摘要:

Ongoing deformation and fault slip (neotectonics) in the Alaskan/Bering Sea region were modeled with thin‐plate finite‐element methods by computing anelastic strain rate, stress, and velocity fields which represent time averages over several seismic cycles. Models included variable crust and lithosphere thickness, heat flow, and elevation and incorporated all faults known or thought to be active. Three sets of experiments, totaling 46 models, were computed with systematic variations of shear traction on the megathrust, fault friction within North America, internal friction of lithospheric blocks, and mantle creep strength. Model quality was scored by comparing predictions to geologic data on fault slip rates, the most compressive horizontal principal stress direction, and geodimeter and very long baseline interferometry (VLBI) baseline rates. Best results were obtained with a driving traction of only 10∼15 MPa on the Aleutian megathrust and fault friction of only 0.17 within North America; this is clearly a “low‐stress” model. The fanning ofaway from the syntaxis was reproduced but does not imply high deviatoric stress magnitudes within the North American plate. The Aleutian forearc west of Kodiak Island moves westward along strike at rates which increase to 65 mm yr−1in the Komandorsky region. However, no crust is presently escaping westward from the Alaskan syntaxis. Instead, it is shortened and stacked by thrusting, principally in the vicinity of the Chugach and Wrangell Mountains. Because stresses in this model are low and are delicately balanced against topography, it is likely that the present regime represents a transient response to Plio‐Pleistocene glacial mass redistribution and differs from typical tectonic flow

ISSN:0278-7407    

DOI:10.1029/95TC02426

年代:1996

数据来源: WILEY

摘要:

Simultaneous inversion of P wave arrivals of 262 local earthquakes, recorded in the region of the Dead Sea, was carried out to model the velocity stratification in the underlying crust and uppermost mantle. The resulting tomographic model delineates several large magmatic domes, rising from the lower crust, at depth of approximately 20 km, and ascending more than 12 km into the overlying intermediate crust. Commonly, the domal ascent took place along the boundary faults of the Dead Sea rift. The domes are elliptical in shape, measuring nearly 5 × 15 km in diameter, and are spaced about 20 km apart. Earthquake foci are abundant around the diapirs but rare within them. The occurrence of these magmatic diapirs under the Dead Sea implies anomalously high temperatures with metastable stratification in the lower crust and suggests magmatic upwelling

ISSN:0278-7407    

DOI:10.1029/95TC03497

年代:1996

数据来源: WILEY

摘要:

Both oceanic and continental rifts show regular along‐axis segmentation, but the relationship, if any, between the two is poorly understood. The tectonically active East African Rift system encompasses systematic along‐axis variations in extension and magmatism, making it possible to explore the links between strain, magmatism, and the length scales of faulting and depositional systems during rift development. We summarize Quaternary along‐axis segmentation within the Afar Rift system, which is transitional to seafloor spreading, and compare it with segmentation in a “continental” rift sector, the Main Ethiopian Rift (MER) system to the south. We use high‐resolution satellite imagery calibrated by field studies and digital topography data to delineate faults and magmatic centers in unmapped areas and to compare with existing geological reports. From south to north we see a significant and systematic decrease in the lengths of young basin‐bounding normal faults (∼50 km long in MER to ≤ 15 km long in northern Afar), in the lengths and widths of rift basins (from 50 to 100 km long × 30 to 80 km wide in MER to ≤ 20 km long × ≤ 5 km wide in northern Afar), and in the relief of the uplifted rift flanks surrounding the basins (from ≥ 1000 m high in MER to ≤ 100 m high in northern Afar). Erosional escarpments along the older, outer rift margins of Afar reveal long, high border fault segments similar to those found in the less evolved MER, suggesting that the Afar Rift was originally bounded by much longer faults. In northern Afar, there is a drastic increase in the volume of Quaternary basalts, with segmentation now dominated by 50‐ to 80‐km‐long volcanic ridges, which are similar in size, morphology, and spacing to the second order, nontransform offset segmentation of slow‐spreading mid‐oceanic ridges. The along‐axis changes in rift segmentation are concomitant with decreasing crustal thickness (i.e., cumulative strain), decreasing effective elastic thickness estimates, and increasing magma supply. These temporal and spatial patterns suggest that segmented continental rifts can evolve into segmented oceanic rifts dominat

ISSN:0278-7407    

DOI:10.1029/95TC02292

年代:1996

数据来源: WILEY

摘要:

The Kenya Rift in central Kenya (between ∼1° and 2° north latitude) is flanked by vast areas of Proterozoic crystalline rock where traditional stratigraphic markers useful for constraining Mesozoic to Recent structures are absent. Apatite fission track age and length data from about 100 samples collected from (1) relief profiles in north‐south trending mountain ranges, and (2) east‐west transects between mountain ranges constrain the structural framework of Cretaceous through Tertiary faulting in these flanking areas. Data from the profiles reveal regionally consistent age/elevation trends characterized by three distinct intervals, of 600 to 1200 m altitude, within which the apatite apparent ages are nearly concordant. The three isochronous age/elevation intervals yield apparent ages of ∼180, ∼115, and ∼65 Ma and record times of cooling starting at ≥220, 140–120, and 70–60 Ma. We interpret the cooling to be related to episodes of relatively rapid denudation separated in time by periods of slower exhumation. The isochronous intervals of crust are regionally consistent and relative offsets between the intervals allow fault displacements and block tilts to be estimated, when used in conjunction with the variation of fission track age and length along the transects. The fault block geometry revealed by the displaced isochronous layers indicates that normal faults related to rifting, with relative displacements>1 km, extend at least 100 km east of the Kenya Rift Valley. The data indicate a consistent direction of block tilting for the segment of the rift studied, which is similar to the asymmetric extension observed for other sections of the East Africa Rift System and other narrow rifts. The regional block faulting in this area probably occurred during early to middle Tertiary time associated with the late stages of extension in the Anza Rift and/or the early stages of extension of the Kenya Rift, in a setting similar to the present morphology of northern Tanzania. Therefore focusing of faulting within a central rift valley in Kenya appears to have followed a phase of regional extension with

ISSN:0278-7407    

DOI:10.1029/95TC02744

年代:1996

数据来源: WILEY

摘要:

Structure developed in association with oblique motions on the dextral, strike‐slip Finlay‐Ingenika fault (FIF) in the McConnell Creek area, north‐central British Columbia, is dominated by subvertical to vertical strike‐slip faults. Some of the faults cut the area into discrete, fault‐bounded blocks several kilometres wide. Stress tensors for 24 sites were inverted from the regional cleavage which predates the block‐bounding faults. The site‐mean stress tensors indicate that the fault‐bounded blocks were rotated clockwise about a subvertical axis during progressive oblique motions on the FIF. Rotation varies systematically, being maximal (58.7±3.3°) close to the FIF and minimal (0.0±1.6°) about 20 km away. Paleomagnetic samples were collected from 13 sites in the widespread Early Jurassic to Cretaceous plutonic rocks. Interpretable magnetic components of presumed Late Cretaceous age were obtained from six sites. The observed paleopole is significantly different from the Late Cretaceous reference pole for cratonal North America (CNA). After corrections for the local block rotations, the precision is much improved (Kincreased from 48 to 383), and the paleopole moves closer to the reference pole but is not coincident with it. Rotation about the Eulerian pole for the best‐fitting small circle to the Tintina trench and northern Rocky Mountain trench fault zone, however, brings the observed pole into coincidence with that for CNA and requires ∼670 km of dextral displacement on the fault zone. It is evident therefore that local structures associated with large dextral strike‐slip faults could account for at least part of the paleomagnetic disparity between some western parts of the Ca

ISSN:0278-7407    

DOI:10.1029/95TC03499

年代:1996

数据来源: WILEY

摘要:

Global Positioning System geodetic measurements at thirteen locations in Indonesia and four in Australia reveal that the Australian continent has accreted the Banda island arc to its margin. Small relative velocities of five sites on west Java, south Kalimantan, Bali, and south Sulawesi define a rigid Sunda shelf that moves relative to northern Australia in a manner consistent with pole locations from NUVEL‐1 Australia‐Eurasia but at a rate that is about 7% slower. Block‐like northward motion of the southern Banda arc toward the Sunda shelf at nearly the same rate as Australia suggests that the Timor trough is now inactive as a thrust. Little of the convergence of Australia with Eurasia is accommodated by strain within the Banda arc structure. Most of the convergence appears to occur as northward translation of the rigid arc with shortening on the Flores and Wetar thrusts and possibly on faults within the back arc

ISSN:0278-7407    

DOI:10.1029/95TC03850

年代:1996

数据来源: WILEY

摘要:

The amphibolite “sole” that occurs along the basal thrust of the Josephine ophiolite (Madstone Cabin thrust) is distinct from those described for ophiolites such as the Bay of Islands and Oman. Antigorite‐bearing serpentinite mylonites occur directly above the amphibolite, and the presence of antigorite + brucite indicates formation between 300° and 400°C. The antigorite mylonites also contain relict lizardite/chrysotile, suggesting formation of antigorite by prograde metamorphism along the base of the ophiolite. Further evidence for prethrusting low‐temperature serpentinization comes from a lizardite/chrysotile shear zone within fresh peridotite directly above the thrust that is cut by undeformed diabase dikes having geochemical affinities to the upper pillow lavas of the Josephine ophiolite. The prograde metamorphism of the serpentinite at the base of the ophiolite was apparently synchronous with greenschist facies retrogressive metamorphism of the underlying amphibolite. The amphibolite is chemically diverse and has been divided into two groups. Group 1 amphibolite appears to be derived from mafic plutonic rocks and has immobile trace element abundances indicating affinities to volcanic arc magmas. Group 2 amphibolite is metavolcanic (contains some quartzite) and has magmatic affinities to mid‐ocean ridge basalt and within‐plate basalt, an association that is distinctive of older basement rocks elsewhere in the Western Klamath terrane. We suggest the amphibolite was derived from the footwall of the Madstone Cabin thrust, including country rocks for the Chetco complex (Group 2) and possibly older intrusive phases of the Chetco complex itself (Group 1). Previous workers have shown that the base of the amphibolite is intruded by gneissic gabbro of the Chetco complex, that intrusion of the Chetco complex overlapped in time with deformation of the amphibolite, and that during and after emplacement the Josephine ophiolite and overlying flysch were intruded by abundant arc‐related dikes and plutons. Emplacement of the Josephine ophiolite appears to have occurred by underthrusting of a cold ophiolite by an acti

ISSN:0278-7407    

DOI:10.1029/95TC02525

年代:1996

数据来源: WILEY

摘要:

Permian chert in the North Fork terrane and correlative rocks of the Klamath Mountains province has a remanent magnetization that is prefolding and presumably primary. Paleomagnetic results indicate that the chert formed at a paleolatitude of 8.6° ± 2.5° but in which hemisphere remains uncertain. This finding requires that these rocks have undergone at least 8.6° ± 4.4° of northward transport relative to Permian North America since their deposition. Paleontological evidence suggests that the Permian limestone of the Eastern Klamath terrane originated thousands of kilometers distant from North America. The limestone of the North Fork terrane may have formed at a similar or even greater distance as suggested by its faunal affinity to the Eastern Klamath terrane and more westerly position. Available evidence indicates that convergence of the North Fork and composite Central Metamorphic‐Eastern Klamath terranes occurred during Triassic or Early Jurassic time and that their joining together was a Middle Jurassic event. Primary and secondary magnetizations indicate that the new composite terrane containing these and other rocks of the Western Paleozoic and Triassic belt behaved as a single rigid block that has been latitudinally concordant with the North American craton since Middle Jurass

ISSN:0278-7407    

DOI:10.1029/95TC03054

年代:1996

数据来源: WILEY

摘要:

In the Penninic‐Austroalpine boundary zone of Eastern Switzerland a clear distinction can be made between upper and lower stacks of nappes with contrasting tectonic evolution that are separated by the Turba mylonite zone (TMZ). The Austroalpine nappes, Platta ophiolites, and Malenco‐Forno‐Lizun ophiolites belong to the upper stack. These nappes were formed by predominantly west directed, late Cretaceous shortening and imbrication. The lower stack, comprising the Suretta nappe, Schams nappes, Avers schists, and Oberhalbstein flysch units, includes Tertiary sediments and was deformed and metamorphosed in early Tertiary time. Kinematic indicators in the mylonites of the TMZ, representing the boundary between the two stacks, show a top‐to‐the‐east directed movement, which is clearly distinct from the regionally prevalent top‐to‐the‐north direction of thrusting in the Tertiary. The main foliation in the shear zone is discordant with respect to the structures of the hanging wall, and concordant to the footwall. Vitrinite reflectance in shaly sediments above and below the TMZ in the northern part indicates that the TMZ coincides with a drop in peak metamorphic temperature from about 350°C in the top of the footwall to about 280°C at the base of the hanging wall. Therefore the TMZ is interpreted as an east dipping, low‐angle normal fault. The age of its activity is bracketed between about 45 and 30 Ma, based on overprinting relations with radiometrically dated features (45–30 Ma: main foliation in the footwall, older than TMZ activity; 30 Ma: Bergell granodiorite intrusion, younger than TMZ activity). Extension along the TMZ was followed by further north‐south shortening, as indicated by postmylonitic folding of the TMZ. Normal faulting along the TMZ accommodated relative uplift of the Lepontine structural dome, a large metamorphic complex in the Central Alps. Unroofing of this dome started before 30 Ma and continued in the Miocene with activity of further normal faults situated at

ISSN:0278-7407    

DOI:10.1029/93TC02312

年代:1996

数据来源: WILEY

摘要:

Subsurface structure contour maps and cross sections show that the northern Los Angeles basin is underlain by a south facing monocline complicated by secondary faults and folds. The monocline and associated structures form a structural high, the northern Los Angeles shelf, which marks the northern margin of the Los Angeles central trough. Analysis of “growth strata,” sediments deposited during structural growth, shows that during the Miocene, the predominant structural style was extension. Growth strata show that at approximately the beginning of the Pliocene (5 Ma), contraction reactivated the Miocene normal faults as reverse faults and formed the monocline, its secondary structures, and the Los Angeles central trough. Lensing‐out of earliest Pliocene (Delmontian Stage) growth sandstones onto the flanks of the monocline and secondary structures (the South Salt Lake, the East Beverly Hills, and the Las Cienegas anticlines) indicates that these contractional structures all began to form at this time. All of the secondary structures became largely inactive prior to the deposition of upper Pico member of early Pleistocene age (1.2 Ma). However, thick accumulations of growth strata of the upper Pico member of the Fernando Formation within the Los Angeles central trough attest to continued monoclinal folding after the secondary structures became largely inactive. The growth strata record both vertical and horizontal components of structural growth of the monocline so that the dip of the blind fault zone causing the monocline (the Los Angeles fault) can be calculated. In the East Beverly Hills area, the dip of the Los Angeles fault is 61° and at Las Cienegas, the dip is 62°. These are maximum values based on the assumption that the growth strata bed lengths record all shortening. The Pliocene‐Pleistocene average fault slip rates for the Los Angeles fault are 1.1 – 1.3 mm/yr in the East Beverly Hills and 1.3–1.5 mm/yr at Las Cienegas. The resulting Pliocene‐Pleistocene horizontal convergence rates are 0.5 – 0.6 mm/yr and 0.6 – 0.7 mm/yr respectively. The Pliocene‐Pleistocene growth strata show increased dip with age, indicating that the monocline grew by progressive limb rotation rather than by kink‐band migration. Therefore, fault‐bend and fault‐propagation fold models based on kink‐band migration are inadmissible solutions to explain the growth of the monocline. We suggest a basement‐involved shear zone to explain the geometry of

ISSN:0278-7407    

DOI:10.1029/95TC02523

年代:1996

数据来源: WILEY