摘要:

The accumulation of a large database on the timing and kinematics of late Cenozoic deformation in the Death Valley region of southeastern California indicates a complex three‐dimensional history. On the basis of paleogeographic reconstructions we suggest the system was initiated as a localized pull‐apart between two conjugate strike‐slip faults, the Garlock and Furnace Creek faults, and evolved into a system characterized by distributed transtension related to the eastern California shear zone. Our reconstructions differ from previous models in the incorporation of significant vertical axis rotations of a number of crustal blocks to explain paleomagnetic data from the region. The model may resolve (1) a long‐standing problem of the eastern termination of the Garlock fault which is explained here as a complex system of splays that initially terminated in the pull‐apart between the Furnace Creek and Garlock systems; and (2) the complex architecture of the Black Mountains which is explained here in terms of initial extreme attenuation between the Garlock and Furnace Creek systems with overprinting by a fold and normal fault system that operated simultaneously as a result of distributed transtension. This model suggests much of the displacement field is taken up in rotations and translations, and the actual crustal thinning in our model is relatively small (50–66% of original

ISSN:0278-7407    

DOI:10.1029/96TC01633

年代:1996

数据来源: WILEY

摘要:

A major problem posed by the geology of Crete is the horizontal contact of an upper unit without Miocene metamorphism onto a metamorphosed lower one with Early Miocene high pressure/low temperature (HP/LT) parageneses. This very sharp contact is roughly parallel to the major Oligo‐Miocene thrust planes which were reactivated as a large‐scale detachment which allowed exhumation of high‐pressure units. We describe the extensional deformation and the metamorphic evolution of the lower plate. Most first‐order deformation features relate to the retrogression from high‐pressure to low‐pressure conditions. A N‐S pervasive stretching is observed everywhere, often associated with a top‐to‐the‐north sense of shear. The extreme variation of thickness of the Phyllite‐Quartzite nappe (upper part of the lower plate) is probably the result of large‐scale boudinage similar to the one seen in large outcrops. The most important observation is the systematic occurrence of fresh carpholite immediately below the base of the Tripolitza nappe except in northwestern Crete where a late extensional shear zone is present. Deeper in the nappe pile carpholite is systematically retrograded. This observation reveals a drastically different PT history for the upper part of the Phyllite‐Quartzite nappe. It also suggests that the late extensional shear zone found along the northern side of Crete cuts inside the metamorphic structure and brings the nonmetamorphosed Tripolitza nappe directly in contact with the deeper parts of the Phyllite‐Quartzite nappe. PT‐t paths suggest a fast temperature decrease in the top of the Phyllite‐Quartzite during retrogression and, hence, during the top‐to‐the‐north shear. The deeper part of the Phyllite‐Quartzite nappe shows a low‐temperature regime throughout, but its PT path includes an isothermal decompression in the first stage. We produce a tentative map of domains having experienced similar PT trajectories during decompression. The overall cool regime is related to the continuous underthrusting of cola continental units during exhumation. Isothermal decompression observed in the core of the Phyllite‐Quartzite Nappe implies fast exhumation during extension and the faster cooling of the upper part is related to a continuous displacement toward the north of a cooler unit during exhumation. Single grain39Ar‐40Ar ages obtained on phengites (15–25 Ma) in various structural sites are in good agreement with these conclusions and with the geological context suggesting that underthrusting of cold units at the front the accretionnary complex occurred contemporaneously with unroofing below a north dipping detachment near the top of the wedge. The age of this detachment is bracketed between the end of the high‐pressure event (20 Ma) and the deposition of the brecci

ISSN:0278-7407    

DOI:10.1029/96TC01417

年代:1996

数据来源: WILEY

摘要:

Structures produced by salt and mobile shales are commonly similar; however, the material behavior of the two is different. Salt mobility is a fundamental material property, shale mobility only occurs if overpressured fluids are present. Dewatering of shales will stop their mobility, while renewed burial or the onset of an internal overpressuring process (e.g., diagenetic release of water or hydrocarbon generation) may renew mobility. Consequently, structures in mobile salt will envolve continuously until salt withdrawal produces touchdowns sufficient to stop salt mobility. Shale mobility may follow a deformation sequence similar to salt, or it may display a more episodic evolution reflecting critical overpressuring events. While salt mobility is confined to specific lithological units, the same is not the case for shale. The overpressuring of shales is strongly dependent on depth, so that mobile shale zones may cut across time/bedding boundaries. The differences in mechanical behavior lead to differences in structural style although many basic aspects of gravity tectonics remain the same. (1) Prekinematic structures and synkinematic deformation occur in both salt and shale tectonics. However, much of the prekinematic deformation in mobile shale‐dominated deltas may be lost by burial and conversion of the prekinematic sequence into mobile shales. (2) Fault‐dominated depocenters occur in salt and shale tectonics. In the Niger delta, overpressured shales vary in thickness from thin decollement zones to massive chaotic zones some 4–6 km thick. Fault‐controlled basins develop over mobile shales. Maximum basin depth is approximately the thickness of the mobile shale plus the thickness of the overlying pregrowth fault strata (i.e., some 4–8 km). (3) Diapirs are common to both salt and shale tectonics. Normal faults associated with reactive diapirism are common in both. Salt has the potential to almost completely evacuate a particular volume, resulting in local touchdowns or welds. Shale may also create touchdown areas, but complete collapse is uncommon because a large volume of immobile dewatered shale is usually left behind. (4) Salt nappes can cover extensive areas in the Gulf of Mexico. Some limited shale tongues occur in the Niger delta. They form imbricate thrusts that pass down dip into gravity flows and slumped blocks of shale derived from the exposed mob

ISSN:0278-7407    

DOI:10.1029/96TC01416

年代:1996

数据来源: WILEY

摘要:

Intraplate extension, in a frame of a global compressional stress field, seems linked to local lithospheric perturbations (lithospheric thinning or thickening) able to modify the resulting state of stress [Zoback, 1992]. The Baikal Rift Zone (BRZ), Siberia, is located north of the India‐Asia collision zone and exhibits no direct communication with any oceanic domain. It can thus be fully considered as an area of continental extension, dominated by the “global compressional intraplate stress field” resulting from plate driving forces. In order to address the problem of its dynamics and kinematics and their links with the India‐Asia collision, a comprehensive stress tensor analysis is presented, based on 319 focal mechanisms of earthquakes located along the whole Baikal rift. The stress field is varying at different scales of observation: when looking at central Asia (several thousands kilometers), the maximum horizontal stressSHmaxdirections remain rather constant (with a fan‐shape geometry) when the tectonic regime goes from compressional (Himalayas) to extensional (Baikal). When observing the Baikal rift (about 1000 km long), clear variations of the stress regime are observed, from an extensional regime in the central part of the rift to wrench ones in its northern and southern ends. Finally, at the scale of 100 km, systematicSHmaxreorientations occur close to major rift faults. We thus infer that the interaction between collisional processes and inherited structures may have a strong influence on rift dynamics. We then use computed stress tensors to predict slip vectors on major rift faults. Deformation patterns show two distinct parts of the rift: the South Baikal Rift (SBR) is characterized by a constant trending (around N100°E) slip vector, meanwhile the North Baikal Rift (NBR) exhibits a complex block rotation behavior involving at least three crustal blocks. We propose to interpret these surficial structures and motions as the result of an interaction between the regional compression coming from the India‐Asia collision and the geometry of the hardly deformable Siberian platform. This particular setting can explain most of the surficial deformation patterns, which suggest a large‐scale cracking of the lithosphere in the Baikal region. Other possible sources of stress could also be considered, like deep mantellic upwelling, or trench suction linked to the Paci

ISSN:0278-7407    

DOI:10.1029/96TC00624

年代:1996

数据来源: WILEY

摘要:

With more than 4000 m of Oligocene‐Aquitanian sediments, the Vistrenque graben (SE France) is the deepest synrift depocenter of the Gulf of Lion passive margin, NW Mediterranean. Detailed analysis of industrial seismic reflection profiles and borehole data show that the steep Nîmes fault, which bounds the graben to the NW, forms at depth a low‐angle (25°) crustal ramp. Along‐strike changes of hangingwall geometry allow us to infer along‐strike changes of fault shape: A rollover structure and divergent Oligocene‐Aquitanian basin fill are associated with a listric geometry of the fault in the southern part of the graben, while a pseudo‐rollover and compensation graben result from a two‐segments planar geometry of the fault in the northern part. Preexisting structures inherited from Mesozoic extension and Late Cretaceous‐Eocene Pyrenean thickening controlled the location of the Nîmes fault and the transfer zones which divide the graben into different compartments. Since both hangingwall and fault profile are well constrained, restoration techniques can be used to estimate the prerift topography. The Vistrenque graben was formed close to sealevel, but at the front of a>1 km‐high elevated area resulting from the Pyrenean orogeny. In the studied transect, the Nîmes fault formed the landward (NW) boundary of the basement faulted domain of the margin. Extensional deformation was restricted to this domain during most of the rifting interval. Small amounts of extension were transmitted landward to Mesozoic cover décollement rooted in the Nîmes fault, only during short episodes, probably resulting from gravitational instability during margin collapse. The Nîmes low‐angle crustal ramp, as well as the other crustal ramps of the margin of similar orientation, are probably newly formed extensional structures rather than reactivated Pyrenean thrusts. Their activation at a low‐angle may have been allowed by crustal weakening resulting from the previous Pyrenean thickening. Upper crustal extension corresponding to the graben formation was transmitted basinward through an intracrustal detachment, or/and distributed in the lower crust across the margin. In contrast to the more stretched areas of the margin which do not display thick synrift series due to their initial high surface elevation, the Vistrenque basin fill records the whole rifting episode because of its location at the

ISSN:0278-7407    

DOI:10.1029/96TC01097

年代:1996

数据来源: WILEY

摘要:

The pattern and characteristics of thrust‐related very low grade metamorphism in the marginal part of an orogenic wedge have been investigated by combining a clay mineral crystallinity survey with detailed structural mapping of the thin‐skinned foreland thrust belt along the external part of the Scandinavian Caledonides. This external part is composed of late Neoproterozoic to Ordovician sedimentary sequences of the autochthonous cover and Lower Allochthon, which are overlain by the higher Caledonian nappes (Middle and Upper Allochthonous units). Stages of the Scandian phase of thrust wedge development are described which are related to the very low grade metamorphic history. The initial stage involved the emplacement of cooled nappes belonging to the Middle and Upper Allochthons, with very low grade peak metamorphic conditions attained within the underlying Lower Allochthon (cover) sediments as they were progressively buried and deformed beneath the thrust wedge. During this initial emplacement the isotherms are considered to have been undisturbed and dipping parallel to the wedge surface. The following stages of wedge development consisted of extensive post‐metamorphic imbrication of the underthrusted cover sediments, with a transition from basal accretion and uplift at the rear, to accretion and forward propagation at the wedge's toe. During accretion into the wedge, the externally dipping isograd surfaces were extensively displaced from deeper levels toward higher tectonic horizons. The last stage of wedge development considered here was characterized by late out‐of‐sequence thrusting with enhanced (epizonal) metamorphic grades developed in the vicinity of the fault zones, which either resulted from further displacement of the isograds toward higher levels, or from localized heating via intense fluid activity. Overall, the pattern of metamorphic grade, fabric relationships, and physical calculations of heat transfer based on the geometry of the thrust wedge, suggest that neither inverted temperature gradients nor shear heating were likely causes of the metamorphism in this flat‐lying part of the orogenic wedge. The description of inverted very low grade metamorphic isograds in other marginal parts of the Scandinavian Caledonides, which have been previously attributed to either the rapid emplacement of hot thrust nappes, or the effects of dissipative shear heating, are discussed in terms of variations in both the critical wedge geometry and its controlling boundary

ISSN:0278-7407    

DOI:10.1029/96TC00983

年代:1996

数据来源: WILEY

摘要:

In understanding of ophiolite emplacement requires knowledge of the elapsed time between igneous crystallization and intraoceanic thrusting, and the rate and duration of that thrusting. Hornblende40Ar/39Ar ages demonstrate that the igneous oceanic crust in Oman crystallized and cooled to ∼825 K in 1–2 m.y. Hornblende ages from metamorphic rocks and from cross‐cutting dikes require that the basal metamorphic thrust fault beneath the ophiolite also cooled below ∼825 K in 1–2 m.y. Motion along the sole thrust accounted for 200 km of displacement at a rate of 100–200 mm/yr. On the basis of age relationships and thermal considerations, we favor a two‐stage model for the initial stages of Samail ophiolite emplacement: intraoceanic thrusting over<2‐m.y.‐old lithosphere at 150 km/m.y. parallel to a spreading ridge for 1–2 m.y., followed by equally rapid and brief thrusting over cold and old lithosphere. Preservation of the Samail ophiolite is a direct result of its young age and positive buoyancy at the time of ocean closure, and we propose that all ophiolites that originated near spreading centers and were emplaced onto continents were young at the time of int

ISSN:0278-7407    

DOI:10.1029/96TC01973

年代:1996

数据来源: WILEY

摘要:

Geophysical data combined with pressure‐temperature‐time data form the basis for a reconstruction of the kinematic evolution of the Scandinavian Caledonides collisional orogenic belt. The reconstruction suggests that the early stages of collision between Baltica and Laurentia involved intracontinental subduction of a minimum of 150 km of Baltic continental crust to a depth of at least 100 km. The subduction juxtaposed strong, Laurentian upper mantle rocks in the upper plate against weak, Baltic crustal rocks in the lower plate. Continued plate convergence, coupled with the buoyancy of the Baltic rocks, resulted in thrust imbrication of a substantial portion of the lower plate and extrusion of the crustal rocks from the subduction zone. Thrusting also resulted in incorporation of fragments of the Laurentian upper mantle into the Baltic crust. The reconstruction also suggests that direct interaction between the Laurentian and Baltic crustal rocks, and the time of greatest crustal thickness in the orogen, occurred after extrusion of the Baltic crust from the subduction zone. One implication of this suggestion is that the eclogite/coesite assemblage presently exposed at the surface in the hinterland of the Caledonides probably formed early in the collision sequence during subduction rather than during the time of greatest crustal thickness. The reconstruction also suggests that exposure of the high‐pressure mineral assemblage was a two‐phase process, involving extrusion from the subduction zone by thrusting during convergence and unroofing by a combination of extension on low‐angle normal faults and erosion. The kinematic model derived for the Scandinavian Caledonides provides a possible scenario for the evolution of other collisional orogenic belts, containing very high‐pressure mineral assemblages formed in situ in continental rocks and associated ultamafic rocks of ma

ISSN:0278-7407    

DOI:10.1029/96TC00828

年代:1996

数据来源: WILEY

摘要:

The metamorphic core of the Himalayan orogen, or Greater Himalayan sequence, is a northward tapering prism bound at the bottom by a N dipping family of thrust faults (the Main Central thrust system) and at the top by a N dipping family of normal faults (the South Tibetan detachment system). Research in the central Annapurna Range of Nepal demonstrates a close temporal and spatial association between contractional and extensional deformation on these bounding fault systems and within the metamorphic core throughout much of the Early Miocene. The Main Central thrust system is represented here by a 2‐ to 3‐km‐thick zone of high strain that developed during two or more episodes of movement. Most of its displacement was concentrated along the Chomrong thrust, a sharp, late‐metamorphic discontinuity that places middle amphibolite facies rocks of the Greater Himalayan sequence on top of lower amphibolite facies rocks of the Lesser Himalayan sequence. The earliest demonstrable movement on this thrust system occurred ∼22.5 Ma; the most recent movement may be as young as Pliocene. The oldest element of the South Tibetan detachment system in this area is the Deorali detachment, which appears to have been active at the same time as the earliest shortening structures of the Main Central thrust system. Fabrics related to the Deorali detachment are disrupted by a previously unrecognized, SW vergent, thrust structure, the Modi Khola shear zone. The effect of this structure, which is constrained to be between 22.5 and 18.5 Ma, was to shorten rock packages that had been extended previously during movement on the Deorali detachment. Transition back to a local extensional regime after 18.5 Ma was marked by development of the Machhupuchhare detachment and related splays. Geologic evidence for rapid, two‐way transitions between contraction and extension in the Annapurna Range indicates that extensional deformation in convergent settings does not only represent gravitational collapse at the end of an orogenic cycle; it also appears to be an important factor in mountain range

ISSN:0278-7407    

DOI:10.1029/96TC01791

年代:1996

数据来源: WILEY

摘要:

40Ar/39Ar data (hornblende, biotite, muscovite, and K‐feldspar) and U/Pb data (zircons) were obtained from the Nanga Parbat‐Haramosh Massif (NPHM), NW Pakistan, along three transects in the southern regions of the NPHM. We have based our interpretations on our new data as well as geochronologic dates from previous studies in the northern regions of the massif. Geochronologic data show that the NPHM has experienced exceptionally high denudation and cooling rates over the past 10 m.y. U/Pb ages determined through sensitive high‐resolution ion microprobe (SHRIMP) “depth‐profiling” experiments on metamorphic zircons and conventional U/Pb monazite dates suggest that the timing of metamorphism varied across the massif. In addition, we have documented that the massif has experienced postmetamorphic, differential cooling both along and across strike. Thermochronologic data on currently exposed surface rocks suggest that cooling occurred more recently and at greater rates in the south‐central regions of the massif (representing deeper crustal levels) than along the margins and northern regions of the massif. Within the Tato region, cooling following peak metamorphic temperatures of 600°–700 °C was as high as 140 °C/m.y. following partial melting of pelitic units. Biotites from this area record plateau ages of 0.9 ± 0.1 Ma. Along the Astor and Indus gorges, cooling was less rapid (approximately 70°–80°C/m.y.) following peak metamorphism as indicated by U/Pb monazite ages of 6–8 Ma and40Ar/39Ar muscovite cooling ages of 2.2–3.4 Ma. Cooling over the last 3 m.y. occurred at rates of 100°–140 °C/m.y. The overall cooling age pattern within the massif is interpreted syntaxial growth through the development of north plunging antiforms prior to 3 Ma, followed by reverse faulting along east dipping fault zones. Along the Raikot River transect the biotite cooling age pattern is consistent with the folding of isotherms during folding of the foliation surfaces. The age pattern was disrupted at 1 Ma due to faulting along the Raikot and Tato faults. An electronic supplement of Tables A1, A2, and A3 may be obtained on a diskette or Anonymous FTP from KOSMOS.AGU.ORG (LOGIN to AGU's FTP account using ANONYMOUS as the username and GUEST as the password. Go to the right directory by typing CD APEND. Type LS to see what files are available. Type GUEST and the name of the file to get it. Finally, type EXIT to leave the system.) (Paper 95TC00032, Geochronologic constraints on syntaxial development in the Nanga Parbat region, Pakistan, David M. Winslow, Peter K. Zeitler, C. Page Chamberlain, and Ian S. Williams). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N. W., Washington, DC 20009; $$15.0

ISSN:0278-7407    

DOI:10.1029/96TC00032

年代:1996

数据来源: WILEY