摘要:

The accretionary margin of the Hikurangi forearc on the southeast coast of the North Island of New Zealand is part of the leading edge of the Australian plate, which is overriding the obliquely converging Pacific plate. We investigate the last 10 m.y. of deformation history of the innermost (western) quarter of the total width of the forearc through analysis of the sedimentologic and structural evolution of the Eketahuna area on the east coast of the North Island. The Eketahuna area is ideal for such a study because emergence of the margin in the Quaternary has exposed a complete late Neogene rock record. This record has allowed us to chronicle the strain history. From 10 Ma to about 2.5 Ma this forearc region was the locus of subsidence and marine deposition. In the latest Pliocene this part of the margin began to shorten through folding and reverse faulting, bringing an end to basin filling. The period of shortening was brief, and by the late Pleistocene, reverse faulting had ceased and was immediately succeeded by dextral strike‐slip faulting, in some cases along the same faults. Presently, the dominant strain regime in the inner quarter of the forearc is strike‐slip faulting. This structural history illustrates that, over time, the pattern of strain partitioning has changed in the Hikurangi forearc. The switch from crustal shortening to dextral shear along the major faults in this area in the last 1 m.y. may be a response to more than 10° of clockwise rotation in the southern Hikurangi forearc in Pliocene and Pleistocene time. This rotation is a consequence of the fact that the accretionary margin is undergoing continuous deformation between the obliquely converging Australian and Pacific plates in this area at the southernmost end of the Kermadec‐Hikurangi subduction system. The inboard portion of this young accretionary margin is exceptionally well exposed today, probably in part because of the late Neogene subduction of relatively thick, buoyant crust of the Hikurangi‐Chatham

ISSN:0278-7407    

DOI:10.1029/94TC01506

年代:1995

数据来源: WILEY

摘要:

The Coastal Range in eastern Taiwan was originated from an oblique collision between the Luzon volcanic arc and Asian continent since the late Neogene. In this collision terrane, two intra‐arc basins, the Pliocene Chingpu and Pleistocene Chengkung basins, were developed on the eastern part of the Neogene Chimei and Chengkuangao volcanic islands, respectively, prior to their accretion to eastern Taiwan. The tectonic evolution of these Neogene volcanic islands and associated intra‐arc basins is reconstructed by stratigraphic and sedimentological analysis, igneous rock geochemistry, and comparison with observations in modern collision zone in the regions off southeastern Taiwan. In the Coastal Range, the intra‐arc basin sequences are 1.5–10 km wide and 40 km long, comparable in size to their modern analogues in the active collision zone. The basin axis trends subparallel to the volcanic ridge. In both basins, deepwater flysch overlies shallow marine reef carbonates, which in turn rest on volcanic basement, indicating rapid arc collapse (minimum rate of 1 km/m.y.) soon after the arc‐continent collision. The arc collapse occurred earlier in the north (Chimei, between 5.1 and 3.5 Ma) and later in the south (Chengkuangao, between 2.9 and 1.8 Ma), in concert with a southward propagation of the oblique collision. Sedimentation ended about 2 Ma and 1 Ma in the Chingpu and Chengkung basins, respectively, coeval with rotation of the Neogene volcanic islands. This suggests that the rotation inverted the intra‐arc basin into thrusting, uplifting, and final emergence. Thus the duration of sedimentation for the intra‐arc basins spanned only about 0.8–3.1 m.y. On the basis of land geology, offshore observations, and a clay model experiment simulating oblique arc‐continent collision, a model for the intra‐arc basin evolution in eastern Taiwan is proposed. During the collision, strike‐slip faults would have been developed in the eastern part of volcanic islands to induce transtension movements, thus forming pull‐apart, intra‐arc basins on the collapsed volcanic island. This mechanism is believed to be responsible for the formation of the Pliocene Chingpu and Pleistocene Chengkung basins as well as the present‐day offshore intra‐arc basins found on the Lutao and Lanhsu volcanic islands. The two intra‐arc basins on Lutao and Lanhsu are predicted to be short lived. As collision continues, these two basins, together with their underlying northern part of the Luzon arc, will be rotated, thrust, and uplifted in the next 1 m.y. and, finally, will become part of the southern

ISSN:0278-7407    

DOI:10.1029/94TC02452

年代:1995

数据来源: WILEY

摘要:

A number of different Euler poles have been proposed by various authors during the past 20 years to describe the relative motion of Caribbean (CA) and North American (NA) plates. These different solutions are a consequence of the small number of focal mechanism solutions of earthquakes, the short length of the plate boundaries of the Caribbean that consists of an active spreading center, the complicated and multibranched fault systems of several parts of the plate boundaries, and biases introduced in using slip vectors along the Middle American Trench to close plate motion circuits in computing CA‐NA Euler poles. Slip vectors from focal mechanisms are now the strongest constraints on the direction of current plate motion. We use 66 slip vectors derived from 117 focal mechanisms of shallow earthquakes located on or near the boundaries between the Caribbean and either the NA or South American (SA) plates to calculate a best fitting, present‐day rotation pole for CA‐NA. It is located at 68.4°N, 126.3°W, near Barrow, Alaska. In making this calculation, we effectively extended the length of the plate boundary considered in the calculation by using data from the CA‐SA plate boundary and rotating them to the CA‐NA reference frame using the Nuvel 1 pole for NA‐SA motion and 20 mm/yr as the rate of CA‐SA plate motion along the southeastern Caribbean. The result is not very sensitive to CA‐SA rates. The azimuth of plate motion is about N70°E along most of the CA‐NA boundary. For example, along the Puerto Rico Trench, several focal mechanisms indicate highly oblique motion with a small component of plate convergence along shallow, south dipping thrust faults. The highly oblique plate motion along the inner wall of the Puerto Rico Trench is indicative of strong coupling of that forearc with the Caribbean plate and of the potential of that zone to generate great earthquakes. Slip vectors are very consistent with one another in areas in which at least one of the interacting plates is oceanic. Focal mechanisms and the distribution of earthquakes exhibit greater scatter, however, in areas such as Hispaniola where continental or thick island arc lithosphere is being deformed and is present on both sides of that complex plate boundary. In addition, other mechanisms indicate northerly subduction of the Caribbean beneath southeastern Hispaniola along the western Muertos Trough, normal faulting to the east of the Lesser Antilles associated with the bending of the downgoing plate, and normal faulting or reverse faulting in more localized areas near the plate boundary. As part of the analysis, we also calculated Euler poles for CA‐SA motion at 52°N, 81°W and Cocos‐Caribbean (CO‐CA) at 22°N, 120°W. These poles are consistent with recent models of transtensional development since about 10 m.y. ago along the plate boundary zone of the southeastern Caribbean and with CO‐CA motion that involves subduction along the Middle America Trench as well as right‐lateral, strike‐slip faulting and extension along the volcanic zone of Central America. The Euler pole for CO‐CA is also consistent with recent

ISSN:0278-7407    

DOI:10.1029/94TC02547

年代:1995

数据来源: WILEY

摘要:

The thermal histories of mylonitic rocks from the footwall of the northern Snake Range decollement (NSRD), Nevada, were calculated using multiple diffusion domain analyses of potassium feldspar Arrhenius data and40Ar/39Ar age spectra in order to characterize the cooling (exhumation) history of these mylonitic rocks and thereby constrain the origin and movement history of the NSRD. The calculated thermal histories, along with reported apatite fission track ages, indicate the three following episodes of rapid cooling (10–55°C/m.y.) related to extensional denudation: (1) middle Eocene (48–41 Ma), (2) late Oligocene (30–26 Ma), and (3) early Miocene (20–16 Ma). The first episode of rapid cooling may be related to the earliest period of normal faulting in east central Nevada and initiation of slip along the NSRD. The second episode signals a pulse of slip along the NSRD, and the third episode marks the cessation of lower plate mylonitic deformation and the onset of the major pulse of uplift and rotation of lower plate rocks from beneath the NSRD and/or younger east dipping normal faults. The thermal history data provide evidence for a temperature difference across the range within the same structural horizon; the northwestern flank of the range dropped below 300°C by 46 Ma and reached 115°C by 20 Ma, whereas lower plate rocks in the eastern parts of the range were at temperatures above 300°C until 19 Ma. If isotherms were subhorizontal, these relations suggest that either lower plate units had an eastward dip prior to the onset of Tertiary mylonitic deformation or that layering dipped eastward as a result of top‐to‐the‐east shear within an east dipping shear zone. Either case results in differential cooling (uplift) of lower plate rocks from beneath the NSRD. A temperature‐distance‐age graph, constructed based upon the calculated thermal histories, shows differential uplift and rotation of moderately to steeply east dipping lower plate units from beneath the NSRD. Furthermore, this reconstruction shows that the dip of lower plate units increases and then decreases in time and space. This reconstruction implies that the NSRD initiated at moderate to steep angles that shallowed at depth, listric style, into a shallow dipping shear zone. These reconstructions provide a spectacular example of the rolling hinge/is

ISSN:0278-7407    

DOI:10.1029/94TC01508

年代:1995

数据来源: WILEY

摘要:

The northwestern Yalakom fault system (YFS) lies on the eastern margin of the Coast Belt in southwestern British Columbia. The Yalakom fault is the most significant structure in the fault system, with at least 115 km of dextral slip, mostly of early Tertiary age. The NW YFS is up to 25 km wide and is bounded on the southwest by the Tchaikazan fault zone, which had 5 km or more of dextral slip. Kinematic and paleostress analysis of mesofaults and map‐scale structures shows that secondary faults within the NW YFS had both dextral and sinistral oblique slip. Three large synclines had increasing amounts of clockwise, vertical axis rotation from southeast to northwest. Block rotation between two synthetic strike‐slip faults with large offset (Yalakom) and moderate offset (Tchaikazan) is similar to current kinematics between the San Andreas and San Jacinto faults in southern California. The NW YFS was active from latest Cretaceous(?) to (mainly) Eocene time and postdates early Late Cretaceous thrust faulting to the southwest. Early stage mesofaults are compatible with the thrust belt and suggest that open folds developed in the foreland of the thrust belt before strike‐slip faulting commenced. The abrupt northwest termination of the YFS may be related to extension within the adjacent Tatla Lake metamorphic complex. Some dextral slip may have been transferred farther northwest through the Coast Belt, but these structures have not been documented in west central British Columbia. Most of the 120 km of dextral slip on the YFS was transferred to the north via a mosaic of normal and strike‐slip faults in central British Columbia. Thus the YFS forms the southwestern margin of a broad zone of transtensional structures that extends to the Tintina‐Rocky Mountain Trench fault zone; these structures accommodated northwestward displacement of the northwestern Cordillera in Paleo

ISSN:0278-7407    

DOI:10.1029/94TC01503

年代:1995

数据来源: WILEY

摘要:

The Tertiary fold‐and‐thrust belt of Spitsbergen can be divided into a western basement‐involved fold‐thrust stack and a central‐eastern foreland fold‐and‐thrust belt. In western Nordenskiøld Land the first‐order structure is an ENE‐verging basement‐cored fold and fault complex involving Paleozoic to Tertiary strata. The northern part reveals an upright, monocline geometry of east tilted sedimentary cover units with associated layer parallel to low‐angle thrusts and folds. These structures consist of two populations oriented both parallel to (NNW–SSE) and oblique to (WNW–ESE) the general structural trend of the fold complex. In the central and southern parts of west Nordenskiøld Land the fold complex involves tilted basement cut by steep, transverse faults and late normal faults. The east limb of the fold complex displays repeated basement and Paleozoic strata (Orustdalen formation) in its core and Mesozoic (Triassic) strata influenced by map‐scale chevron folds and two decollement levels, all located above an eastward rotated, major detachment fault, the Kleivdalen Thrust. Establishing fold‐fault relations includes a three‐stage structural history in the fold complex as follows: (1) a phase of early NNE–SSW shortening associated with WNW–ESE folds and thrusts and (2) a dominant ENE–WSW, basement‐involved shortening leading to the first‐order, NNW–SSE‐striking fold complex, followed by (3) approximately E–W extension. The resulting structures and structural variability along strike as well as across strike appear to have been controlled by basement and Carboniferous basin structures underlying the Permian‐Cretaceous platform strata. Restored stratigraphic sections based on thrust‐repetition of basement and cover (e.g., within a type section of the Carboniferous Orustdalen formation) support such an interpretation. A tentative inversion tectonic model reproduces the position(s) of local and major thrust ramps and associated folds, as a resu

ISSN:0278-7407    

DOI:10.1029/94TC01677

年代:1995

数据来源: WILEY

摘要:

Slab breakoff is the buoyancy‐driven detachment of subducted oceanic lithosphere from the light continental lithosphere that follows it during continental collision. In a recent paperDavies and von Blanckenburg[1994] have assessed the physical conditions leading to breakoff by quantitative thermomechanical modeling and have predicted various consequences in the evolution of mountain belts. Breakoff will lead to heating of the overriding lithospheric mantle by upwelling asthenosphere, melting of its enriched layers, and thus to bimodal magmatism. Breakoff will also lead to thermal weakening of the subducted crustal lithosphere, thereby allowing buoyant rise of released crustal slices from mantle depths. In this paper we present a test of this model in the Tertiary evolution of the European Alps. In the Alps, both basaltic and granitoid magmatism occur between 42 and 25 Ma, following the closure of oceanic basins by subduction and continental collision. The granitoids are now well established to result from mixing of basalt with assimilated continental crust. To identify the tectonically crucial origin of the partial mantle melts, we have compiled all published geochemical and isotopic data of numerous mafic dykes occurring throughout the whole Alpine arc. Their trace element and isotopic composition suggests that they have been formed by low‐degree melting of the mechanically stable lithospheric mantle. We see no evidence for melting of asthenospheric mantle. It was thus not decompressed to depths shallower than 50 km. Once initiated, rapid lateral migration of slab breakoff will result in a linear trace of magmatism in locally thermal weakened crust. This explains why all Alpine magmatic rocks intruded almost synchronously along a strike‐slip fault, the Periadriatic Lineament. A compilation of ages from Penninic high‐pressure rocks subducted to depths of up to 100 km shows that subduction took place at circa 55–40 Ma, followed by uplift at 40–35 Ma. From the short time interval between their uplift and the onset of magmatism we infer that both processes have been induced by the breakoff. The slab breakoff model fulfills its predictions in the case of the Alps and therefore supports the assumptions made in the theoretical model on a geological basis. We believe that the characteristic association of magmatic activity with the return of high‐pressure rocks to the surface allows the identification of this process in the Earth's m

ISSN:0278-7407    

DOI:10.1029/94TC02051

年代:1995

数据来源: WILEY

摘要:

The Variscan French Massif Central experienced two successive stages of extension from Middle Carboniferous to Early Permian. In the northern Massif Central, the first stage began in the late Visean, immediately after nappe stacking, and is well recorded by Namurian‐Westphalian synkinematic plutonism. The Middle Carboniferous leucogranites widespread in the NW Massif Central (Limousin and Sioule area) were emplaced within a crust extending along a NE–SW direction. At the same time, the hanging wall or “Guéret extensional allochton” moved toward the SE. Several examples of the synextensional plutonism are also recognized in central Limousin: Saint Mathieu dome, La Porcherie, and Cornil leucogranites. These examples illustrate the relationship between granite emplacement and crustal scale deformation characterized by NW–SE stretching and NE–SW shortening. In the central and southern Massif Central (Cévennes, Châtaigneraie, and Margeride areas), plutonism is dominantly granodioritic and exhibits the same structural features: NW–SE maximum stretching and overturning to the SE. Middle Carboniferous (Namurian‐Westphalian) extension was parallel to the Variscan belt both in the Massif Central and southern Armorican area. This extensional regime was active from the late Visean in the north, while compression dominated in the southernmost domains (Montagne Noire and Pyrenées). The second extensional stage occurred from Late Carboniferous to Early Permian. This event was responsible for the opening of intramontane coal basins, brittle deformation in the upper crust, and ductile normal faulting localized on the margin of cordierite granite‐migmatite domes. Data from the coal basins show that the half‐graben is the dominant structural style, except for basins located along submeridianal left‐lateral faults which have pull‐apart geometries. Late Carboniferous extension occurred along the NE–SW direction. The NE–SW maximum stretching direction can be found in the whole Massif Central but is more developed in the eastern part. The extensional direction is transverse to the general trend of the belt, and top‐to‐the‐NE shearing is dominant. Correlations of these two extension directions with neigh

ISSN:0278-7407    

DOI:10.1029/94TC02021

年代:1995

数据来源: WILEY

摘要:

Paleomagnetism of the Upper Devonian reef complexes in the Canning Basin, Western Australia, was reinvestigated with a primary aim of testing its magnetization age. Samples were collected from back reef and reefal limestones, basinal red beds, syndepositional allochthonous blocks, and sediment infillings in syndepositional cracks and voids. A characteristic remanence, similar to that identified in a previous study, was found in six autochthonous sites in outcrop and four diamond drill cores, with a mean direction of declinationD= 27.5°, inclinationI= −19.9° (half angle of 95% confidence cone α95= 15.2°). The primary origin of this remanence was demonstrated by both a syndepositional breccia test and an allochthonous block test. The corresponding Late Devonian pole position (62.0°S, 23.2°E with half angle of 95% confidence cone A95= 14.6°) overlaps with the previously reported paleopole from this reef complex and other Late Devonian paleopoles from Australia. The evidence indicates that the spread of late Middle to Early Carboniferous paleopoles from Australia is caused by a phase of rapid apparent polar wander, rather than the tectonic discordance of southeastern Australia in relation to th

ISSN:0278-7407    

DOI:10.1029/94TC01622

年代:1995

数据来源: WILEY

摘要:

The tectonic setting of a transcontinental belt of 1500–1300 Ma granitic intrusions that extends from southern California to Labrador is controversial; however, the granites are conventionally considered to be “anorogenic.” Detailed field, microstructural, and geochronologic data from the 1425 Ma Beer Bottle Pass pluton, southern Nevada, indicate that major mylonite zones recording dextral‐contractile strains were probably active during and/or shortly after pluton emplacement and suggest that the anorogenic interpretation for this pluton requires reevaluation. Mylonite zones up to 100 m thick strike northeast, dip moderately northwest, and contain a consistent west plunging elongation lineation. Mylonites occur along 15% of the exposed granite‐wall rock contact and extend into both the pluton and the wall rock. Mesoscopic and microscopic kinematic indicators record an oblique, dextral/reverse (pluton side down) movement sense. Synkinematic mineral assemblages of hornblende and biotite and dynamic recrystallization of feldspars suggest that deformation occurred minimally under amphibolite facies conditions. A K/Ar biotite date of 1399±32 Ma, obtained from a sample of mylonitic granite, suggests that deformation took place during or soon following pluton crystallization. We reject forcible emplacement of the Beer Bottle Pass pluton as a mechanism for formation of the mylonite zones because (1) rocks near the granite‐wall rock contact are largely unstrained, (2) the mylonite zones conform only locally to the pluton‐wall rock contact, (3) mylonite zones strike at high angles to, and truncate, the intrusive pluton‐wall rock contact, (4) the pluton‐side‐down shear sense is more compatible with a uniform sense simple shear zone than a forcibly intruding pluton, and (5) fabrics indicative of noncoaxial deformation dominate over flattening fabrics. We suggest that the Beer Bottle Pass pluton is fundamentally synkinematic with respect to either (1) a local, contractile deformational event or (2) regional strains produced by distant plate tectonic processes operative between 1500 and 1300 Ma as suggested by Nyman and Karlstrom (1994). The timing and kinematics of shear zones associated with other circa 1400 Ma plutons in the western United States (e.g., Graubard and Mattinson, 1990; Nyman et al., 1994; Kirby et al., this issue) support the latter interpretation. Our kinematic data are more consistent with a contractile or transpressive tectonic setting at circa 1400 Ma (Nyman and Karlstrom, 1994; Nyman et al, 1994) than with models involving regional extension (

ISSN:0278-7407    

DOI:10.1029/94TC02446

年代:1995

数据来源: WILEY