摘要:

Detailed SPOT image analysis which completed field data and a microseismicity study was done on the central Caucasian mountain belt (Georgia, Armenia, and eastern Turkey), north of the front of the Arabian collision in order to clarify the relationships between compression, extension, and volcanism. In fact, this region is characterized by relatively complex active tectonics, associating both N‐S compressive (E‐W thrusts and folds) and E‐W extensional (N‐S normal faults and dikes) structures accompanied by considerable Neogene to Quaternary volcanism, and NE‐SW left‐lateral and NW‐SE right‐lateral strike‐slip faults. These different structures are all intricately imbricated and can be observed at different scales. The general layout implies an important variation in the stress state; for instance, this passes from a N‐S compression in the Spitak earthquake fault zone to an E‐W extension at the Aboul‐Samsar volcanic axis. These results agree well with the N‐S convergence between Arabia and Eurasia, and the E‐W lateral expulsion of the Anatolian and Iranian blocks. A detailed cartographic network of active structures was drawn and their kinematic relationships were specified. In places, recent offsets on strike‐slip faults were estimated at about 500 to 1000 m. Suitable sites for future determination of fault velocity displacements were also selected. This study shows that some strike‐slip faults behave partly like faults which transform the E‐W extension of the N‐S striking normal faults into a N‐S compression along the E‐W thrusts. One of the characteristics of this region is the continuous important volcanic activity at least from the Jurassic until now. This persistence evidences a lithospheric thinning, which remained in spite of the recent Arabian‐Eurasian collision because of the E‐W extension linked to the opposite lateral expulsion of the Anatolian and Iranian blocks. This geodynamic evolution can explain the juxtaposition and superimposition of volcanic structures a

ISSN:0278-7407    

DOI:10.1029/93TC00514

年代:1993

数据来源: WILEY

摘要:

From two examples of orogenic domains, some general mechanisms significant of late orogenic tectonic processes in mountain belts are characterized. The Basin and Range province and the Variscan belt in the French Massif Central have both suffered important compressional orogenic crustal thickening, and the results of late orogenic processes can be observed in the field. Both areas are covered by deep seismic profiling providing constraints on the geometry of a crust which has been restored to a normal thickness. Late orogenic features from the two domains are compared at different scales and their tectonic significance for extension mechanisms is discussed. At the scale of the orogenic domains, the most prominent tectonic features are the metamorphic core complexes (MCC) which expose deformed rocks from the middle crust generally affected by high‐temperature, low to medium pressure metamorphism, partial melting, and widespread granite emplacement. In these MCC, large‐scale extensional shear zones present an intense mylonitic deformation characterized by low dipping foliations and pervasive stretching lineations. They show a complete evolution from early deep‐seated ductile deformation (generally achieved under high‐temperature, low to medium pressure metamorphism) to a late shallow brittle stage characterized by cataclastic deformation. The late detachment stage generally controls the development of asymmetrical extensional sedimentary basins filled by continental deposits. Two main geometries of MCC are defined that are characterized by differing geometry and kinematics of low‐angle shear zones. In the first case, two low‐angle shear zones with opposite vergence develop along the flanks of a roughly symmetrical MCC (often one system is dominant over the other). The second geometry characterizes asymmetrical MCC bounded by a single normal shear zone which is upwarped during uplift and doming of the core caused by tectonic denudation. Detailed strain analysis performed in several extensional shear zones shows that the deformation regime is heterogeneous and results from general noncoaxial flow. Deformation along the shear zones evolves progressively from slight homogeneous pure shear strain to intense heterogeneous noncoaxial shear strain. Strain distribution within the lower crust is less well constrained by field observation; however, analogies between COCORP and ECORS deep seismic reflection profiles give important constraints on crustal structure. Wide zones of highly subhorizontally layered lower crust and a flat high‐amplitude reflection Moho characterize both evolved orogenic domains suggesting that major deformations and flow occur within the lower crust during extension. A kinematic model involving heterogeneous crustal deformation and regional scale flow fits relatively well with late orogenic structures observed in continental domains. A weak, hot upper mantle allows large‐scale flow of lower crust material from zones of deep ductile extension to uplifted domains of upper crustal denudation. Heterogeneous strain is accommodated by low‐angle extensional shear zones from localized zones of extension in the brittle crust to ductile lower crust. Combined pure and simple shear occurs along localized shear zones, whereas at the scale of the whole lithosphere, deformation nearly corresponds to a vertical pure shear. Such deformation processes which affect a thick and hot crust seem to be common in both compared domains suggesting that late orogenic extensional processes are slightly dependent of the type of contractional tectonics. Thus, as much in the Andean‐type west American Cordilleran belt as in the collision‐type Variscan belt, late orogenic processes produced similar

ISSN:0278-7407    

DOI:10.1029/93TC01129

年代:1993

数据来源: WILEY

摘要:

Since 1964 there have been six earthquakes ofMw≥ 5.5 in east Africa whose centroid depths have been demonstrated to be in the range of 25–40 km. These depths are significantly greater than the 5‐ to 15‐km range typical of most other regions of continental extension. The March 10, 1989 earthquake (Mw6.1) in Malaŵi is the first such deep event to have occurred within the main topographic expression of the late Cenozoic east African rift system. Its focal mechanism and depth (32 ± 5 km) allow it to be plausibly associated with slip on a deep part of a major normal fault zone bounding the Malaŵi rift. We cannot determine whether the earthquake occurred in the crust or mantle or whether the postulated fault zone exists as a continuous seismogenic surface from the upper crust to depths of ∼30 km: it is possible that the fault zone exists as an aseismic shear zone in the lower crust. In the Malaŵi rift the width of the half graben (up to 50 km), the effective elastic thickness of the lithosphere (∼35 km), and probably the largest fault segment lengths (>50 km) are greater than is typical in rifts outside Africa. We suggest that these features and the greater earthquake depths are all related to the likelihood that the upper part of the lithosphere is colder and stronger than is typical elsewhere. These observations are consistent with earlier suggestions that normal faulting and significant strength can exist throughout the bulk of the crustal thickness. If this is the case, wide half graben can form without requiring shear strengths on the bounding faults to be greater than 1–10 MPa (10–100 bars), which is the typical level of stress drop obse

ISSN:0278-7407    

DOI:10.1029/93TC01064

年代:1993

数据来源: WILEY

摘要:

Geologic and geodetic studies in California indicate that about 1 cm yr−1of right‐lateral shear occurs across what has been referred to as the Eastern California Shear Zone. Northwest trending zones of dextral, sinistral, and normal faults splay eastward from the San Andreas system, continuing through the Mojave Desert, east of the Sierra Nevada, and northward along the Central Nevada and Walker Lane fault zones. Aerial photography, field investigations, and fault studies in southern and central Oregon, compiled with a comprehensive analysis of previous studies nearby, indicate that latest Pleistocene and Holocene fault activity is concentrated along four zones that stretch northward into the Cascade volcanic arc and across the northwestern edge of the Basin and Range Province. The Oregon zones appear to continue the activity in eastern California and northwestern Nevada northward and provide a connection to seismically active zones in southern and central Washington. Several techniques are applied to fault data from the Oregon zones in an attempt to estimate the overall direction and rate of motion across them. The orientations and styles of faults younger than middle Tertiary are used with models of oblique rifting to estimate that the motion of western Oregon is ∼N60° ± 20°W, relative to North America. Summation of geologic moment tensors from faults with latest Pleistocene and Holocene slip yields a direction ∼N90° ± 30°W at a rate of ∼0.5 mm yr−1. This result is a minimum since many fault scarps have not been preserved or recognized, and additional deformation is recorded as folding and tilting. Crustal strain associated with slip during 76 of the largest crustal earthquakes in the past 120 years located along this broad zone from northern California and Nevada, across Oregon, to Washington and Vancouver Island, indicates motions at rates of 3 ± 1 mm yr−1in a direction N55° ± 10°W. Although the motion across central Oregon is much slower, its similarity in style with regions to the north and south suggests that the regional averages are meaningful. Oregon fault zones, taken together, may accommodate as much as 6 mm yr−1oriented ∼N60° to 70°W. A tectonic model of fault activity reveals that this proposed shear zone through Nevada, Oregon, and Washington can account for 10% to 20% of the total Pacific‐North American transform motion and much of the lateral component of relative motion between the Juan de

ISSN:0278-7407    

DOI:10.1029/92TC02950

年代:1993

数据来源: WILEY

摘要:

The Cuyama basin in the southern Coast Ranges of California underwent a transition from strike‐slip to compressional faulting, synchronous with the deposition of the Pliocene‐Pleistocene Morales Formation. Paleomagnetic stratigraphy was used to date the Morales Formation and, by inference, the beginning of compressional tectonics. Sections sampled below the Whiterock and Morales thrusts in the western Cuyama basin are predominantly normal and are correlated with the Gauss chron (3.57–2.60 Ma). An abrupt appearance of clasts derived from the overlying thrust sheet in the section below the Whiterock thrust suggests uplift of the Caliente Range during the middle to late Gauss chron. Seismic reflection data indicate that the eastern sections were deposited earlier. In conjunction with fossil evidence, the eastern sections are correlated to a time between the late Gilbert and early Matuyama chrons. The presence of a crystalline boulder bed midway in an eastern Cuyama basin section indicates uplift of the Mount Pinos‐Frazier Mountain highlands during the Matuyama chron between 2.60 and 0.78 Ma. Paleomagnetic directions of the Morales Formation document approximately 23° clockwise rotation of the Cuya

ISSN:0278-7407    

DOI:10.1029/93TC00314

年代:1993

数据来源: WILEY

摘要:

This paper presents a study of the ductile and brittle deformation on Naxos and Paros islands (Cyclades, Greece). Previous maps and studies of the two islands have shown that a major low‐angle fault zone separates surface rocks above the contact from an initially deep‐seated unit below, showing a metamorphic evolution from high to low pressures. Structural analysis, as well as available stratigraphical, metamorphic, and geochronological data taken together demonstrate that this fault zone is a major normal‐sense detachment zone dipping to the north. Rapid denudation of footwall rocks subsequent to high temperature metamorphism, at an estimated rate of 1.8–9.5 mm/yr, attests for tectonic unroofing during regional‐scale top‐to‐the north ductile shearing. The change from ductile to brittle behavior of the footwall rocks together with a progressive localization of high strain intensity deformations just below the hangingwall is explained by the progressive cooling of the uprisen footwall of the detachment. Mio‐Pliocene clastic sediments in the hangingwall represent the infilling of half grabens opened in between major normal faults that are synthetic to the underlying ductile shear zone. These sediments are as old as (Aquitanian‐25 Ma), or younger than the earliest recognized evidence of ductile extension in the footwall. This provides a minimum age for the onset of extension in the Cyclades, which appears significantly older than maximum ages reported up to now (13–5 Ma). Structural data strongly suggest that the detachment fault was initially rather low dipping (≈35°). An evolutionary model is proposed, in which migmatite domes in the footwall correspond to the uprise of the lower ductile crust between two separating upper crustal blocks, during a process of asymmetric boudinage of the crust. This detachment model applies to a previously thickened continental lithosphere, which then suffers thermal relaxation and weakening, allowing extensional deformation to reach a climax during and subsequent to high temperature metamorphism. In the Cyclades, crustal‐scale extension started after Early Cenozoic thrusting, while the crust was still thick, or less likely, before late underthrusting b

ISSN:0278-7407    

DOI:10.1029/93TC01131

年代:1993

数据来源: WILEY

摘要:

The Denali fault system (DFS) extends for ∼1200 km, from southeast to south central Alaska. The DFS has been generally regarded as a right‐lateral strike‐slip fault, along which post late Mesozoic offsets of up to 400 km have been suggested. The offset history of the DFS is relatively unconstrained, particularly at its western end. For this study we calculated relative motion vectors at discrete points along the length of the DFS, based on the well‐understood kinematic interaction between the North American, Pacific, and Kula plates, and the following assumptions: (1) The arcuate geometry of the DFS has existed essentially unchanged since the Late Cretaceous; (2) The Yukon‐Tanana terrane and other terranes north of the DFS were fixed, in situ, prior to the accretion of the southern Alaskan terranes; and (3) Tangential and normal relative motion vector components calculated for points along the DFS using the plate model of Kelley [1993] describe the plate kinematics of the DFS since the Late Cretaceous. The consequent kinematic model for the DFS predicts that left‐lateral stresses have acted upon the western end of the DFS for much of its history, and conflicting senses of shear exist between the eastern and western ends of the system. The offset history of the western end of the Denali fault system should be significantly different than the history of the central and eastern sections; consequently, individual crustal blocks in southeast and southwest Alaska may have undergone, respectively, clockwise and counterclockwise rotations. The sense of rotation predicted by our model is in agreement with rotations determined by paleomagnetic studies and provides an alternative model to the “Alaskan oroclin

ISSN:0278-7407    

DOI:10.1029/93TC00674

年代:1993

数据来源: WILEY

摘要:

Subsidence on rifted conjugate continental margins around the North Atlantic is analyzed to derive the amount and areal distribution of stretching in the crust and in the lower lithosphere during continental rifting. Study areas are the Grand Banks and Orphan Basin regions of the eastern Canadian continental margin and the Goban Spur and Galicia Bank regions off western Europe. In all areas, maps of synrift and postrift sediment thickness and bathymetry were used to derive maps of post‐ and synrift subsidence. A two‐layer lithospheric stretching model with independent amounts of stretching in the crust and in the lower lithosphere was assumed to be applicable, with the rifting history approximated by several instantaneous episodes of extension. This model was used to derive estimates of stretching at all points on a 0.05° geographical grid, where subsidence values were available within the study regions. The models are constrained with seismic measurements of crustal thickness. The results imply that pure shear stretching predominates at a lithospheric scale, while simple shear is more localized laterally and confined to the crust. In places there is significant decoupling between crustal and mantle stretching. Near the continent‐ocean boundary, final continental breakup may be localized on one side of the rift between conjugate margin pairs, rather than symmetrically located. Total extension of the margins is compatible with that estimated from normal fault geometries and indicates that the continent‐ocean boundary has been extended up to 350 km seaward of its original position, which should be considered in plate kinematic reconst

ISSN:0278-7407    

DOI:10.1029/93TC00915

年代:1993

数据来源: WILEY

摘要:

Western Iberia is bounded by a nonvolcanic rifted continental margin made up of three apparently independent segments. The age of breakup decreases from south to north. Seismic refraction and reflection profiles, and magnetic and gravity data from each segment, show a consistent pattern of geophysical observations across the ocean‐continent transition (OCT) zone, which is a few tens of kilometers wide. We emphasize here the discovery of thin (2–4 km) oceanic crust underlain by 7.6 km s−1material within the OCT. The available evidence favors the suggestion that the 7.6 km s−1layer is serpentinized peridotite and that the thin oceanic crust is primarily the result of a poor magma supply for a few million years immediately after continental breakup. This thin crust may be the source of some ophiolites which exhibit thin crustal sections and continental margin aff

ISSN:0278-7407    

DOI:10.1029/93TC00059

年代:1993

数据来源: WILEY

摘要:

Structural and geochronologic data from the 111–114 Ma Okanogan Range batholith in north central Washington are used to characterize the timing and style of deformation during the early stages of the mid‐to Late Cretaceous North Cascades‐southeastern Coast Belt (NC‐SECB) orogen. The Pasayten fault zone bounds Jurassic‐Cretaceous sedimentary and volcanic rocks of the Methow basin to the west against predominantly Mesozoic igneous rocks of the Intermontane terrane to the east. The Pasayten fault zone accommodated intrusion of the western units of the Okanogan Range batholith during high‐angle slip, then approximately 20 km of left‐lateral strike‐slip at 109–95 Ma. South of the Pasayten fault zone, the Red Shirt and Methow River thrust zones accommodated (1) west vergent contraction at amphibolite grade that began by 113 Ma and ended by 112 Ma, based on new U‐Pb zircon dates for deformed and undeformed granitoids, and (2) northwest vergent contraction in the Methow River thrust zone followed by west vergent contraction in the Red Shirt thrust zone, both at greenschist grade prior to 104 Ma. These contractional displacements thrust the Methow basin 10–20 km southeastward beneath the western Okanogan Range batholith. Deformation in the NC‐SECB thus began by earliest Albian time, earlier than previously thought, and was characterized by coeval sinistral transcurrent and west vergent contractional faulting. Sinistral strike‐slip in the Pasayten fault zone may have reflected the pre‐100 Ma sense of oblique plate convergence or relatively minor southward tectonic escape of the adjacent Methow basin during contractional deformation. The slip history of the Pasayten fault zone is undocumented between approximately 80 Ma and 60 Ma when, on the basis of paleomagnetic data, some workers have proposed that the Methow basin moved 1700 km northward to its present position relative to the Intermontane terrane. However, significant Late Cretaceous to Early Eocene dextral slip on the Pasayten fault zone is unlikely because the western intrusive units of the Okanogan Range batholith continue without disruption acr

ISSN:0278-7407    

DOI:10.1029/93TC01061

年代:1993

数据来源: WILEY