摘要:

Surface deformation and a tsunami accompanied the destructive April 22, 1991, Costa Rica‐Panamá earthquake (Ms = 7.5). Along a 135 km stretch of Caribbean coast, coseismic uplift was measured between the lower and upper limits of sessile intertidal organisms stranded on coral reefs, the preearthquake and postearthquake high tide levels located from driftwood lines on beaches, and the preearthquake and postearthquake tide levels as pointed out by local residents. The nature and distribution of offshore vertical displacements were further constrained from analysis of measured run‐up heights and reported arrival times of the tsunami. Uplift detected along the coast jumped, within 4 km, from zero to 157 cm near Limón and generally decreased over a distance of 70 km southward to the border with Panamá. These data map an axis of uplift that intersects the coastal beach ridge just north of the port of Moín and runs offshore to the east and south roughly parallel to the coast. No surface faulting was found. The earthquake and tsunami were generated by backarc thrusting along faults that bound the north Panamá deformed belt and dip from the Caribbean Sea beneath Costa Rica and northern Panamá. Combined geodetic and seismological data indicate that the main rupture dips landward at an angle of about 30° and is approximately 40 km wide and 80 km long. Dislocation models suggest 2.2 m of slip on a causative thrust fault striking between 105° and 120°. We estimate that the repeat time for this type of earthquake is 200 to 1100 years. The historical record and new isotopic data favor the middle

ISSN:0278-7407    

DOI:10.1029/92TC00609

年代:1992

数据来源: WILEY

摘要:

Geophysical and regional geologic data provide evidence that parts of the oceanic crust in the abyssal basins of the Bering Sea have been created or altered by crustal extension and back‐arc spreading. These processes have occurred during and since early Eocene time when the Aleutian Ridge developed and isolated oceanic crust within parts of the Bering Sea. The crust in the Aleutian Basin, previously noted as presumably Early Cretaceous in age (M1–M13 anomalies), is still uncertain. Some crust may be younger. Vitus arch, a buried 100‐ to 200‐km‐wide extensionally deformed zone with linear basement structures and geophysical anomalies, crosses the entire west central Aleutian Basin. We suggest that the arch and the inferred fracture zones in the Aleutian Basin are early Cenozoic structures related to the early entrapment history of the Bering Sea. These structures lie on trend with known early Cenozoic structures near the Bowers‐Shirshov‐Aleutian ridge junction and on the Beringian continental margin (with possible continuation into Alaska); the structures may have coeval and cogenetic(?) histories for early Cenozoic and possibly younger times. Cenozoic deformation within parts of the Bering Sea region is principally extensional, although the total amount of extension is not known. As examples, the Komandorsky basin formed by back‐arc seafloor spreading, the Aleutian Ridge has been extensively sheared, and extensional block faulting is common. Sedimentary basins of the Bering shelf have formed by extension associated with wrench faulting. The Cenozoic deformation throughout the Bering Sea region probably results from the interaction of major lithospheric plates and associated regional strike‐slip faults. We present models for the Bering Sea over the past 55 m.y. that show oceanic plate entrapment, back‐arc faulting and spreading along Vitus arch, breakup of the oceanic crust in the Aleutian Basin at fracture zones, and back‐arc spre

ISSN:0278-7407    

DOI:10.1029/92TC00214

年代:1992

数据来源: WILEY

摘要:

A multichannel seismic line acquired in the western Solomon Sea images reflectors within the New Britain accretionary wedge which we interpret as accreted duplexes of the downgoing oceanic plate. The seaward edge of the largest duplex is located just 6 km from the toe of the accretionary wedge suggesting recent incorporation into the wedge. These data therefore provide an excellent opportunity to determine the dynamics of oceanic basement accretion. The oceanic basement is represented by a high‐amplitude, low‐frequency reflector that can be traced up to 40 km arcward of the toe of the accretionary wedge on several seismic lines in this survey. Another prominent reflector, similar in character to the basement reflector, lies approximately one second higher in at least one seismic section. This reflector is likely to be the top of a basement duplex. Structures imaged above the basement duplex define a preduplex accretionary wedge, whereas those at the toe indicate a new episode of postduplex accretion. We have tested this interpretation using both magnetic modeling and interval velocity analysis. Magnetic models of the wedge that include basement duplexes provide a good match to the observed magnetic data and provide limits on the size and interval velocities of the slivers. Interval velocities between the two strong reflectors, calculated using stacking velocities and constrained through magnetic modeling, exceed 5000 m/s and are consistent with our interpretation of ophiolitic slivers within the accretionary we

ISSN:0278-7407    

DOI:10.1029/91TC02901

年代:1992

数据来源: WILEY

摘要:

Using multichannel seismic data from the western Solomon Sea we have imaged several 2.5 to 3‐km‐thick oceanic basement slivers within the New Britain accretionary wedge which may serve as a modern analogue for the detachment of some ophiolitic slivers. The most recently accreted basement sliver is located just over 5 km from the thrust front. An approximate area balance indicates the subjacent section of the downgoing plate from which this sliver originated is between 11 km and 15 km arcward of its present position. Though we have not imaged the basement ramp, sedimentary strata beneath the basement sliver pinch out between 9 and 12 km from its seaward edge, suggesting that detachment occurred in that vicinity. Normal faults reactivated in response to the flexural bending of the downgoing oceanic plate are a prominent feature of this region. Significantly, almost all basement normal faults near the recently accreted basement sliver, including those imaged beneath the toe of the wedge, dip arcward. Based on these data, we suggest that small (1–3 km thick) basement slivers are decoupled along favorably oriented, preexisting zones of weakness formed by normal fault detachments within the oceanic bas

ISSN:0278-7407    

DOI:10.1029/91TC03093

年代:1992

数据来源: WILEY

摘要:

The northeast Brazilian rift basins provide important data critical to the understanding of continental rifting processes associated with the opening of the South Atlantic. These basins represent the locus of intersection of the Southern and Equatorial branches and some basins yield substantial chronostratigraphic data that constrain the temporal and spatial interaction of the rift phases. Similar data are not found in its counterpart in Africa, especially for the Neocomian. These Early Cretaceous rift basins of northeast Brazil illustrate key three‐dimensional geometries of intracontinental rift systems, mainly controlled by the basement structural framework. During the main rift phase (Syn‐rift II, Neocomian‐early Barremian) extensional deformation was distributed over three main rift axes: (1) the Gabon‐Sergipe Alagoas (GSA) trend, (2) the Recôncavo‐Tucano‐Jatobá (RTJ) trend and (3) the Cariri‐Potiguar (CP) trend. During this phase, extensional deformation jumped west from the easternmost basins (GSA trend) to a series of NE trending intracratonic basins (RTJ and CP trends), characterized by a set of asymmetric half grabens separated by basement highs, transfer faults, and/or accommodation zones. These basins are typically a few tens of kilometers wide and trend NE‐SW, roughly perpendicular to the main extension direction during the Neocomian. Preexisting upper crustal weakness zones, like the dominantly NE‐SW trending shear zones of the Brazilian/Pan‐African orogeny, controlled the development of intracrustal listric normal faults. Internal transverse structures such as transfer faults and accommodation zones were also controlled by the local basement structural framework. The megashear zones of Pernambuco (Brazil) and Ngaundere (Africa) seem to have behaved like a huge accommodation zone, accommodating extensional deformation along the RTJ/GSA trends with simultaneous extension along the CP trend. During the late Barremian (Syn‐rift phase III), a significant change in rifting kinematics occurred, when the CP trend was aborted and major rifting initiated at the Equatorial branch. During the Aptian, while the Equatorial branch and Benue trough (Africa) experienced the main rift phase, the RTJ trend was aborted and the GSA trend developed a transitional phase between the rift and drift stage. The GSA trend and the offshore Potiguar basin represent the site of continued evolution into passive margin basins followin

ISSN:0278-7407    

DOI:10.1029/91TC03092

年代:1992

数据来源: WILEY

摘要:

Prefolding magnetization components have been resolved at 38 sites of Liassic “Adnet” limestones in the central Northern Calcareous Alps (NCA) and at one site in the Drauzug. The site mean directions are northeasterly and down, and are closer to coeval European directions than to Southern Alpine “African” directions. The western end of both the NCA and of the Drauzug yield different site mean directions and negative fold tests indicating remagnetization. The sampling localities have been rotated relative to one another during thrusting, however, comparison of paleomagnetic poles with the European apparent polar wander path implies an average clockwise rotation of about 30° for the NCA relative to Europe. Larger‐scale clockwise rotations in excess of 80°, relative to the Southern Alps, are implied by comparison with the Southern Alpine (African) polar wander path. Facies analysis of Jurassic and Lower Cretaceous basinal sediments in the central part of the NCA suggests that Liassic subsidence was related to presently E‐W sinistral strike‐slip faults, whereas Malm‐Berriasian subsidence was controlled by presently E‐W south‐dipping normal faults. Both paleomagnetic and facies data tend to indicate that the NCA did not undergo the “African” rotation seen in Southern Alpine paleomagnetic data. This implies the existence of a short‐lived tract of Triassic?‐Jurassic ocean floor between the NCA and the Southern Alps, which linked the Pindos Ocean to the Ligurian trough and allowed Adria to rotate independently from the NCA during the Late Jurassic. Eo‐Alpine deformation and metamorphism are associated with the Early Cretaceous clo

ISSN:0278-7407    

DOI:10.1029/91TC03089

年代:1992

数据来源: WILEY

摘要:

Jurassic and Cretaceous paleomagnetic data from 46 sites in the Venetian Alps, Trento Plateau and SE Lombardian Basin have been compiled. Nine of these site mean directions have been previously published. The sites can be divided into four age groups (Senonian, Hauterivian/Barremian, Kimmeridgian/Tithonian, and Callovian/Oxfordian). For three of the age groups, positive regional fold tests indicate (at the 99% confidence level) that the characteristic magnetization components predate the deformation. Structural mapping in the Venetian Alps allowed us to assign sites to individual thrust sheets, and we observe no significant relative rotation of thrust sheets in the nappe pile. Indeed, we observe remarkable consistency in mean direction throughout the entire sampling area for each of the four age groups. The Senonian and Hauterivian/Barremian pole positions lie close to coeval poles on African apparent polar wander paths (APWPs). This result is consistent with previous analyses of middle and Late Cretaceous data from the Venetian Alps. The new Jurassic poles deviate significantly from recently published African APWPs, reflecting either the poor definition of the African APWP in the Late Jurassic or about 20° anticlockwise rotation of the Southern Alps relative to Africa during Early Cretaceous time. The former of the two possibilities is favored by a new Late Jurassic/Early Cretaceous pole from North America, and by Early Permian paleomagnetic data from the Southern Alps which are consistent with coeval African data, implying that the movement of the Southern Alps was similar to that of Africa since Early Permian

ISSN:0278-7407    

DOI:10.1029/92TC00212

年代:1992

数据来源: WILEY

摘要:

Upper Jurassic and Lower Cretaceous basinal strata are preserved in a discontinuous belt along the inboard margin of the Alexander‐Wrangellia‐Peninsular terrane (AWP) in Alaska and western Canada, on the outboard margin of terranes in the Canadian Cordillera accreted to North America prior to Late Jurassic time, and along the Cordilleran margin from southern Oregon to southern California. Nearly all of the basinal assemblages contain turbiditic strata deposited between Oxfordian and Albian time. Arc‐type volcanic rocks and abundant volcanic detritus in many of the assemblages suggest deposition within or adjacent to a coeval arc complex. On the basis of the general similarities between the basinal sequences, we propose that they record involvement of the AWP in the Late Jurassic‐Early Cretaceous evolution of the Cordilleran margin. A geologically reasonable scenario for the accretion of the AWP includes (1) Middle Jurassic accretion to the Cordilleran margin, in particular the Stikine and Yukon‐Tanana terranes, in a dextral transpressional regime, (2) Late Jurassic‐Early Cretaceous overall northward translation of the AWP and evolution of a series of transtensional basins within a complex dextral strike‐slip system along the Cordilleran margin, and (3) mid‐Cretaceous structural imbrication of the AWP and inboard terranes that either terminated or resulted in a change in the character of deposition in the marginal basins. Mid‐Cretaceous deformation along the inboard margin of the AWP was broadly synchronous with contractional deformation throughout the Cordillera and most likely due to changes in subduction zone parameters along the Cordilleran margin, outboard of the AWP, rather than col

ISSN:0278-7407    

DOI:10.1029/92TC00241

年代:1992

数据来源: WILEY

摘要:

Yukon‐Tanana Terrane (YTT) underlies much of central and western Yukon and east central Alaska. Its history and tectonic evolution, particularly prior to mid‐Mesozoic time, has been largely obscured by younger magmatism and tectonism. The application of geochronological and isotopic techniques over the past decade, together with detailed field studies in certain critical areas of the terrane, has shed new light on the early history of YTT. Much of YTT is a product of episodic continental arc magmatism, with three main pulses in Late Devonian‐Early Mississippian, mid‐Permian, and Late Triassic‐Early Jurassic time. From Late Devonian to mid‐Mississippian time, subduction was north or northeast dipping, but arc polarity was apparently reversed by mid‐Permian time. The main, subhorizontal structural fabric characterizing much of YTT was produced between mid‐Permian time and the onset of renewed magmatism in Late Triassic time and probably reflects a major continent‐continent collision. Although the Triassic‐Jurassic magmatism is also considered to be arc related, it occurred over a very broad area of not only YTT, but also Quesnellia, and the Stikine, Nisling, Cache Creek, and Slide Mountain terranes. This magmatism appears to have coincided with final amalgamation of the Intermontane Superterrane, and the arc polarity and the position and orientation of the associated subduction zone is still controversial. Available evidence suggests that Nisling Terrane is closely related to YTT and mainly consists of older strata that underlie the Devonian and younger units generally considered to be more typical of YTT. There are close similarities between YTT and a number of other “pericratonic” terranes in the central and eastern parts of the Cordillera, and it is likely that these terranes originally formed a single arc and arc basement assemblage which has now been fragmented and dispersed b

ISSN:0278-7407    

DOI:10.1029/91TC01169

年代:1992

数据来源: WILEY

摘要:

On Ellesmere Island, the Clements Markham fold belt of the Franklinian mobile belt is flanked to the northwest by a Caledonian terrane, Pearya. On the basis of the Proterozoic to mid‐Ordovician stratigraphic and structural similarities with Spitsbergen and on the mid to late Silurian relationship between Pearya and Ellesmere Island, some strike‐slip displacement must have been involved in the accretion of Pearya to its present position. However, the structural style of the Clements Markham fold belt on Ellesmere Island is characterized by chevron folds with axial planar cleavage (flexural folding) and shallow plunging fold axes. The present terrane boundary faults show steeply plunging lineations indicating a vertical component of motion strongly predominating the strike‐slip component. Thus the first detailed structural study of the area has produced no positive structural evidence for late Silurian transcurrent shear or transpression. Therefore decoupling of a strike‐slip component of plate motion from a distributed compressive component is suggested for northern Ellesmere Island. Accretion of Pearya most likely occurred along one of the major fault zones to the southeast of the thrust faults marking the present terrane boundary. However, indications of strike‐slip displacement have not yet been observed along these faults possibly due to later thrust faulting. The Silurian Imina and Lands Lokk formations form an accretionary wedge of orogen‐derived turbidites which were deposited on oceanic or highly attenuated continental crust. Thus the time interval spanning the deposition and deformation of the turbidite formations dates the period of accretion and of predominantely orthogonal continental convergence. In the light of current ideas concerning the episodic and diachronous development of orogenic belts, the Silurian deformation in northwestern Ellesmere Island may be interpreted as the beginning of the orogenic cycle that culminated in the Late Devonian ‐ early Carboniferous Elle

ISSN:0278-7407    

DOI:10.1029/92TC00277

年代:1992

数据来源: WILEY