摘要:

Salt marsh subsurface deposits (0–4 m depth) in Netarts Bay, a small coastal lagoon of northern Oregon, record six events of marsh burial in the last several thousand years. Five of the buried marsh surfaces show sharp, nonerosional upper contacts with either anomalous sand layers (tsunami deposits) or tidal flat mud deposits. These sequences indicate episodic, abrupt subsidence of the marsh surfaces to low intertidal levels. In contrast, lower marsh contacts with underlying intertidal muds are gradational, indicating gradual uplift and development of the marsh. Three independent measures of deposit elevation relative to mean tidal level (percent organics, diatom assemblages, and percent eolian sand) have been used to estimate vertical displacements of marsh surfaces. Abrupt subsidence displacements of 1–1.5 m alternate with gradual uplift displacements of the order of 0.5–1.0 m. The vertical tectonic movements are interpreted to reflect coseismic strain release (abrupt subsidence) following interseismic strain accumulation (gradual uplift), associated with interplate coupling between the Juan de Fuca Plate and the North American plate in the Cascadia subduction zone. Recurrence intervals between subsidence events range from possibly less than 300 years to at least 1000 years, with the last dated event likely occurring 300–400 radiocarbon years before present (RCYBP). Significant14C age overlaps of at least four subsidence events recorded at Netarts and reported for southern Washington and other northern Oregon bays (at 300–500, 1000–1300, 1400–1800, and 3000–3300 RCYBP) suggest the potential for event synchroneity over at least 200 km of the central part of the Cascadia subduction zone. Additional work is needed to test the synchroneity of these episodic events of cosei

ISSN:0278-7407    

DOI:10.1029/TC009i001p00001

年代:1990

数据来源: WILEY

摘要:

The Maraetotara Plateau‐Cape Kidnappers area in southern Hawke's Bay, New Zealand, lies in the forearc region of the obliquely convergent Hikurangi margin, where the Pacific Plate is presently subducting beneath the leading edge of the Australian Plate. Uplift accompanied by extensional faulting has been the dominant style of deformation in this area since mid‐Pleistocene time. Principal strain directions and strain ratios, determined from mesoscale fault arrays associated with major faults, document the regional extension. In the Cape Kidnappers area, fault arrays record plane strain with northwest horizontal extension and vertical shortening. In the Maraetotara Plateau area, fault arrays record triaxial strain with roughly equal magnitudes of horizontal extension in two principal strain directions, either east‐west and north‐south, or northwest and northeast, and vertical shortening. The morphology, seismicity, and nature and distribution of active faults of the southern Hawke's Bay region suggest that the Maraetotara Plateau‐Cape Kidnappers area is the on‐land expression of an outer‐arc high, which is developed between the accretionary wedge on the east and the forearc basin on the west. The cause of uplift and extension in the Maraetotara Plateau‐Cape Kidnappers area may be either upper plate deformation related to structural evolution of the outer‐arc high and underlying backstop, underplating on the Hikurangi subduction zone, or a combination of these processes. The uplift and horizontal extension occurring in the Maraetotara Plateau‐Cape Kidnappers area contrast sharply with active deformation in adjacent parts of the forearc, reflecting a distinct partitioning of strain in this part of the obliquely conver

ISSN:0278-7407    

DOI:10.1029/TC009i001p00023

年代:1990

数据来源: WILEY

摘要:

Three seismically defined lower crustal blocks (LCBs) have been recognized to underlie the familiar tectonic‐stratigraphic zones of the Canadian Appalachians. Tectonic development of the orogen is inferred to have been strongly influenced by collision of the two outboard LCBs (Central and Avalon LCBs) against the sharply irregular North American craton (Grenville LCB), as evidenced by an abrupt offset in the trend of the orogen localized at this irregular edge. The LCBs are restored to a precollisional configuration by (1) removing Carboniferous strike‐slip motion on major faults linking through the Magdalen pull‐apart basin; and (2) removing Devonian(?) offset along a hypothesized fault through the present‐day Strait of Canso (Canso fault) to realign the Central and Avalon LCBs, presumed to have been continuous prior to accretion to North America. The restoration leads to (1) a qualitative explanation for deep seismic reflection observations across the Magdalen basin which suggest that portions of the underlying crust do not correspond to any of the three principal LCBs; and (2) a plate tectonic model for the orogen which suggests the backarc basin separating the Taconian (Ordovician) arc from the eastern margin of the early Paleozoic Iapetus ocean was at least as wide (>450 km) as the offset in the irregular Grenville LCB, inherited from rifting of Iapetus. The present‐day Southwest Newfoundland Transform Margin may be a segment of the Canso fault reactivated during Mesozoic opening of the Atlan

ISSN:0278-7407    

DOI:10.1029/TC009i001p00045

年代:1990

数据来源: WILEY

摘要:

The orientation of the principal axes of the regional stress tensor in and near the western Great Basin boundary is calculated from the inversion of earthquake focal mechanism data. Data were taken from three regions along the western Great Basin‐Sierra Nevada boundary: Mammoth Lakes, Round Valley, and Chalfant Valley, and at two regions within the Great Basin interior: Mono‐Walker Lake and west central Nevada. The depth range is similar for all data, earthquakes are essentially restricted to the uppermost 16 km of the crust. The stress tensor changes from the Great Basin boundary to the Basin interior, for example, the extension direction changes by about 50° from roughly E‐W at the boundary in the west to roughly NW‐SE inside of the basin. In the boundary region the mean T axis inferred from focal mechanisms rotates clockwise by 27° from N64°E in the Mammoth Lakes area to N91°E in the Chalfant Valley, about 50 km southeast of Mammoth Lakes. However, the components of the best fitting stress tensors in the two regions vary only by 12° so that a common stress tensor with a greatest principal stress azimuth of N10°W and a least principal stress azimuth of N80°E, both components nearly horizontal (favoring strike slip), describes the seismic deformation of the boundary region quite well. The stress tensors for the Mono region and west central Nevada have similar greatest and least principal stress azimuths (northeasterly and northwesterly, respectively), however, the plunges of the greatest principal stress are different: nearly horizontal for the Mono area and about 45° for central Nevada, reflecting the strong contribution of normal faulting in the central Nevada earthquake data. The resulting Φ values, which describe the relative sizes of the three stress components, vary from 0.5 to 0.65, which indicates that the magnitudes of the three principal stresses are

ISSN:0278-7407    

DOI:10.1029/TC009i001p00063

年代:1990

数据来源: WILEY

摘要:

The Anadyr‐Koryak region in northeastern USSR consists of a series of exotic terranes, most of which arrived from the Pacific and accreted to the Eurasian margin in Cretaceous and Tertiary times. Eight major terranes can be recognized: (1) The Kanchalan and (2) Pekulnei terranes include possible ancient blocks in their cores. The (3) Penzhina, (4) Ust' Belaya, and (5) Vaega terranes (which are parts of the Talovo‐Mainsky zone) have chaotic structure incorporating Paleozoic and early Mesozoic oceanic, island arc, continental rise, and other lithologie assemblages indicative of a subduction melange environment. The (6) Mainits and (7) Ekonai terranes (two parts of the Khatyrka megaterrane) contain late Paleozoic and Triassic island arc and back‐arc assemblages with faunas of Tethyan provenance. The (8) Olyutorsky terrane is composed mostly of Late Cretaceous island arc assemblages which accreted to the Eurasian margin in the middle Tertiary. Computed trajectories of Kula/Eurasia and Pacific/Eurasia relative motions suggest that some (or most) of these terranes traveled thousands of kilometers from the central Pacific until they eventually collided with, and were attached to, the Eurasian m

ISSN:0278-7407    

DOI:10.1029/TC009i001p00081

年代:1990

数据来源: WILEY

摘要:

Paleomagnetic and structural analyses have been conducted on three well‐exposed sections through a 6–8 km thick pile of molassic red beds (Murree formation) in the lowermost tectonic unit of the Hazara‐Kashmir Syntaxis. Micropaleontological age determinations of the lowermost Murree sediments indicate Late Paleocene deposition (55 m.y.). From south to north, the sections are situated in the Jhelum, Neelum (both in Azad Kashmir), and Kaghan (northeastern Pakistan) valleys. Thermal demagnetization experiments suggest that haematite with high unblocking temperatures carries stable characteristic remanence directions. The relationship between finite strain and magnetic fabric was established by mapping deformed reduction spot strain markers and by measuring the anisotropy of magnetic susceptibility (AMS). For the Jhelum valley the weakly tectonized Murree beds are characterized by flattened AMS ellipsoids resulting from diagenetic compaction. Inclination values suggest that the Murree foreland basin started to develop at about 8°N during the early suturing of India and the development of island arcs to the north. India has moved northward by at least 2600 km since collision with Eurasia in the Paleocene. Declination values suggest 45° of clockwise rotation of the block supporting the Murree formation relative to the Indian craton. For the Neelum and Kaghan valleys, quantitative strain mapping shows a progressive increase of deformation northward. NRM directions rotate passively toward the cleavage plane which parallels the foliation of the AMS ellipsoids. NRM directions and AMS ellipsoids are deformed because of superposition of tectonic strain on a primary compactional strain. The AMS pattern is interpreted in light of this superposition, and a regional deformation path from south to north is suggested. A tectonic rotation model is proposed which is consistent with the transport directions around the Hazara‐Kashmir Syntaxis and the rotation of thrust sheets indicated by the NRM data. This model relates the convergent transport directions with the mean indentation direction of India

ISSN:0278-7407    

DOI:10.1029/TC009i001p00103

年代:1990

数据来源: WILEY

摘要:

Paleomagnetic data from 231 samples from 31 sites in rocks of Upper Jurassic to Miocene age in Sarawak (Malaysian Borneo) reveal a trend of increasing counterclockwise (CCW) declination deflection with age. Six sites in Tertiary hypabyssal intrusions show 8° to 52° of CCW deflection. The intrusion deflected 52° CCW was K‐Ar dated at 26 m.y. (Upper Oligocene), while one deflected 22° CCW gives a 17 m. y. age (Lower Miocene). Three sites in the Upper Eocene to Miocene(?) Silantek Formation show an average 40° of CCW deflection. Prefolding directions, showing 90° of CCW deflection, are isolated in 4 sites (including two positive fold tests) in Upper Jurassic and Cretaceous rocks of the Bau Limestone and Pedawan Formations. A postfolding, Cenozoic remagnetization with an average of 60° of CCW deflection is found in five Bau Limestone sites. Three sites in the Upper Jurassic Kedadom Formation show an average of 50° of CCW deflection. CCW declination deflections found in Mesozoic and Cenozoic rocks as far as 400 km east and 150 km south of Sarawak, in Kalimantan (Indonesian Borneo), also fit the trend of deflection versus age. On the basis of the regional consistency of declination deflection versus age, along with geologic evidence the data are considered to be evidence of a regional (rather than a local block or distributed shear) rotation. The domain of CCW rotation extends into West Malaysia, suggesting that West Borneo and the Malay Peninsula may have been a stable block during the latest Cretaceous and Cenozoic. West Malaysia and Borneo may have had different histories in the rest of the Mesozoic. The data imply up to 108° CCW rotation of Borneo with respect to stable Eurasia, sometime during the Cretaceous and Cenozoic. Cenozoic rotation may also have occurred between Indochina and Borneo. The sense of rotation shown by the data does not support the “propagating extrusion tectonics” model for Cenozoic

ISSN:0278-7407    

DOI:10.1029/TC009i001p00123

年代:1990

数据来源: WILEY

摘要:

The central part of the Rhodope‐Pontide fragment, one of the major tectonic units in Turkey, provides critical data for evaluating the Cimmeride and Alpide evolution of the Mediterranean Tethysides. Tectonic events that affected the central part of the Rhodope‐Pontide fragment since the end of the Paleozoic, generated east‐west trending belts with the event of every episode redeforming and partly obliterating the structures of previous episodes. This evolution may be conveniently described in terms of three major episodes: (1) Two different realms of pre‐Dogger oceanic rocks are present in the area. The northern realm coincided with main branch of Paleo‐Tethys that was being actively destroyed by south dipping subduction. The southern realm, the Karakaya ocean, a back arc basin related to this subduction, began opening by rifting of a retroarc carbonate platform during the Permo‐Triassic. To the west a continental domain with sparse magmatism seperated the two oceanic areas. Toward the east the two oceans become united by the wedging out of the continental domain. These two pre‐Dogger oceans closed during the Lias, and their remnants were emplaced between the southern margin of Laurasia and the fragments of the Cimmerian continent. (2) The second episode partly overlapped the first with rifting south of the Cimmerian continent fragment during the Lias. This rifting was followed by a transgression which covered the ruins of the Cimmeride orogenic belt by the Malm. This rifting concurrently led to the development of the northern branch of the Neo‐Tethys and a south facing Atlantic‐type continental margin. A southerly thickening sedimentary prism developed on this margin during the Lias to early Cretaceous interval. (3) The floor of the northern branch of Neo‐Tethys began to be consumed along the north dipping subduction zone beneath the previosly constructed continental margin. This convergent margin generated a magmatic arc to the north and to the south a subduction accretion complex, present width of which approaches 80 km. To the south of the subduction accretion complex yet another convergent margin is indicated by the presence of the remnants of an ensimatic arc. The northern branch of the Neo‐Tethys was totally consumed in the late Cretaceo

ISSN:0278-7407    

DOI:10.1029/TC009i001p00141

年代:1990

数据来源: WILEY

摘要:

Geological and geophysical data, coupled with recent plate tectonic reconstructions, suggest that the Cenozoic geologic history of the northern Venezuela‐Trinidad area has been dominated by strike‐slip displacement of discrete crustal blocks. Allochthonous terranes within the area include metavolcanic rocks of the Cretaceous Villa de Cura Group and metamorphic rocks of the Precambrian to Cretaceous Cordillera de la Costa. A relatively competent crustal block (Margarita Block) is defined by an outline around the metamorphic basement of Margarita Island, the Araya/Paria peninsula, the Northern Range of Trinidad, and Tobago Island. Reconstruction of the Margarita Block to its original position requires at least partial closure of the Falcon Basin, closure of the Bonaire and Cariaco basins, and restoration of about 50 km of motion on both the Oca and Bocono faults. Post middle Eocene eastward translation of the Caribbean plate caused eastward motion of the Margarita Block. A minor change in relative plate motion during the late Oligocene or early Miocene produced a right step in the Moron fault, forming the Cariaco pull‐apart basin and El Pilar fault zone. Maximum offset on El Pilar fault is estimated to be no more than 125 km, though displacement along the entire fault zone may have been greater. Transpressional stresses between the Caribbean plate and northern South America caused folding of the Serrania del Interior of Venezuela and the Central Range of Trinidad. Eastward migration of transpressional stresses at the southeastern corner of the Caribbean‐South American plate boundary is being accommodated by formation of oblique thrusts, transpressive anticlines, and downwarping of the crust. Bouguer gravity data suggest that Jurassic‐aged Atlantic oceanic crust is being depressed as the Caribbean plate expands into the Demerara Plateau area. This study suggests that the faults and transtensional/transpressional/compressional structures identified in this study are the result of stresses produced during the large eastward translation of the Caribbean plate since the Paleocene, and are not the product of a she

ISSN:0278-7407    

DOI:10.1029/TC009i001p00161

年代:1990

数据来源: WILEY

摘要:

Earthquake locations, depths and focal mechanisms from the Southern California Regional Network (1977–1985) are used to identify the orientation and sense of slip of active subsurface faults in the Easternmost Transverse Ranges (ETR). The ETR are separated from the Salton Trough province by the southernmost strands of the San Andreas fault (SAF). Much of the seismicity in the ETR is concentrated well northeast of the SAF at relatively shallow depths under the Little San Bernardino Mountains. Many of these earthquakes reflect slip on steeply dipping, left‐lateral faults striking northeast to east, at relatively high angles to the adjacent SAF. Focal mechanisms in the ETR show predominantly strike‐slip, normal, or oblique‐normal faulting, and share common near‐horizontal T axes striking WNW. P axes range from near vertical to near horizontal and strike mostly NNE. In contrast, reverse and strike‐slip focal mechanisms that exhibit persistent north trending, near‐horizontal P axes characterize the San Gorgonio Pass area immediately to the west. These different patterns of strain geometries are inferred to represent changes in local stress regime and clearly establish a boundary between contrasting tectonic styles of contemporary secondary deformation along the SAF. This boundary, which in the Coachella Valley may be the Mission Creek fault, is also distinguished by abrupt changes in (1) rate and depth of seismic activity; (2) topography; (3) Quaternary vertical deformation; (4) strikes and dips of major branches of the SAF; and (5) seismic velocities in the crust and upper mantle. The preponderance of secondary normal faulting in the ETR versus secondary reverse faulting in the San Gorgonio Pass region suggests that fault‐normal stress is much less across the SAF adjacent to the ETR. If a friction law where strength is proportional to normal stress applies to the SAF, then a smaller tectonic shear stress would be required tor slip in large earthquakes along the Salton Trough segment. In this case, the southernmost SAF may have shorter repeat times, smaller average displacements, and lower moment release rates than earthquakes that rupture through the major restraining bend in Sa

ISSN:0278-7407    

DOI:10.1029/TC009i001p00185

年代:1990

数据来源: WILEY