摘要:

A regional Miocene‐Pliocene unconformity in the Coastal Range of eastern Taiwan provides information about vertical motions that affected the colliding volcanic arc during initiation of arc‐continent collision. The unconformity separates precollisional volcanic and volcaniclastic rocks of the Miocene Luzon arc and forearc basin from an overlying thick sequence of syncollisional, Pliocene‐Pleistocene sedimentary rocks derived from the metasedimentary accretionary wedge of Taiwan. The duration of time represented by the unconformity averages 1–2 m.y. and locally ranges up to 4 m.y. The basal unit of the younger syncollisional sequence consists of bathyal, mega‐slump‐folded mudstone, pebbly mudstone, and thin‐bedded turbidites. Sharp stratigraphic juxtaposition of these deep‐marine deposits directly above shallow‐marine limestone and epiclastic rocks (upper part of the older arc sequence) requires extremely rapid subsidence during initiation of the collisional basin. The Coastal Range unconformity is interpreted to represent a period of latest Miocene nondeposition and/or erosion in the Luzon arc that was related to initial subduction of the Eurasian continental margin along the Manila Trench. This period was followed in Pliocene time by very rapid subsidence, or collapse, of the arc platform to 1–2 km water depth. There are two dissimilar mechanisms of basin subsidence that may have controlled initiation of the collisional basin: (1) extension, rifting, and crustal thinning in the Luzon arc massif to form a series of small rift or pull‐apart subbasins or (2) flexural subsidence of oceanic lithosphere in a foreland‐style basin due to crustal loading in the expanding accretionary wedge of proto‐Taiwan. Although data are insufficient to provide a unique solution, the flexural subsidence model is supported by systematic younging of the initial subsidence event from west to east, which may record eastward migration of an outer flexural bulge. Notable differences between the unconformity and paleogeography of the early collisional basin and analogous present‐day basins in the offshore region to the south suggest that arc‐continent collision and basin development in eastern Taiwan have experienced secular ch

ISSN:0278-7407    

DOI:10.1029/91TC02463

年代:1992

数据来源: WILEY

摘要:

Patterns of microearthquakes and Quaternary surface deformation suggest that the tectonic setting of the SW Sacramento Valley is similar to areas of the western San Joaquin Valley known to be underlain by seismogenic blind thrust faults. On the basis of previous work and analysis of geologic and seismic reflection data, the following late Cenozoic tectonic features and processes are identified: (1) uplift of the northern Coast Ranges beginning approximately 3.4 Ma, and eastward propagation of uplift into the southwestern Sacramento Valley by 1.0 Ma; (2) uplift and homoclinal flexure of Plio‐Pleistocene strata at the eastern Coast Ranges mountain front; (3) uplift and folding above blind thrusts approximately 15 km east of the mountain front in the southwestern Sacramento Valley. Similar associations of structures and processes have been observed in thrust belts in Pakistan, the Peruvian Andes, and the Canadian Cordillera and are commonly attributed to thrusting within an intercutaneous wedge or triangle zone. By using other thrust belts as analogs, the propagation of an eastward tapering triangle zone is interpreted to be the principal mechanism for uplift and homoclinal flexure at the eastern Coast Ranges mountain front. Seismic reflection profiles reveal that (1) the triangle zone consists primarily of east‐vergent blind thrusts and (2) west‐vergent backthrusts exposed in the the eastern Coast Ranges and southwestern Sacramento Valley are rooted in the east‐vergent thrusts. Transfer of slip from the east‐vergent blind thrusts to the west‐vergent backthrusts occurs locally beneath the southwestern Sacramento Valley. Fault‐bend folding in the hanging walls of the backthrusts has created a north‐northwest striking chain of low hills approximately 15 km east of the mountain front. The folds deform 3.4–1.0 Ma fluvial sediments and thus are middle Pleistocene in age or younger. Local variations in strike suggest that the fold chain is segmented, like the New Idria‐Coalinga‐Kettleman Hills segmented fold chain in the southwestern San Joaquin Valley (Stein and Ekström, 1989). These data have implications for seismic hazard assessment. Anecdotal accounts indicate that two M = 6.0+ events of the 1892 Winters‐Vacaville earthquake sequence probably occurred beneath the eastern Coast Ranges (Dale, 1977; Toppozada et al., 1981). Ground cracking was observed following the main shocks along the mountain front in the southwestern Sacramento Valley. We propose that the earthquakes were generated by slip on a blind thrust beneath the Coast Ranges, and that the ground cracking in the valley represents propagation of the eastward tapering triangle zone. The 1892 earthquake sequence suggests that blind thrusts beneath the southwestern Sacramento Valley are active and capable of generating moderate to lar

ISSN:0278-7407    

DOI:10.1029/91TC02494

年代:1992

数据来源: WILEY

摘要:

Paleomagnetic data from volcanic and crystalline rocks elucidate the evolution of a major Miocene accommodation zone in the northern Colorado River extensional corridor. The accommodation zone is a 10‐km‐wide belt of intermeshing conjugate normal faults that facilitates reversals in the dominant tilt direction of fault blocks and dip direction of major normal fault systems. Tilt‐corrected means (e.g., N = 28 sites, D = 353.4°, I = 61.3°, α95= 6.7°, k = 17.4) from Miocene volcanic strata overlap expected Miocene directions at the 95% confidence level. These data and geologic relations suggest that at exposed structural levels the accommodation zone did not facilitate distributed strike‐slip displacement between opposing tilt block domains. Vertical axis rotations are probably negligible in most of the corridor, as the Miocene structural grain generally mimics that in the zone. Discrepancies between characteristic remanent magnetizations (ChRM) in crystalline rocks and expected directions are therefore attributed to rotations about horizontal (i.e., tilting or flexing) axes. ChRMs from Cretaceous and Miocene intrusions suggest 50°–90° of tilting of large crystalline terranes on either side of the accommodation zone. The magnitude of tilting inferred from the paleomagnetic data is similar to that of Tertiary strata in nearby fault blocks, implying that these crystalline terranes are parts of highly tilted fault blocks rather than flexed lower plate rocks. Major low‐angle normal faults that bound highly tilted parts of these crystalline terranes probably nucleated at steep dips and were rotated to gentle dips by block tilting. Paleomagnetic data indicative of negligible tilting (e.g., Miocene intrusions, northern Black Mountains crystalline terrane, N = 13 sites, D = 359°, I = 55°, α95= 9°, k = 24) and geologic relations imply that lower plate rocks may surface in both the east and west tilted domains 35–50 km away from the zone. The trend toward shallower structural levels, with respect to Miocene extension, and tapering of highly extended terrane toward the accommodation zone imply that the magnitude of upper crustal extension decreases toward the zone. Temporal patterns of major extension, especially an apparently continuous northward younging across both the east and west tilted domains, further suggest that the accommodation zone served as a long‐lived rupture barrier between conjugate normal fault systems rather than as a short‐term boundary between opposing systems that propagated towar

ISSN:0278-7407    

DOI:10.1029/91TC00869

年代:1992

数据来源: WILEY

摘要:

Franciscan metagraywacke immediately below the Del Puerto ophiolite, an outlier of the Coast Range ophiolite in the northern Diablo Range, was sheared during top‐to‐the‐east displacement on the Coast Range fault. This represents normal faulting and extensional offset. It was accompanied by attenuation of the Coast Range ophiolite and Great Valley sequence in the hanging wall along layer‐parallel normal faults that sole into the Coast Range fault. Extension occurred as the Franciscan Complex moved relatively west, out from under North American lithosphere and across the subducting ocean plate below. This effected a lengthening and thinning in the wedge of material above the down‐going plate, presumably in response to instability brought about by subduction shallowing (Krueger and Jones, 1989) and accretion of the Franciscan Central belt in the latest Cretaceous to early Paleocene. As a result, blueschist facies terranes of the uppermost part of the Franciscan Complex are now juxtaposed directly against hanging wall units that bear only low‐grade m

ISSN:0278-7407    

DOI:10.1029/91TC01880

年代:1992

数据来源: WILEY

摘要:

The Pacific plate obliquely converges with the Australian plate at latitude 39°50′S along the Hikurangi margin off the east coast of the North Island of New Zealand. An extensive and youthful subaerially exposed forearc on the east coast of the North Island in the Hawke's Bay area provides the opportunity to document contemporaneous forearc deformation in this obliquely convergent margin setting. Geologic mapping and analysis of strain at both mesoscale and megascale indicates that strain is partitioning into domains of extension, contraction, and strike‐slip. The domains are elongate and trend parallel to the margin. Measurements of net shortening and transcurrent slip in the forearc show that the obliquely convergent motion is transferred across the plate interface. Deformation rates calculated for the past 1–2 m.y. for structures in all six forearc domains account for 50–70% of the margin‐parallel motion required by Pacific‐Australian plate convergence and about 6% of the plate motion perpendicular to the plate boundary. At the surface in the forearc, this obliquely convergent motion is manifest not by transpressional faults but rather by paired structural domains that consist of a strike‐slip fault zone and an accompanying contractional fault‐and‐fold zone on the trenchward side. There are two such transcurrent faulting‐and‐contraction couplets, one where the backstop daylights at the arcward edge of the forearc and another couplet trenchward of a relatively undisturbed forearc basin. The small amount of shortening, relative to strike‐slip, in the onshore part of the forearc suggests that shortening perpendicular to the plate boundary may be concentrated offshore and that most of the component of plate motion perpendicular to the plate boundary may be accommodated by slip along the su

ISSN:0278-7407    

DOI:10.1029/91TC02363

年代:1992

数据来源: WILEY

摘要:

The Thor‐Odin ‐ Pinnacles area is a structural culmination in the Shuswap complex of the southern Omineca Belt of the Canadian Cordillera. It comprises amphibolite‐facies rocks that were deformed during Mesozoic‐Paleocene compression and were exhumed in the footwalls of Eocene normal faults during crustal extension. The Ladybird leucogranite suite coincides with the extended terrane in the southern Omineca Belt. It is generally restricted to a midcrustal level which lies in the hanging walls of deep‐seated thrust faults and the footwalls of extensional faults. Field relationships of the leucogranites and U‐Pb geochronology place timing constraints on compressional and extensional shear zones. The last thrust motion on the Monashee décollement occurred in the latest Paleocene, and the shear zone had stopped by 58 Ma. Crustal‐scale normal faults were active in the early Eocene, indicating that crustal extension closely followed the compressional regime. Geological and geochronological data are consistent with an anatectic crustal origin for the Ladybird granite. The granites apparently postdate the thermal peak of metamorphism (Carr, 1990) and were generated during the final stages of thrusting, perhaps due to decompression melting as the midcrustal rocks were carried up a thrust ramp and unroofed and/or due to the introduction of hydrous fluids into the system. In situ magma and hot intrusions probably played an important role in the nucleation of extensional shear zones. The extensional regime then facilitated the intrusion of vast late‐synkinematic to posttectonic plutons. U‐Pb systematics reveal that zircons in high‐temperature shear zones may have suffered high‐temperature Pb loss, perhaps due to deformation‐ or fluid‐enhanced diffusion, and that monazite systematics from samples from high‐grade terranes are complex. Magmatic monazite populations contain crystals of different ages that do not coincide with zircon ages and apparently represent neither a crystalli

ISSN:0278-7407    

DOI:10.1029/91TC01644

年代:1992

数据来源: WILEY

摘要:

Catalogs of moment tensors for more than 8000 earthquakes provide a more objective and complete description of the earthquake source than do focal mechanisms derived from first motions, and therefore moment‐tensors provide a valuable resource for tectonic analysis. We here present background information about the properties of moment tensors and examples of moment tensor analysis. We also introduce a new statistic, the seismic consistency Cs, which measures the similarity of earthquakes within a group. Csis 1.0 if earthquakes are all alike and 0.0 if they cancel one another. Triangle diagrams provide a practical method for defining the fraction of normal, strike‐slip, and thrust fault components for an earthquake and are a new graphical method for displaying source properties of groups of earthquakes. We apply these methods to the Harvard centroid moment tensor catalog to study the characteristics of shallow earthquakes (<50 km depth) within 200 km of typical ridge‐transform and subduction zone plate boundaries. In this way, we have reached four major conclusions. First, even along relatively simple plate boundaries, there is considerable variation in the type and orientation of earthquake mechanisms. Second, along individual plate boundaries, groups of thrust, normal, or strike‐slip earthquakes generally have Csequal to 0.8 or higher. Thus for many types of tectonic analyses it is unnecessary to add moment tensors to study seismic deformation; rather, it is sufficient to add scalar earthquake moments of the individual events. Third, moment tensors for some individual earthquakes are quite different from those produced by slip along a planar fault. However, summing up moment tensors shows that the deformation pattern produced by groups of earthquakes is generally more like fault slip than the pattern of typical earthquakes within the group. Fourth, by dividing the sum of moments by the velocity of plate motion and the length of the boundary, we calculate Rmom, the efficiency of seismic moment produced along individual plate boundaries. For the 12.75 years of data available, normal fault earthquakes along spreading ridges produce moment less efficiently than strike‐slip earthquakes along transforms. These in turn produce moment less efficiently than thrust earthquakes along subduction zones. For ridge‐transform earthquakes, boundaries with fast plate velocities produce seismic moment less efficiently than do boundaries with slow

ISSN:0278-7407    

DOI:10.1029/91TC02888

年代:1992

数据来源: WILEY

摘要:

If a rock body becomes fully magnetized and then is folded, faulted, or otherwise tilted, its vector of remanent magnetization will tilt with it. However, if a rock body is tilted while it is very hot, and then cools and becomes magnetized, its vector of magnetization will show little if any of the effects of tilt. It is shown, for a wide variety of circumstances, that tilting a large batholith should not be expected to result in equal tilt of that batholith's magnetic direction. This follows from several elementary considerations: (1) Batholiths cool slowly, and slow cooling lowers the blocking‐temperature range for acquisition of remanent magnetization. (2) If a tilted block is large (of the order of 25 km in width) and has been tilted through even a moderate angle (25° will do), its “lower” end originally must have been very hot; quite possibly too hot to retain a remanent magnetization. (3) Tilt entails uplift, and uplift rearranges the geothermal gradient. Rapid uplift accompanied by erosion is particularly effective in raising the temperature of the upper levels of the crust, thereby delaying acquisition of magnetization until tilting is nearly complete. Other thermal constraints on the tilting of remanence vectors in batholiths are discussed; in general, it appears that quite special circumstances are required to tilt a large pluton without leaving a clear magnetic record. Few, if any, Cordilleran batholiths studied to date show this kind of evidence o

ISSN:0278-7407    

DOI:10.1029/91TC02340

年代:1992

数据来源: WILEY

摘要:

A four‐layer model of the upper 150 km of the Earth is used to calculate the viscous response of continental crust and the underlying mantle to tectonic denudation. The model comprises a strong upper crustal layer, a weak lower crustal layer, a very strong mantle lithosphere layer, and a weak mantle asthenosphere layer, which is in accord with experimental constraints on strength‐depth profiles for continental lithosphere. The strength of each layer is represented by its effective viscosity. Flow in the crust and mantle is driven by buoyancy forces, which arise from the unloading of an allochthon along a detachment fault by a series of instantaneous displacements (earthquakes or rapid creep events). Numerical solutions, obtained by using a finite element technique, predict footwall uplift, Moho deflection, and surface topography that are consistent with observations from the Basin and Range province of the western United States. The calculated curvature of the footwall uplift is also similar to that observed and is sensitive to the geometry of the detachment fault. Such bending need not be elastically controlled; hence the curvatures of footwall domes do not clearly place limits on the effective elastic thickness of the extending crust. The upward deflection of the Moho and the surface topography are sensitive to the viscosity structure and enable us to bound the range of the various viscosities. By matching observations from the Basin and Range province, which indicate no Moho deflection and low magnitude of surface topography (≤3–5 km), we estimate the upper crustal, lower crustal, and mantle lithospheric viscosities in the ranges 1021–1023Pa s, 1019–1021Pa s, and 1021–1023Pa s,

ISSN:0278-7407    

DOI:10.1029/91TC02341

年代:1992

数据来源: WILEY

摘要:

On the basis of thermal and mechanical modeling, it is concluded that the subduction of continental lithosphere can lead to its breakup and the formation of a new plate contact within the middle or lower crust. As a result, (part of) the subducting continental crust is transferred to the upper plate. Breakup is caused by the resistive forces acting upon subducting continental crust, due to the buoyancy of crustal material and to friction at the plate contact, as well as the decrease in strength of the subducting crust once it has been subducted to a depth of a few tens of kilometers. The crustal thickness and the thermal and compositional structure of the continental crust just before the onset of subduction have a large influence on the depth to which continental crust can be subducted (prior to its breakup). The depth to which continental crust can be subducted coherently decreases as the surface heat flow or the crustal thickness of the subducting continental plate increases. In many cases, breakup is found to occur at a time when the upper surface of the continental plate has been subducted to a depth of 25–50 km. The subduction of a cold continental shield or of continental lithosphere with a relatively small crustal thickness, on the other hand, may lead to the subduction of both upper and lower crust to mantle depth

ISSN:0278-7407    

DOI:10.1029/91TC01039

年代:1992

数据来源: WILEY