摘要:

Metamorphic core complexes that originated in the middle crust have been exposed by uplift and extension. Indicators of paleohorizontal planes are few or absent, thus it is difficult to determine the degree of tilting that might have accompanied this process. However, if aberrant paleomagnetic directions (paleodirections) are observed in core complexes or their associated intrusive rocks, and if large‐scale rotation and translation can be excluded, then these paleodirections can be used to calculate the sense and magnitude of tilting. We describe paleodirections from mid‐crustal rocks in southeastern British Columbia, where uplift and extension occurred in Eocene time about a central culmination aligned north‐south. Several large extensional faults dip both to the west and to the east of this central culmination. The Eocene Granby pluton (49.3°N, 118.5°W), situated to the west of the main culmination, has a mean direction of magnetization (D°,I°) 309°, 41° (α63° = 7° the standard error) and paleopole at 44°N, 138°E (A63° = 7°). To the east of the main culmination, a 20 km long section of altered Eocene Syringa dikes and associated metadiorite (49.3°N, 117.7°W) has a mean direction of 56°, 47° (α63° = 3°) and paleopole at 56°N, 319°E (A63° = 3°). The paleopoles are very different from one another, and from those calculated from contemporaneous (53 to 49 Ma) rocks of the North American craton to the east. They also differ from those observed from coeval bedded volcanic rocks to the west of the sampling area. Hence, these large aberrancies cannot be caused by translations. We suggest that they are caused by tilting which occurred during the extension process. The tilts are in opposite senses on either side of the central culmination. They are estimated to be down 38°±8° at 99°E for the Granby Pluton, and down 41°±4° at 270°W for the dikes and metadiorite. These tilts are consistent with the Wernic

ISSN:0278-7407    

DOI:10.1029/TC009i005p00925

年代:1990

数据来源: WILEY

摘要:

We have analyzed Satellite Laser Ranging (SLR) observed baseline rates of change and compared them with rates determined from sea floor spreading rates and directions, and earthquake solutions. With the number of years of observation now over six for many of the baselines, the inaccuracy of determining baseline rates of change has diminished so that in some cases it is less than a few mm per year. Thus a direct comparison between baseline rates of change and rates of change established using geophysical information (which we call geological rates) is now feasible. In most cases, there is good agreement between the rates determined from SLR and geological rates, but in some cases there appear to be discrepancies. These discrepancies involve many of the data for which one end of the baseline is either Quincy (California), Huahine (French Polynesia) or Simosato (Japan). We have devised a method for looking at the discrepancies for these SLR observatories which allows us to calculate the motion not modelled by the geologic information. The results are discussed in terms of what is known about plate margins, and other information.

ISSN:0278-7407    

DOI:10.1029/TC009i005p00935

年代:1990

数据来源: WILEY

摘要:

Previous analyses of the accretionary history of NW Washington‐SW British Columbia have suggested the possibility of large‐scale, dextral‐slip faulting events to explain the present day regional outcrop pattern. The contact zone between the crystalline Skagit Complex and the volcano‐sedimentary Methow sequence has been considered to be a major fault, named the Ross Lake fault by Misch (1966), and has been considered to be a possible large‐scale strike‐slip fault by later workers. Detailed mapping of this contact zone in the Ross Lake area shows that the brittle faults and mylonites thought by Misch to express dextral shearing along the contact are not continuous, throughgoing structures, and that the contact between Skagit orthogneisses and Methow sequence strata is primarily intrusive. Furthermore, the mylonites show both left‐ and right‐lateral shear. The Skagit‐Methow contact is therefore not a major terrane boundary but rather a tectonized intrusive contact that has not accommodated significant motion between the two regions. Based on these observations and regional synthesis of timing relations, deformation in the contact zone is interpreted to express regional ENE‐WSW shortening, not major dextral slip, in the early Tertiary. Since no terrane‐bounding fault exists, the Cretaceous‐Tertiary history of the Ross Lake area can be interpreted as follows: (1) Jurassic(?)‐Early Cretaceous forearc/rift basin sedimentation (deposition of the lower Methow sequence); (2) 110–95 Ma, imbrication of the lower Methow sequence and its basement of oceanic crustal rocks by east‐vergent thrusting, with concomitant development of a west‐derived foredeep in the upper Methow sequence; (3) 95–85 Ma, arc magmatism, with genesis of the Skagit migmatite from Methow and older basement protoliths; and (4) Paleocene‐Early Eocene folding, faulting, uplift, and exposure of a relatively intact 25+ km crustal section, termed the Skagit‐Methow crustal

ISSN:0278-7407    

DOI:10.1029/TC009i005p00953

年代:1990

数据来源: WILEY

摘要:

The Cape Blanco region of south coastal Oregon sits on the upper plate of the Cascadia subduction zone about 60–70 km east of the base of the continental slope. Though the region has no historic coseismic deformation, late Quaternary deposits and landforms show abundant evidence of uplift, folding, and faulting. A set of five late Quaternary marine terraces and one uplifted Holocene beach berm are preserved in the Cape Blanco region. Stratigraphic and altitudinal surveys of these physiographic features, combined with several numerical and correlation age determinations, permit a reconstruction of tectonic deformation near Cape Blanco in approximately the last 200,000 years. The most cogent aspects of the neotectonics in this part of the Cascadia subduction zone are that an east‐west trending anticline deforms the lower three terraces (80–125 ka) as well as accounting for uplift of the Holocene storm berm. Latest movement on the anticline was no more than 2,000 years ago, but the anticline also deforms sediments as old as Middle Miocene. The anticline is consistent with a north‐south principal contraction axis at Cape Blanco in about the last 100,000 years. An older terrace at Cape Blanco (approximately 200 ka) is offset by two reactivated faults whose movements prior to the Eocene served to suture several terranes to the North American continent. Tectonic tilting in the older terrace is not consistent with a north‐south principle axis of contraction, but the axis of tilt is parallel to north‐northwest striking structures developed in the Quaternary fold belt offshore on the continental shelf. The observed deformation indicates that regional stress in the vicinity of Cape Blanco in the late Quaternary has been accommodated by a variable patter

ISSN:0278-7407    

DOI:10.1029/TC009i005p00983

年代:1990

数据来源: WILEY

摘要:

The subsidence history of Kilohigok Basin (1.9 Ga), N.W.T., Canada reflects the opening and closing of an ocean basin along the present Thelon Tectonic Zone. The basal sedimentary sequence represents a passive‐margin shelf that faced SE toward the orogen. Following passive‐margin subsidence, tectonic loading occurred resulting in rapid subsidence of the outer shelf margin and simultaneous uplift and erosion of the platform interior; a flexural moat formed subparallel to the Thelon Tectonic Zone. The shelf was subsequently drowned and successively buried by deep‐water flysch, shallow‐marine shelf sediments, and fluvial molasse, forming a foreland basin tapered to the NW. Direct structural evidence for the tectonic load is provided by the leading edge of a thin‐skinned thrust‐fold belt exposed in oblique cross‐section within the SE extremity of the basin. Upright folds at high structural levels can be traced downward to recumbent, tight folds and thrusts near the basement/cover contact documenting increasing shear strain with depth. Overall northwest vergence and about 50% shortening is indicated by fold‐thrust geometry and finite stretching lineations, above a brittle/ductile decollement that roots within the Thelo

ISSN:0278-7407    

DOI:10.1029/TC009i005p01015

年代:1990

数据来源: WILEY

摘要:

The Yucatan Basin preserves a record of the Late Cretaceous to Paleogene Caribbean‐North American convergent history that is largely unaffected by Neogene strike‐slip tectonics of the current plate boundary. An examination of seismic basement within the Yucatan Basin, based upon available seismic reflection data including extensive multichannel data, shows that the basement comprises nine domains distinguished on the basis of internal reflection character and surface topography. These domains encompass three distinct crustal types or blocks. The first underlies the western flank of the basin and represents the offshore continuation of the adjacent Yucatan platform. The second includes the topographically heterogeneous domains of the eastern two‐thirds of the basin, and is dominated by a subsided volcanic rise or arc (Cayman rise) resting upon probable oceanic crust of pre‐Tertiary age. The eastern edge of the rise and adjacent basins dips northeast beneath the Cuban margin along a sediment filled trench. The third type of crust occupies a rectangular deep within the western third of the basin. Available evidence indicates that this crust is oceanic in character, and represents a large, mature pull‐apart basin set within a wide paleo‐transform zone between the western platform and eastern oceanic basin. This zone defines the northwestern portion of the Caribbean‐North American convergent plate boundary. Paleocene to Middle Eocene transform motion was left‐lateral along north‐south to NNE‐SSW trends, with a displacement of about 350 km. A long Middle Eocene transcurrent fault of about 50 km left‐lateral displacement cuts the basin diagonally from SW to NE and continues onland in Cuba as La Trocha fault. This reconstruction is consistent with known Eocene regional tectonics, but the timing of regional events raises questions about present interpretations of plate geometry in the n

ISSN:0278-7407    

DOI:10.1029/TC009i005p01037

年代:1990

数据来源: WILEY

摘要:

The 120‐km‐long Kalabagh fault zone is formed by transpressive right‐lateral strike‐slip along the western Salt Range‐Potwar Plateau allochthon in northern Pakistan. Lateral ramping from a decollement thrust along an Eocambrian evaporite layer produced NNW‐ to NW‐trending folds and NE‐ to N‐dipping thrust faults in a topographically emergent zone up to 10 km wide. Piercing points along the main Kalabagh fault indicate 12–14 km of middle to late Quaternary right‐lateral offset. The older right‐lateral Surghar fault displaced axes of frontal folds of the eastern Surghar Range by 4–5 km. Total displacement is reduced northward in the Kalabagh fault zone where north‐dipping thrust faults splay to the west. Cumulative right‐slip offset in the Kalabagh fault zone is comparable to displacement along the Salt Range frontal thrust, at a minimum average displacement rate of 7–10 mm/year near the Indus River since 2 Ma. In the basement, which dips 2–3° north along the Kalabagh fault, a NNW‐trending discontinuous ridge beneath the lateral ramp is interpreted from residual gravity anomalies. The eastern flank of this basement ridge probably ramped allochthonous strata upward from a depth of over 5 km in the Kalabagh fault zone. Kalabagh faulting displaced and uplifted Holocene terrace deposits and shifted the course of the Indus River eastward. A high slip rate and associated seismicity indicate that the Kalabagh fault zone should be consider

ISSN:0278-7407    

DOI:10.1029/TC009i005p01061

年代:1990

数据来源: WILEY

摘要:

Structural analysis of metamorphic tectonites, low‐angle faults, and planar dilational dike swarms in the southern Baboquivari Mountains reveals a complex Mesozoic through middle Tertiary history comprising multiple periods of deformation. Locally preserved rootless folds probably represent Jurassic soft sediment deformation or tectonic deformation that pre‐dated the main episode of Late Cretaceous through early Tertiary tectonism. The latter involved thrust faulting, isoclinal folding, ductile strain, and transposition of bedding into foliation; greenschist to lower amphibolite facies regional metamorphism; and late‐ to post‐metamorphic intrusion of an early Tertiary peraluminous granite pluton. Intrusion of the pluton was accompanied by minor folding of foliation in the country rock and subsequent brittle extension with emplacement of a granitic dike swarm. Middle Tertiary metamorphic‐core‐complex deformation involved mylonitization accompanied and closely followed by detachment faulting and intrusion of microdiorite and lamprophyre dikes. The southern Baboquivari Mountains thus record two independent tectonic episodes, the first compressional and the second extensional. Both episodes are marked by a progression from ductile to brittle deformation. Orientations of late‐metamorphic extension fractures and the granitic dike swarm indicate NE‐SW crustal extension in Late Cretaceous through early Tertiary time. The middle Tertiary extension direction, NNE‐SSW, is indicated by the attitudes of post‐mylonite mafic dikes. Poles to these dikes are parallel to mylonitic lineation. These extension directions documented in the southern Baboquivari Mountains are typical of the southern Papago terrane of south‐central Arizona. In contrast, the rest of southern Arizona is characterized by NNW‐SSE extension during Late Cretaceous through early Tertiary time and ENE‐WSW extension during middle Tertiary time. The anomalous extension directions of the southern Papago terrane probably reflect deviations from regional strain patterns in a terrane with inherited anomalous crustal

ISSN:0278-7407    

DOI:10.1029/TC009i005p01077

年代:1990

数据来源: WILEY

摘要:

The Western Gneiss Region (WGR) in Norway experienced high‐pressure metamorphism during Silurian‐Devonian continent‐continent collision. The eclogite‐bearing lower crust is separated from the middle and upper crust by major detachment zones formed during extensional collapse of the orogen; formation of the Devonian basins is related to the extension. The footwall of the detachment zones comprises three structural and metamorphic zones. The upper zone, zone 1, is characterized by penetrative homogeneous down‐to‐the‐west simple shear developed under retrograde greenschist‐facies metamorphism. Zone 2 suffered inhomogeneous simple shear of the same polarity. Petrography and mineral chemistry data from the lower zone, zone 3, show a record of initial eclogite facies metamorphism at 600°C and>16 kbar, which was decompressed almost isothermally to amphibolite‐facies conditions at 550°C and 10–12 kbar. Both the eclogite‐ and amphibolite‐facies metamorphism developed in a regime of pure shear with vertical shortening. The rapid decompression records an approach of approximately 20 km to the surface, related to uplift that was probably the result of the removal of a thickened thermal boundary layer in the mantle lithosphere. The pure shear regime, which developed initially in the lower crust, was truncated by zones of simple shear as the lower crust was uplifted to middle and upper crustal levels. Extension by simple shear in the upper crust was rooted in the lower crust where extension occurred by pure shear. The shear zones in zones 1 and 2 did not penetrate the pure shear regime of the lower crust. A considerable amount of tectonic stripping of the orogenic welt predates deposition of the De

ISSN:0278-7407    

DOI:10.1029/TC009i005p01097

年代:1990

数据来源: WILEY

摘要:

The Cenozoic Provencal orogeny and the subsequent opening of the Ligurian‐Provencal basin were caused by a geodynamic mechanism that originated in the Cretaceous. It is evident from the arguments presented that (1) regional doming preceded rifting of the continental margin and this uplift widened and/or migrated during the ensuing opening of the Ligurian‐Provencal basin; (2) clastic sedimentation on the Provencal platform was initiated during the early Cenomanian; (3) deformation in the Provencal block, which varies from Late Cretaceous to Miocene, reflects a migration of the north‐south compression from west to east accross the block; (4) the Languedoc‐Provencal fold belt was initiated during the Eocene as a result of tectonic uplift and sinistral displacement of the Paleozoic basement toward the north. Significant geologic features of the region associated with these events include (1) similarities in shape of the Languedoc virgation and the Provencal thrust front with the shape of geophysical features over the Golfe du Lion; (2) the unusual width of the young, stretched continental margin in the Golfe du Lion; (3) the distinctive V‐shaped form of the Ligurian oceanic crust; and (4) the concordance in age of the earliest post‐rift deposits, which increasingly young toward the east, and magnetic anomalies that parallel the basin axis. These events and structures are attributed to the uplift of a Paleozoic block situated to the south in the Golfe du Lion and previously buried under the Mesozoic cover. The development of this crustal uplift since upper Aptian, and its northeastwards migration and/or gradual widening account for both sedimentary events and tectonic deformation that occurred on the Provencal platform until upper Eocene, and the later stage of rifting and formation of the Provencal margin. In the Ligurian‐Provencal basin, the synthesis of marine data as well as relevant terrestrial data indicates that rifting propagated towards the Gulf of Genoa, while the Corsica‐Sardinia block rotated t

ISSN:0278-7407    

DOI:10.1029/TC009i005p01113

年代:1990

数据来源: WILEY