摘要:

Lower crustal reflections seen on British Institutions Reflection Profiling Syndicate (BIRPS) deep seismic reflection data, recorded offshore the United Kingdom, are commonly short, convex‐up, and crosscutting. Consequently, the lower crust should not be described as layered, as much of the reflecting structure is not fully resolved and may be quite complex. Moreover, the lower crustal reflective zone (LCRZ) is not a uniformly reflective layer, but contains reflective bands that wrap around nonreflective (“transparent”) zones. Because transparent zones probably correspond to a lack of a reflecting fabric, understanding why these features are unreflective may help constrain the cause of the reflectivity of the lower crust. Although transparent zones may have a variety of explanations (e.g., large shear pods or frozen magma bodies), the geometry of reflective bands and transparent zones is perhaps best explained by the geometry of shear zones and low‐strain lozenges predicted both by combining theoretical lithospheric strength profiles with slip‐line field theory and by drawing analogies with structures developed in highly tectonised high‐grade metamorphic terrains. This interpretation implies that lower crustal reflections may be from shear zones and also has implications for the way the lower crust deforms. In particular, extension may be accommodated by the relative movement of low‐strain lozenges along intervening zones of simple shear. The lower crust may be stretched during extension, but it must also transfer deformation between the upper crust and the mantle. The observation on BIRPS data of mantle reflections, interpreted as mantle shear zones, implies that deformation in the mantle is localised. However, the manue reflections are never colinear with faults in the upper crust, requiring, at least locally, a component of sub‐horizontal bulk strain within the lower crust, which may greatly increase the amount of shearing therein and hence perhaps may increase the reflectiv

ISSN:0278-7407    

DOI:10.1029/TC007i005p00929

年代:1988

数据来源: WILEY

摘要:

One‐ and two‐dimensional finite difference models are used to generate theoretical pressure‐temperature‐time (PTt) paths for rocks uplifted from deep and intermediate crustal levels during extension via ductile stretching of the entire lithosphere (“pure shear”) and via movement of crustal blocks along rooted large‐scale low‐angle normal faults (“simple shear”). The temperature‐time paths of rocks uplifted by these pure shear and simple shear mechanisms have similar morphologies, although rocks from pure shear settings reach their final depth at higher temperatures than do rocks uplifted by simple shear, and experience a greater amount of posttectonic cooling. The depth‐temperature paths of rocks uplifted from deep crustal levels (30 km depth) via pure shear extension are often characterized by a period of nearly isothermal (high dP/dT) uplift during the early stages of extension. A modified pure shear extension model that includes enhanced heating in the lower lithosphere during extension generally produces PTt paths characterized by a more protracted period of nearly isothermal uplift. In contrast, simple shear uplift along low‐angle normal faults produces almost linear depth‐temperature paths with moderate dP/dT, and isothermal uplift is not observed. For a given initial geotherm, the single factor controlling the PTt paths of rocks unroofed below gently dipping normal faults that dip less than 30° is the rate at which unroofing takes place (unroofing rate is defined as the rate of uplift relative to the surface), and the syntectonic PTt paths are insensitive to fault dip or horizontal displacement rate. Current geothermometric and geobarometric techniques yield typical uncertainties in real pressure‐temperature and temperature‐time data of ±1 kbar and ±50°C. Thus it will be difficult to distinguish between these various extensional mechanisms on the basis of PTt data derived from a single sample or from a single structural level, although use of data from samples collected from a broad range of structural levels may improve resolution. Ultimately, the best prospect for distinguishing between simple shear and pure shear mechanisms through petrologic work lies in improving the precision of metamorphic pressure and temperature

ISSN:0278-7407    

DOI:10.1029/TC007i005p00947

年代:1988

数据来源: WILEY

摘要:

A conceptual model is proposed for the generation of low‐angle normal faults in Metamorphic Core Complexes. The model is based on three assumptions: (1) the isostatic response to normal fault motion is of regional extent; (2) when a fault segment is significantly rotated from the optimum angle of slip, relative to the crustal stress field, it is replaced by a new planar fault oriented in the optimum direction; and (3) the fault cuts the entire upper crust and fault motion always nucleates in the same region at the base of the upper crust. The stress field is considered to be uniform through the crust and the regional isostatic response of the crust to loads is computed using the thin plate flexure approximations. Active low‐angle normal faults are difficult to reconcile with rock mechanics theories, earthquake focal mechanism studies, and geochronologic results indicating rapid cooling of core complex rocks. The model does not require active fault slip on low‐angle faults. The flexural response to normal faulting is shown to be significantly affected by anelastic behavior of the crust and by loading due to sedimentation. The anelastic response to large bending stresses results in significant reduction in the effective elastic rigidity of the upper crust. This can explain the observed short wavelength of topographic response to normal fault loads. The model results in: (1) a nearly flat‐lying abandoned normal fault (or “detachment”) below slices of upper plate rocks and sedimentary infill; (2) a strong contrast in metamorphic grade across the abandoned “detachment”; (3) rapid movement of lower plate rocks from midcrustal depths to shallower depths. These results are qualitatively in agreement with geologic observations for

ISSN:0278-7407    

DOI:10.1029/TC007i005p00959

年代:1988

数据来源: WILEY

摘要:

The actively subsiding North China sedimentary basin is associated with an unusually high level of seismic activity. This oil‐ and gas‐producing basin has been the site of nine large (M ≥ 7), destructive earthquakes since 1600 A. D. An analysis of faulting during the Tangshan earthquake sequence, which includes some of the largest shocks that have occurred in this basin during the past 400 years, showed that the dominant pattern of deformation during this sequence was associated with displacement on right‐stepping, right‐lateral strike‐slip faults. A large amount of subsidence (∼1.0–1.5 m) occurred in pull‐apart regions between steps of the north‐northeast (NNE) trending strike‐slip faults. The directivity of the P wave radiation pattern and the distribution of aftershocks of the Bohai Gulf earthquake of July 18, 1969, also indicate strike‐slip faulting on planes of similar NNE trend. Together with the four largest shocks of the Hsingtai sequence in 1966, earthquakes with right‐lateral slip on NNE trending faults account for about two‐thirds of the seismic moment released in this region for the past 100 years. The average regional strain due to seismic slip has a dominant component of dextral simple shear (on NNE striking planes) that is approximately twice as large as north‐south extension or east‐west shortening. Based on a combination of published field and borehole data, the dominance of horizontal dextral simple shear over subsidence seems to have begun no later than the mid‐Pliocene and the total amount of extension over the entire basin is only about 20–30%. This pattern of deformation, the spatially variable heat flow values, the scattered locations of rapid deposition of Quaternary sediments, and the fluctuating rate of sedimentation in space and time during the Neogene in this basin are difficult to explain by pure extension and thermal subsidence. We propose that these observations, and the shape of the whole North China basin (that of a “lazy Z”), are a consequence of pulling‐apart associated with right‐lateral movement on the NNE trending strike‐slip fault systems and that the basin as a whole has formed and evolved as a composite pull‐apart basin due to right‐lateral slip o

ISSN:0278-7407    

DOI:10.1029/TC007i005p00975

年代:1988

数据来源: WILEY

摘要:

Bathymetric, gravity, and magnetic data collected in the southwest Pacific Ocean over the northern New Caledonia ridge show that the main geological units known from the island of New Caledonia extend northward from this island, beneath the Grand Lagon Nord, the Grand Passage, and the d'Entrecasteaux reefs. These data support the model of tectonic evolution of the New Caledonia region proposed by Kroenke [1984]. We interpret a linear axial gravity low that extends from southern New Caledonia to the Grand Passage as evidence for the continuity of the thick pre‐Permian to Jurassic core of the island. The Belep magnetic pattern, which covers the southwestern half of the Grand Lagon Nord, shows linear, high relief (1000–2000 nT) anomalies that are locally associated with a 120‐mGal gravity high. These magnetic and gravity anomalies resemble anomalies measured over the west coast of New Caledonia, suggesting that the Cretaceous to Eocene basaltic complex of the coastal area is overlain by ophiolite remnants as far north as the western d'Entrecasteaux reefs. The similarity between the Belep magnetic pattern and a highly magnetic province evident 200 km southwest across the New Caledonia basin, along the Fairway ridge, indicates that volcanic rocks lie symmetrically on both sides of the New Caledonia basin. We suggest that part of these volcanic rocks were emplaced during the middle Cretaceous when the New Caledonia ridge rifted from the Australian margin. Simple gravity models of an elongated gravity high, having peak values in the range of +130 to +150 mGal, suggest that the ophiolite, which was thrust over New Caledonia during the Upper Eocene, extends along the east side of the Grand Lagon Nord and as far north as the d'Entrecasteaux reefs. Gravity and magnetic models suggest that in the area of the d'Entrecasteaux reefs, the ophiolite belt includes two subunits. The first subunit underlies the Huon‐La Surprise platform and may include highly serpentinized ultramafic rocks without basaltic oceanic crust. The second subunit, which lies beneath the Guilbert ridge, may include ultramafic rocks as well as thin imbricate slices of oceanic crust. At the northern termination of the New Caledonia ridge, seismic and geopotential data evidence a major east‐west trending tectonic zone that separates the basement of the New Caledonia ridge from the d'Entrecasteaux zone, an arcuate oceanic feature extending northward from the ridge. Differences in structure, geophysical signatures and morphology evident between areas north and those south of the Grand Passage, together with the nearness of the Le Noroit massif west of the Grand Passage, suggest that contemporaneously with Eocene to early Oligocene subduction along the western New Caledonia margin, an arc‐ridge collision may have occurred near the northern termination of this subd

ISSN:0278-7407    

DOI:10.1029/TC007i005p00991

年代:1988

数据来源: WILEY

摘要:

Three stages of predominantly mafic (sub‐) seafloor magmatism accompanied the late Paleozoic evolution of both the Croisilles and Patuki ophiolites, Nelson Province, New Zealand. Stage 1 Croisilles and Patuki basaltic pillow lavas show strong light rare earth element (LREE) enrichment and low Zr/Nb ratios resembling alkali basalts from fracture zones and within‐plate (e.g., ocean island) settings. Analogous eruptives are not recognized in the Dun Mountain ophiolite or typical subduction‐related settings. Evolved Stage 2 basic lava flows and volcanic breccias show pronounced LREE‐depletion and high Zr/Nb ratios similar to N‐type mid‐ocean ridge basalts. Cross‐cutting dikes and irregular intrusions (Stage 3 magmas) that range from near‐primary basalts (Mg″ = 74; Al2O3/TiO2= 20.9) to silicic plagiogranites, show decoupled rare earth element and high field strength element levels. They display evidence of extensive, polybaric, closed‐system fractionation adhering closely to geochemical trends of subduction‐related magmas. The composition of the most primitive dikes, and the presence of highly refractory magnesian Cr‐diopside and Cr‐spinel, indicate that Stage 3 magmas were derived by second‐stage melting of depleted upper mantle in a suprasubduction zone environment. Differences in mantle melting regimes (i.e., source composition plus/minus multiple melting) for the three magmatic episodes imply initial generation of the Croisilles and Patuki ophiolitic protoliths (Stage 1 and 2 magmas) in an ocean basin (most likely in a “leaky” transform‐related setting). Subsequent evolution (Stage 3 intrusives) took place in a forearc location after a time interval>50 Ma. The transition from oceanic to forearc regime could have occurred due to stepping‐out of a west dipping Benioff zone related to the Brook Street island arc, trapping a segment of oceanic lithosphere, and creating a tensional situation conducive to second‐stage mantle melting. The distinctive style of tectonic intrusion of Patuki ocean crust by subparallel bands of partially serpentinized ultramafic rocks (residual mantle after extraction of Stage 3 magmas) and formation of their metamorphic border zones (K‐Ar age 202–216 Ma

ISSN:0278-7407    

DOI:10.1029/TC007i005p01015

年代:1988

数据来源: WILEY

摘要:

Data from the Coast Range ophiolite and its tectonic outliers in the northern California Coast Ranges suggest that the lower part of the ophiolite formed 169 to 163 Ma in a forearc or back arc setting at equatorial latitudes. Beginning about 156 Ma and continuing until 145 Ma, arc magmatism was superimposed on the ophiolite, and concurrently, a transform developed along the arc axis or in the back arc area. Rapid northward translation of this rifted active magmatic arc to middle latitudes culminated in its accretion to the California margin of North America at about 145 Ma. This Late Jurassic episode of translation, arc magmatism, and accretion coincided with the Nevadan orogeny and a proposed major plate reorganization in the eastern Pacific basin. The high rate of poleward motion necessary to translate the Coast Range ophiolite to middle latitudes during this time implies that the ophiolite traveled north on a fast‐moving plate of the eastern Pacific basin, here termed plate X. Plate X probably was driven by a cryptic ridge east‐northeast of the Pacific‐Farallon‐Izanagi ridge triple junction. Structural relations indicate that following Late Jurassic time, parts of the Coast Range ophiolite were displaced from the west side of the Great Valley province and incorporated into the Central belt of the Franciscan Complex along steep‐dipping to low‐angle reverse faults having dominant components of dextral shear. A northwest trending eastern zone of these right‐laterally displaced outliers shows strong affinities to the main Coast Range ophiolite of the northwestern Sacramento Valley (the Elder Creek terrane), in that the outliers include ophiolitic breccias of Oxfordian to Kimmeridgian age. A southwestern zone of outliers lacks ophiolitic breccia and instead includes latest Oxfordian or Kimmeridgian to Tithonian, arc‐derived volcanic rocks like those found in the Del Puerto and Stanley Mountain terranes of the main ophiolite. Whereas outliers of the northeastern outlier zone are right‐laterally displaced no more than 260 km from the western side of the Sacramento Valley, outliers of the southwestern zone are displaced a minimum of 169 to 249 km. This displacement occurred between about 60 and 52 Ma. Ophiolitic rocks in the Decatur terrane of western Washington that have recently been correlated with the Coast Range ophiolite and the Great Valley sequence of California were apparently displaced at least 950 to 1200 km from the west side of the Great Valley between early Tertiary and Early Cretaceous time. Derived rates of northward translation for the ophiolite outliers in California are in the range of 1 to 4 cm/yr. Rates for the Decatur terrane are in the range of 2.5 to 4 cm/yr if translation was initiated 90 Ma, but as much as 11.9 to 15 cm/yr if it was not initiated until 60 Ma. The lower rates for the Decatur terrane are consistent with the rates derived for the California outliers and with the northward component of relative motion between the Farallon and North American plates from 90 to 50 Ma. The higher rates require northward transport on the Kula plate or on a fast‐moving microplate. The higher translation rates derived for the Decatur terrane are also consistent with paleomagnetically determined rates for some limestones of the Central belt. This may indicate that outliers of the Coast Range ophiolite dispersed in the Central belt of the Franciscan Complex record only part of the total displacement which occurred along the Late Cretaceous to early Tertiary western marg

ISSN:0278-7407    

DOI:10.1029/TC007i005p01033

年代:1988

数据来源: WILEY

摘要:

Oblique plate convergence can cause forearc, arc, and back arc regions of active margin orogenic belts to behave as semi‐independent terranes relative to bordering continental and oceanic lithospheric plates. These terranes, called here “transpressional terranes,” are bound by the subduction zone on one side, by arc‐parallel strike‐slip faults in the forearc, in the volcanic arc, or in the back arc region on the other and are underlain by throughgoing basal decollements. These terranes undergo shortening in a direction perpendicular to the subduction zone and migrate as coherent structural bodies parallel to the subduction zone along bounding strike‐slip faults. Many terranes of the North American Cordillera may have behaved in this manner and perhaps did not travel across the Pacific basin but rather were always peripheral to North America. We propose that during the Triassic to mid‐Cretaceous, portions of the Cordillera borderland generally were displaced southward with respect to the North American craton due to left‐oblique convergence. Since the mid‐Cretaceous, they moved northward due to right‐oblique subduction. The proposed displacements are consistent with structural, paleomagnetic, and paleontologic data and with the relative and absolute motions of North America and the Pacific basin plates based on hotspot ref

ISSN:0278-7407    

DOI:10.1029/TC007i005p01057

年代:1988

数据来源: WILEY

摘要:

Rb‐Sr biotite ages of Archean and Early to Middle Proterozoic crystalline rocks in northern Wisconsin and adjacent Upper Peninsula of Michigan describe a regionally systematic pattern related to differential uplift. An “age low” occurs in northern Wisconsin where values range from 1070–1172 Ma for rocks with crystallization ages of 1760 to 1865 Ma. These values overlap with the main episode of mafic igneous activity (1090 to 1120 Ma) along the Midcontinent rift system (MRS). We interpret these low biotite ages as registering closure due to cooling below the 300°C isotherm as a consequence of uplift and rapid erosion of an area that we are informally naming the Goodman swell. The position of the Goodman swell is independent of the Archean and Early Proterozoic regional geologic framework but closely linked with the curvilinear (concave to the south) MRS. We interpret the swell to be a forebulge imposed on an elastic crust by loading of mafic igneous rocks along and within the axis of the MRS. The southward concavity of load axes resulted in flexural interaction that localized the swell. Because of rapid uplift the swell may have been a source area for clastic sediments in the rift

ISSN:0278-7407    

DOI:10.1029/TC007i005p01077

年代:1988

数据来源: WILEY

摘要:

A new experimental method for determining the pattern of flow and three‐dimensional progressive finite deformation of fluid elements in low Reynolds number (Re) viscous flow has been applied to creeping flow past a rigid sphere falling in a cylinder. This is intended as a model analogue of natural rock deformation associated with magmatic and solid‐state diapirs rising through ductile crust. The kinematics of finite strain in such analogue experiments may give useful information on the development of natural structures. The experiment and a new method for quantifying the progressive finite deformation of fluid volumes in the transparent Newtonian test fluid used (SGM36) are discussed in detail. The progressive finite deformation of fluid flowing along specific streamlines passing the sphere appears to be complex and largely a function of the initial distance of fluid volumes and the particular streamlines which they follow from the flow axis. The progressive deformation history in the material surrounding a rising spherical diapir is characterized by progressive flattening and rotation of material above and progressive stretching below the sphere. The results compare favorably with the external strain pattern associated with post tectonic, mesozonal diapiric intrusi

ISSN:0278-7407    

DOI:10.1029/TC007i005p01091

年代:1988

数据来源: WILEY