摘要:

An interpretation of Northern Apennine geology is presented which relates the temporal and spatial occurrence of both compressional and extensional deformation features in terms of the changing dynamic evolution within an accretionary wedge, after a model proposed by Platt [1986], followed by the initiation and development of continental rifting. During Cretaceous to Eocene time an accretionary wedge formed as remnant Tethyan oceanic crust subducted beneath the rotating Corsica‐Sardinia microplate. Microplate collision during the Oligocene was characterized by the rapid imbrication of buoyant continental crust of the Italian continental margin, the record of which is preserved within the duplex structure geometry of the Alpi Apuane region. The overthickened wedge geometry returned to a more stable configuration by developing extensional features during the Miocene: both listric normal faults at upper‐crustal levels and shear zones indicating evidence of distributed ductile extensional strain at mid‐crustal levels are recorded. It is proposed that large‐scale regional extension with associated volcanism beginning in the Messinian was represented by the intrusion of asthenospheric material from the subducted plate into the already attenuated accretionary complex. Further rifting, perhaps aided by subduction and back arc processes in the Southern Apennines, led to the formation of the Tyrrhenian Sea as an oceanic basin. Both the Apennines and North American core complexes record evidence of crustal thickening followed by crustal thinning, and finally of continental rifting. This suggests that the similar histories of these regions with vastly different plate tectonic settings may both be explained by processes linked to the changing internal dynamics of accretionary

ISSN:0278-7407    

DOI:10.1029/TC009i006p01275

年代:1990

数据来源: WILEY

摘要:

This paper describes structures within mylonites and metamorphic rocks from the Santa Catalina and Tortolita‐Suizo metamorphic core complexes, southeastern Arizona, United States. In these areas, mylonitic rocks constitute an easterly low‐dipping slab more than 4 km thick. Mapping of principal strain directions and analysis of shear sense have been carried out through the whole pile of mylonites. It is shown that although the stretching lineation appears to have a uniform mean SW‐NE trend, three successive shearing events are involved. The first shearing event is toward the west and is accompanied in part by the emplacement of the Leatherwood Quartz Diorite dated at 64 Ma. The second shearing event is toward the NE and is accompanied by the emplacement of the Wilderness Granitic Suite dated at 49 Ma. The third shearing event is localized and directed toward the SW. The top‐to‐west shearing and top‐to‐NE shearing are possibly related to Laramide thrusting. The last ductile shearing is related to post‐E

ISSN:0278-7407    

DOI:10.1029/TC009i006p01305

年代:1990

数据来源: WILEY

摘要:

The deep crustal seismic line NFP 20‐EAST crosses almost the entire Swiss Alps. Despite the complex geometry of the well‐exposed nappe structure and the considerable axial plunge of some of the units, the Vibroseis survey yielded coherent reflections from several individually identifiable nappe contacts. In the northern part of the survey the Vibroseis data closely match the internal structure of the Helvetic nappes and the underlying autochthonous‐parautochthonous Mesozoic sediments. On the northern flank of the Aar massif, an external basement uplift, these Mesozoic sediments seem to rise from a depth of approximately 7–8 km below sea level to the surface in a series of steps which is interpreted to represent crustal shortening achieved by a combination of folding and thrusting. In the southern part of the survey it was possible to image a number of thin slivers of Mesozoic carbonates pinched between slabs of Penninic basement nappes as well as nappe contacts between lithologically contrasting units. In addition, it seems that the Insubric fault zone, which marks the contact between the Penninic zone and the Southern Alps and which outcrops about 30 km to the south of the survey, shows up as steeply north dipping reflections. The lower European crust in the northern part of the survey is relatively transparent as opposed to the Adriatic lower crust, whose reflective nature may stem from shear zones associated with Mesozoic crustal stretching. The base of both the European and Adriatic crust coincides with a 1‐s‐thick band of laterally discontinuous reflections. This reflection Moho drops to greater depths going from the north toward the center of the Alpine chain, where it disappears with a steep southerly dip. The Moho reappears as a reflection band farther south. This Moho gap is situated above the lithospheric root and may be caused by perturbations related to subduction of lower crustal material. The crustal‐scale structure obtained from the Vibroseis data may be interpreted as a continent‐continent collision with wedge‐shaped indentation of a piece of Adriatic crust into the European crust and vertical escape of the material in the core of the orogen along steep

ISSN:0278-7407    

DOI:10.1029/TC009i006p01327

年代:1990

数据来源: WILEY

摘要:

The Cretaceous rocks of the Dead Horse graben area underwent two superposed deformations. Laramide (latest Cretaceous to early Tertiary) left‐transpression structures are overprinted by a Basin and Range (Miocene to Recent) accommodation zone. During Laramide shortening, two northwest trending sets of monoclines developed: an en echelon set over a left‐slip basement fault and a single monocline draped over the uplifted edge of a rotated basement block. Basin and Range extension caused northerly trending half‐grabens that have opposing asymmetry: downthrown‐to‐the‐east north of the study area and downthrown‐to‐the‐west south of the area. These half‐grabens are separated by an accommodation zone, expressed as a northwest trending, right‐lateral pull‐apart graben. The normal faults of the pull‐apart graben flatten at a shallower depth than those of the half‐grabens. This suggests not only that accommodation zones can be mechanically layered but that this layering can occur on a scale different from the adjacent half‐grabens. The northwest trend and superposition of both sets of structures implies a tectonic control by the preexisting structural

ISSN:0278-7407    

DOI:10.1029/TC009i006p01357

年代:1990

数据来源: WILEY

摘要:

The Palmyride fold belt is a northeast‐trending, 400 by 100 km transpressive belt in central Syria embedded in the northern Arabian platform, bounded to the north by the Aleppo plateau and to the south by the Rutbah uplift. Palinspastically restored cross sections from three transects across the Palmyride fold belt demonstrate a minimum NW‐SE shortening of about 20% or 20 km across the southwestern segment of the belt, diminishing to 1–2 km in the northeast, close to the Euphrates graben system. The cross sections are based on the 1∶200,000 scale geologic map of Syria and previously unavailable seismic reflection and well data, all provided by the Syrian Petroleum Company. These results differ significantly from those predicted by kinematic models of Middle East plate motions. In western Syria and eastern Lebanon the Palmyrides obliquely intersect (at about 45°) the roughly north‐trending Dead Sea transform fault system. The Dead Sea fault system shows well‐documented evidence of 105 km of left‐lateral displacement since mid‐Tertiary time south of its intersection with the Palmyrides, yet only about 25 km of motion has been documented north of that juncture in Lebanon and western Syria. Thus, kinematic models of Middle East plate motions predict 80 km of shortening in Syria, most of which should be accommodated in the Palmyride fold belt. Several possibilities exist to explain the discrepancy between the 80 km of predicted shortening and the only 20 km of shortening measured from restored cross sections. Restored cross sections offer only minimum shortening estimates, so the calculated 20 km may underestimate shortening. Second, evidence of strike‐slip displacement recognized in minimum shortening estimates, so the calculated 20 km may underestimate shortening. Second, evidence of strike‐slip displacement recognized in the field and reported in the literature, and indicated by new focal mechanism solutions of two recent earthquakes in the Palmyrides, indicates that some of the still “missing” displacement may be distributed throughout central and northern Syria as strike‐slip motion oblique to the relative northward convergence of the Arabian plate on the Eurasian plate. Alternatively, previous estimates of slip along the northern segment of the Dead Sea transform fault system may be only minimum estimates. A final possibility is that the Dead Sea transform fault in northwestern Syria has been active for only the past 5–6 m.y. or so, implying that it was either nonexistent or moved only slightly before the Pliocene. This would suggest that there is a total of only 45 km of N‐S convergence to be found in central and northern Syria, about 25 km on the Dead Sea fault system and about 20 km in the Palmyrides. This last possibility requires that the northern and southern segments of the Dead Sea fault system developed independently during most of the past 15–20 m.y. In light of the documented but unquantified strike‐slip motion in the Palmyrides, it seems reasonable that strike‐slip motion does accommodate a significant portion of the convergence between the Arabian and Eurasian plates. It is likely, however, that one or more of the other proposed mechanisms also accounts for a component

ISSN:0278-7407    

DOI:10.1029/TC009i006p01369

年代:1990

数据来源: WILEY

摘要:

The positions of certain nonmarine sedimentary basins in central and southern Alaska are spatially related to the geometry of adjacent curved faults. These interior basins occur along the northern, convex side of these fault systems which change trend from northwest in Canada and eastern Alaska to east‐west in central Alaska and to northeast in western Alaska. The basins probably initiated in early to middle Tertiary time and deposition within all basins continued through the Quaternary, broadly synchronous with Tertiary strike‐slip displacements along the Tintina, Denali, and Border Ranges fault systems. We examine the spatial and temporal relationship between basins and faults by using a mechanical model of curved strike‐slip faults based on boundary elements. We relate slip along the curved faults to Tertiary Pacific‐North American plate motions by using principal stress directions inferred from the plate motion directions. Locations of the predicted mean stress reductions around the fault bends caused by fault slip correlate well with positions of the interior basins. Our results suggest that basin subsidence and location were controlled primarily by the geometry of adjacent curved fault systems and the state of stress in Alaska during the Tertiary. Mechanical interaction between curved Alaskan fault systems increases the tendency for basin subsidence. Stresses inferred from early Tertiary plate motion vectors are not oriented correctly to drive right lateral slip along Kaltag, Farewell, and parallel faults in western Alaska. Furthermore, these stress states would have produced left lateral displacements, for which there is no evidence, along these western faults in their present orientations. We find that a combination of slightly rotated fault geometry and low friction along the faults appears to facilitate right lateral slip along the faults in western Alaska. Our results show that deformation well inside the continent can be explained in terms of the Pacific‐North American plate motions. Thus Tertiary plate motions have resulted in a broad zone of fault slip, uplift, and subsidence in the nonrigid North American plate, producing several lar

ISSN:0278-7407    

DOI:10.1029/TC009i006p01387

年代:1990

数据来源: WILEY

摘要:

Right‐lateral shear along the eastern margin of Asia, from the Eocene to the Present has led to the opening of pull‐apart basins, intracontinental such as the Bohai basin, or oceanic such as the Japan Sea. We suggest in this paper that this right‐lateral shear is a consequence of indentation of Asia by India. As in small‐scale analog experiments, we conclude that antithetic wrench faults accommodate the counterclockwise rotation of large domino blocks between two major left‐lateral shear zones (Tien Shan‐Baikal‐Stanovoy for the northern one, and Qin Ling for the southern one). We discuss the compatibility of this mechanism, which involves a rather small amount of extrusion, with the fast eastward expulsion described for southeast Asia. We re‐emphasize the role played in the opening of marginal basins by the Pacific subduction as a free bound

ISSN:0278-7407    

DOI:10.1029/TC009i006p01409

年代:1990

数据来源: WILEY

摘要:

In many strike‐slip tectonic settings, large rotations of crustal blocks about vertical axes have been inferred from paleomagnetic data. These blocks are bounded by sets of parallel faults which presumably accommodate the relative motion between the blocks as regional deformation progresses. A mechanical model suggests that rotations greater than ϕc= 25° to 45° must be accommodated by more than one set of faults, with the angle ϕcbetween their directions, consequently the sum of the angles between sets must be roughly equal to the total tectonic material rotation. To test this model we investigated the fault geometry and field relationship of fault sets in the Mt. Hermon area in northern Israel, where paleomagnetic declination data imply 69°±13° counterclockwise (CCW) block rotation. The statistical and field relationship analysis of over 315 faults shows that the faulting is predominantly right lateral strike slip consisting of three distinct sets. The oldest set strikes 254°, the second oldest set strikes 295° and the youngest strikes 331°. This last direction is consistent also with the current north‐south direction of the maximum principal stress axis. The angle ϕcbetween the first and second sets is 41° and between the second and third sets 36°, in good agreement with the ϕcangle predicted from mechanical considerations. The sum of the two angles is 77°CCW, in good agreement with the 69°±13°CCW paleomagnetically derived rotation. The results suggest specifically that the sequential development of multiple intersecting fault sets is responsible for the faulting in the Mt. Hermon area and generally that the model of block rotation with multiple faults provides very good simple rules for analyzing very c

ISSN:0278-7407    

DOI:10.1029/TC009i006p01421

年代:1990

数据来源: WILEY

摘要:

The Cantwell Basin is a Paleogene intermontane basin located in south central Alaska. The Cantwell Formation which fills the basin is as much as 4000 m thick and consists of a lower sedimentary sequence of fluvial and alluvial fan origin and an upper sequence of calc‐alkaline volcanic rocks. The east trending Cantwell Basin developed as a southward thickening asymmetrical graben between the Hines Creek and Denali faults. Substantial dip‐slip displacement and local, dextral strike‐slip displacement on the Hines Creek fault during basin subsidence was followed by dip‐slip displacement in late Cenozoic time. Strike‐slip movement along the Denali fault, to the south, exerted primary control on basin subsidence and subsequent structural inversion. During the Paleocene, Kula plate motion was nearly perpendicular to the axis of the Cantwell Basin; however, Kula plate motion was oblique to southeastern Alaska and produced regional dextral slip on the Denali fault system. The Cantwell Basin appears to have developed as a pull‐apart basin in which northwest striking syndepositional normal faults linked the Denali fault with the western part of the Hines Creek fault. In early Eocene time an increased rate of plate convergence resulted in substantial displacement on the Denali fault. Concurrent counterclockwise rotation of southern Alaska produced a regional constraining bend in the Denali fault and resulted in strong compressional deformation of the Can

ISSN:0278-7407    

DOI:10.1029/TC009i006p01433

年代:1990

数据来源: WILEY

摘要:

On the western side of Sacramento Valley in northern California, the Upper Jurassic and Cretaceous Great Valley sequence dips east at 40° to 70° in what has been regarded as a tilted but otherwise undeformed east dipping homocline containing strata collectively up to 15 km in thickness. This paper reports the results of structural mapping in the Paskenta area which suggests that, in contrast, steep dips are caused by westward directed thrusting and folding which led to tectonic thickening of the sedimentary pile. In a mainly east dipping sequence, thrusts are recognized by changes in the structural facing and vergence of both folds and cleavage. Although mainly affecting undated rocks of the Great Valley sequence, one major thrust rides on a serpentine‐matrix melange which forms the substrate of the Great Valley sequence. Differential shortening occurring in different blocks during thrusting was accommodated by left‐lateral movement on the Paskenta and Elder Creek fault zones which acted as tear faults. Ductile and brittle faulting have also occurred throughout the sequence, in the melange, and on the tear faults. From work described in this paper and from other reports of thrusts and deformation in the Great Valley sequence in the foothills west of Sacramento Valley, the hypothesis is advanced that these strata have all been tectonically thickened by westward directed thrusting of possible early Tertiary age. While the regional applicability of this hypothesis awaits future detailed structural work, it is no longer sufficient to assume that east dipping strata have been merely tilted. The proposed thrusting model is compatible with the recent tectonic wedge model proposed for the Coast Ranges‐Sacramento Valley by Wentworth et al. (1984). With respect to such a crustal model in which a tectonic wedge of Franciscan Complex was thrust east above older continental basement but below rocks of the Coast Range ophiolite and Great Valley sequence, thrusts of this paper are back thrusts developed above a gently west dipping floor thrust. Back thrusting, shortening, and structural elevation of the Great Valley sequence are direct consequences of this wedging. The uplifted strata lie east of a poorly defined triangle zone and define the western limb of a regional synclinorium that has some analogies with the Alberta syncline of the Canadian fo

ISSN:0278-7407    

DOI:10.1029/TC009i006p01451

年代:1990

数据来源: WILEY