., rather than continuous deformation. Events were separated by gaps of 15–30 m.y. and include (1) crustal thickening during continental collision at ca.1860 Ma under peak metamorphic conditions of 9.5 ± 0.5 kbar and 950° ± 50°C in the core of the orogen, and prethermal peak conditions of 4.5 kbar and 550°C in the Foreland zone; (2) sinistral transpressional shear at ca. 1845–1822 Ma under post‐peak conditions of 7.3 ± 0.5 kbar and 750° ± 50°C in the core of the orogen, but at thermal peak conditions of ∼5 kbar and 700°C in the Foreland zone; and (3) west‐directed exhumation of the orogen at 1794–1740 Ma, under lower‐grade conditions of up to ∼5 kbar and 600°C. The overall transpressional features of the orogen are shown to be a cumulative result of three distinct tectonic events which can be related to changes in plate/subdu

ISSN:0278-7407    

DOI:10.1029/95TC03771

年代:1996

数据来源: WILEY