摘要:

Average strain rates in sectors of deforming Asia are matched by a fifth‐order polynomial function, and that function integrated, to obtain the relative velocities and rotations occurring within east Tibet, western Sichuan, Yunnan, and south China. The method was applied to strain rates obtained from moment tensor summation of both modern and historic earthquakes but can be applied as well to strains obtained from Quaternary slip rates on major faults. If south China has negligible motion relative to Siberia, then the velocity results indicate that nearly all of the expected motion between India and the south China portion of Eurasia has, in the last 85 years, been accommodated by distributed intraplate deformation in east Burma, Yunnan, western Sichuan, and east Tibet. Calculations indicate that these regions constitute a zone of distributed right‐lateral shear that accommodates an overall north‐south sense of relative motion between east Tibet and south China and India and south China. Line elements parallel to both right‐lateral and left‐lateral faults in east Burma and western Yunnan are rotating clockwise relative to south China, with the line elements parallel to left‐lateral faults rotating most rapidly (2.0 ± 0.5°/m.y.). In eastern Tibet and the Gansu‐Ningxia, NW‐SE trending left‐lateral faults give rotation clockwise relative to south China (1–2.5°/m.y.). In central Tibet and western Sichuan, right‐lateral faults give slight counterclockwise rotation rates relative to south China (0.5–0.75°/m.y.) Instantaneous rotation rates within the deforming region, extrapolated over a 20–40 m.y. time period, are in rough agreement with the paleomagnetic rotations meas

ISSN:0278-7407    

DOI:10.1029/92TC00658

年代:1993

数据来源: WILEY

摘要:

Regional and local stratigraphic, metamorphic, and structural constraints permit reconstruction of the southern Snake Range extensional deformational system in east central Nevada. The dominant structure of the range, the southern Snake Range décollement (SSRD), operated during Oligocene and Miocene extensional deformation to exhume a footwall of multiply deformed metasedimentary and plutonic rocks. Intrusion of three plutons (∼160 Ma, 79.1 ± 0.5 Ma, and 36 ± 1 Ma, respectively) and development of two cleavages preceded the onset of extensional deformation. Plastic deformation of lower plate metasedimentary rocks accompanied the early phases of regional extension and produced bedding‐parallel grain shape foliations and WNW trending stretching lineations. These fabrics parallel the SSRD even in low‐strain domains, suggesting that a significant component of pure shear strain probably accompanied noncoaxial deformation associated with motion on the SSRD, consistent with other lines of evidence. Meanwhile, hanging wall rocks were greatly extended by at least two generations of tilt block‐style normal faults soling into the SSRD, with the earlier faults antithetic to the SSRD and the later faults dipping in the same direction as the SSRD. A retrodeformed regional cross‐section sequence illustrates plausible alternative schemes for reconstructing the southern Snake Range extensional system. In one scheme, the SSRD forms as a crustal scale stretching shear zone separating an upper plate that extends on steeply inclined normal faults from a lower plate that stretches by penetrative flow. In the other, lower plate deformation incorporates a component of coaxial stretching, but the SSRD also functions as a conventional shear zone accommodating through‐going displacement between opposing plates. In either case, as tectonic unroofing proceeds, differential isostatic unloading induces the SSRD to rotate to steeper dips as it migrates into the frictional sliding regime, thus enabling it to remain active as a brittle normal fault until it finally rotates to its present shallow inclination. In either scenario, cross‐section constraints suggest that total extension accommodated by the SSRD was probably betwee

ISSN:0278-7407    

DOI:10.1029/92TC01713

年代:1993

数据来源: WILEY

摘要:

Paleomagnetism of sedimentary rocks of the late Miocene Uchiumigawa Group exposed in southeast Kyushu yielded 17 tilt‐corrected paleomagnetic directions with significant westward declinations. Fourteen of these were of reversed polarity, and three at the intervening horizons were of antipodal normal polarity. The overall formation mean direction is D = 322.0°, I = 48.6° with α95= 6.0°. This is statistically indistinguishable from the mean direction of D = 331.2°, I = 41.3° with α95= 9.9° for the middle Miocene deposits in Tanegashima Island of the northern Ryukyu arc. The common mean direction is D = 333.2°, I = 45.1° with α95= 4.9°. This indicates that both south Kyushu and the northernmost Ryukyu arc have experienced 27° ± 6° of counterclockwise rotation with respect to the Eurasian continent after the latest Miocene, or during the last 6 m.y. This counterclockwise rotation cannot be fully explained by either a conventional model which links an arc rotation with the formation of back‐arc oceanic crust, or the collision of the Kyushu‐Palau ridge with southwest Japan arc. We propose that this intra‐arc rotation took place during the period of extension of the continental crust behind the north Ryukyu arc. This study demonstrates that arc rotation may generally occur when the back‐arc spreading is of a

ISSN:0278-7407    

DOI:10.1029/92TC01712

年代:1993

数据来源: WILEY

摘要:

The Chugach and Prince William terranes compose a Mesozoic through Paleogene accretionary complex rimming the Gulf of Alaska. In eastern Prince William Sound, we studied the Contact fault system, which juxtaposes flysch of the latest Cretaceous Valdez Group (Chugach terrane) with flysch of the late Paleocene through early Eocene Orca Group (Prince William terrane). Near Valdez, we found three lithotectonic belts separated by right‐lateral strike‐slip faults. Anomalous structural trends between the two faults appear to be the result of clockwise block rotation in response to dextral shear. In a second area, to the east near Cordova, early formed folds were rotated clockwise before burial to depths sufficient to form axial‐planar cleavage. Apparently, synchronous thrusting and dextral‐slip faulting occurred during accretion. The relations above, combined with earlier studies, suggest that the fundamental nature of motion on faults composing the Contact fault system is dextral. Near Cordova, deposition, accretion, and block rotation occurred between early Eocene and early middle Eocene time. Near Valdez, block rotation appears to have occurred prior to accretion of the rocks near Cordova. Current plate tectonic models show that convergence became progressively more oblique during Eocene time, correlating well with the initiation of strike‐slip faulting. The presence of early Tertiary dextral‐slip faults, such as these, helps account for the northward displacement of the Chugach‐Prince William terrane with respect to the Peninsular and Wrangellia terranes suggested by paleomagnetic and geo

ISSN:0278-7407    

DOI:10.1029/92TC01324

年代:1993

数据来源: WILEY

摘要:

Data from a digitally recording seismic network in San Juan, Argentina, provide the first images of crustal scale basement faults beneath the Precordillera. This seismicity is near the boundary between the Precordillera (a thin‐skinned thrust belt) and the Sierras Pampeanas (a region of thick‐skinned basement deformation), two seismically active tectonic provinces of the Andean foreland. The seismicity data support models for this region in which crustal thickening, rather than magmatic addition or thermal uplift, plays the dominant mountain building role. The Precordillera seismicity occurs in three segments distributed north to south. The southern segment is an area of diffuse activity extending across the Precordillera and eastward into the Sierras Pampeanas that shows no patterns in map or cross section. The northern and central segments have well‐defined dipping planes that define crustal scale faults extending from 5 to 35 km depth. It is clear from the relative fault geometries that the overlying Precordillera is not simply related to the basement activity. The seismicity here may result from reactivation of an ancient suture between the Precordillera and Pampeanas terranes or be occurring in basement of unknown affinity west of the suture. The seismicity provides the first constraints on basement fault geometries, and we present models integrating this information with the surface geology. These basement faults may have been responsible for the 1944 Ms 7.4 earthquake that destroyed the city of San Juan. The imaging of these faults suggests that seismic risk estimates for San Juan made on the basis of surface geologic studies may be to

ISSN:0278-7407    

DOI:10.1029/92TC01108

年代:1993

数据来源: WILEY

摘要:

Fission track analysis of Wind River Range basement rocks provides information about the timing, magnitude, and cooling that occurred in response to Laramide uplift. Eighty samples were collected from Precambrian rock along six traverses in the northern, central, and southern parts of the range over 5 km of elevation. Apatite fission track ages and track length data suggest that the range did not have a uniform thermal history during late Mesozoic and Cenozoic time. Zircon fission track ages are Precambrian, implying that these rocks have not been at temperatures higher than ∼ 200°C (9–10 km burial depth) since Precambrian time. Apatite ages from the northeastern and southwestern flanks of the range suggesst that cooling may have initiated in the Wind River Range by about 85 Ma but was definitely occurring by 75 Ma. Data from the Green River Lakes area and Fremont Peak in the northern part of the range indicate that cooling was occurring by 62 Ma and was most rapid between 60 and 57 Ma. Apparent rates of exhumation vary from 94 m/m.y. to 246 m/m.y. in this part of the range. The central part of the range records this same period of rapid cooling, but structural and topographic relief are not as great. Ages from the southern part of the range suggest that uplift and erosion may have been initiated earlier there but continued at much slower rates (34 m/m.y.) over a longer period of time than in the north. Samples from a drill hole in the central Wind River Range have apatite ages ranging from 37 Ma to 86 Ma and suggest a significant cooling event at approximately 42 Ma involving nearly 2 km of the rock column. Timing, magnitudes, and apparent rates of exhumation have varied spatially in the Wind River R

ISSN:0278-7407    

DOI:10.1029/92TC01567

年代:1993

数据来源: WILEY

摘要:

Fault planes and slickenslides have been systematically studied in the Troodos and Kellaki ophiolitic sequences and in their overlying Uppermost Cretaceous to Neogene sedimentary cover exposed in southern Cyprus. A relative chronology of three main deformational events has been recognized. The most recent event, occurring during the late Neogene, is characterized by a N‐S striking extension. From late Cretaceous to late Miocene, the deformation became compressional with a N160° strike. An older tectonic event, contemporaneous with the development of the Cretaceous ophiolitic suite, is characterized by a ESE‐WNW extension or extensional strike

ISSN:0278-7407    

DOI:10.1029/92TC00332

年代:1993

数据来源: WILEY

摘要:

Mid‐Cretaceous crustal extension played a fundamental role in the structural evolution of the Yukon‐Tanana terrane (YTT) in the northern Cordilleran interior. In the central portion of the YTT northwest of Delta Junction, Alaska, a mylonitic shear zone juxtaposes greenschist facies rocks in the upper plate against middle to upper amphibolite facies metamorphic rocks in the lower plate, a juxtaposition suggesting elimination of as much as 10 km of crustal section. The mylonites form a partial sheath enveloping a domal footwall structure and kinematic analysis of the mylonite zone yields a uniform transport direction of hanging wall to ESE. These relations suggest analogies to the metamorphic core complexes of the southern Cordillera. However, the YTT structures are entirely ductile, suggesting either a relatively deep erosional level or relatively high geothermal gradients during extension. In the study area remnants of an older preextensional thrusting event are preserved at the highest structural levels at the base of the Seventymile terrane and the leading edge of YTT in the Wickersham terrane. However, most areas display a regional, subhorizontal fabric that is superimposed on older fabrics, and in the study area this latest fabric is subparallel to the mylonitic sheath of the apparent extensional structure. Thus the conventional viewpoint that this latest fabric is related to thrusting needs to be reevaluated and this fabric may be entirely extensional in origin. Further evidence for extension is provided by clear similarities between YTT and characteristic features of other extensional terranes. Thus we suggest that the YTT is a deeply eroded view of highly extended continental crust. The tectonic mechanism for the extensional event and the magnitude of the extension is uncertain because of complications in regional timing relationships and in alternative interpretations of the reconstruction of the crustal section. Three end‐member models based on analogies with Neogene extensional systems are presented as working models to accommodate the alternative interpretations: (1) a Jurassic collision and Cretaceous extension model based on comparisons with the Neogene history of the Mediterranean region; (2) an Early to mid‐Cretaceous syncollisional model analogous to the Carpathian Mountains of eastern Europe; and (3) a syncollisional plateau uplift model with extension driven by gravity sp

ISSN:0278-7407    

DOI:10.1029/92TC00860

年代:1993

数据来源: WILEY

摘要:

We use numerical and laboratory modeling to analyze the mechanical role of backstops in the overriding plate at subduction zones. A backstop is defined as a region within a forearc that has significantly greater shear strength than the sediment lying farther trenchward; it can be thought of as the bulldozer behind an accretionary wedge. We calculate the stress and displacement fields within forearcs for various backstop models using the finite element method, and we simulate deformation over a backstop using a small‐scale laboratory model. In this way we model the effects of the mechanical properties and geometry of a backstop on forearc structures. We find that the growth of an outer arc high, the development of an inner deformation belt with landward vergence, and the seemingly paradoxical presence of an undeformed forearc basin within an otherwise highly deformed forearc can be explained by assuming the existence of a geometrically simple backstop with geologically reasonable contrasts in mechanical properties compared to the sediments just trenchward of it. The structures produced in our numerical models are quite insensitive to the rheology, boundary conditions, and exact mesh geometry employed. The general types of structures observed in the laboratory models depend only weakly upon the strength and the geometry of the backstop. These results suggest that a detailed picture of an underlying backstop cannot be determined from surface information alone. Backstops in which the contact with the accretionary wedge dips arcward rather than trenchward, however, should produce only slightly different forearc structures, with less development of both the outer arc high and the landward‐vergent inner deformation belt. Although natural forearcs are far more complex than our simple models, they exhibit many of the same features, indicating that relatively simple backstop mechanics may be a very important factor in the overall growth of forea

ISSN:0278-7407    

DOI:10.1029/92TC00618

年代:1993

数据来源: WILEY

摘要:

Several zones of major thrust faulting exhibit a juxtaposition of rocks of higher metamorphic grade over rocks of lower grade. This configuration may indicate, but does not require, that temperature gradients were temporarily inverted near the fault. We examine the physical conditions under which such a temperature inversion could occur. The overthrusting of hotter rock on colder rock can cause a temporarily inverted gradient both above and below the fault only if the time taken to underthrust rock from the land surface to a given depth,zƒ, on the fault is less than π times the characteristic time, , for diffusion of heat through the block above the fault, where κ is thermal diffusivity. An inverted gradient cannot form without heat generation in the fault zone unlessV×zƒ× sin δ ≳ 100, whereVis the rate of underthrusting (in millimeters per year),zƒis in kilometers, and δ is the dip of the fault. This simple criterion is sufficient to demonstrate that several examples of inverted metamorphic gradients cannot be explained simply by the thrusting of hot on cold rock without heat sources in the fault zone. Dissipative heating accompanying deformation can cause an inverted temperature gradient within and beneath the thrust zone, but whereas the overthrusting of hot upon cold rocks cools the fault zone, dissipation heats it. Thus the overthrusting of hot on cold rock and dissipative heating affect the temperature gradient and the maximum temperatures differently. We show that the magnitudes of the inverted temperature gradient and of the maximum temperature above the inverted gradient yield independent estimates of the rate of dissipative heating. Discrepancy between these estimates implies that some additional process must have occurred, such as the post‐thrusting disruption of the isograds. If there is such a discrepancy, the maximum temperature probably provides the more reliable estimate of the rate of heating at the fault. We illustrate this analysis by applying it to reported inverted metamorphic zonation in the Pelona Schist, the St. Anthony Complex, the Mt. Everest region of the Main Central Thrust, and the Olympos Thrust. Petrological inferences of maximum temperatures, depths of metamorphism, and magnitudes of apparent inverted gradients, imply that shear stresses of about 100 MPa accompanied thrust faulting in some of these regions, and that some zones of inverted metamorphism have been tectonically thinned after the metamorp

ISSN:0278-7407    

DOI:10.1029/92TC00850

年代:1993

数据来源: WILEY