Convergent Plate Boundary Zone Research Articles

Observations of SKS splitting beneath the Central and Southern External Dinarides in the Adria-Eurasia convergence zone

Seismic anisotropy beneath the greater region of the Central and Southern External Dinarides is estimated from observations of SKS splitting. The area is located in the broad and complex Africa-Eurasia convergent plate boundary zone, where the Adriatic microplate interacts with the Dinarides. We analyzed recordings of 12 broadband seismic stations located in the Croatian coastal region. Evidence of seismic anisotropy was found beneath all stations. Fast axis directions are oriented approximately in the NE-SW to NNE-SSW direction, perpendicularly to the strike of the Dinarides. Average delay times range between 0.6 and 1.0 s. A counter-clockwise rotation in average fast axis directions was observed for the stations in the northern part with respect to the stations in the southern part of the studied area. Fast axis directions coincide with the assumed direction of asthenospheric flow through a slab-gap below the Northern and Central External Dinarides, with the maximum tectonic stress orientation in the crust, and with fast directions of Pg and Sg-waves in the crust. These observations suggest that the detected SKS birefringence is primarily caused by the preferred lattice orientation of mantle minerals generated by the asthenospheric flow directed SW-NE to SSW-NNE, with a possible contribution from the crust.

Postglacial (after 20 ka) dextral slip rate of the offshore Alpine fault, New Zealand

Geological displacement rates for major plate boundary strike-slip faults have seldom been determined with precision from the marine environment. In this paper I present high-quality slip rates derived from the southern Alpine fault using multibeam bathymetric data and dated samples from the Fiordland continental margin. These rates are derived from dextral displacements of relict but well-preserved glacial geomorphology (moraines and outwash fans), interpreted to 17 (+2/−1, calendar) ka. The weighted mean slip rate is 27.2 (−3.0/+1.8) mm/yr on the shelf between Milford and George Sounds, increasing southward to 31.4 (−3.5/+2.1) mm/yr between Caswell and Doubtful Sounds, with uncertainties at the 95% confidence level. These offshore rates are higher than those from an adjacent 80-km-length section of the fault on land in south Westland, and are in good agreement with global positioning system data. The southern slip rates represent some of the highest strike-slip rates on Earth, and ~90% of the total plate motion in southern New Zealand. The southward increase in strike-slip rate on the Alpine fault occurs despite a southward reduction in Pacific-Australian relative plate motion rate and despite the prediction of southward-decreasing plate-parallel motion and southward-increasing normal convergence. The Fiordland region is thus an excellent example of geological strain being highly partitioned within an obliquely convergent plate boundary zone.

Refined slip distribution and moment magnitude of the 1848 Marlborough earthquake, Awatere Fault, New Zealand

The Awatere Fault is an important element of the obliquely convergent plate boundary zone in northeastern South Island, New Zealand, and is comprised of two geo‐morphically distinct sections that join across a complex fault junction in the upper Awatere Valley. The M w c. 7.5 Marlborough earthquake of 1848 ruptured a c. 110 km length of the eastern section of the Awatere Fault, from the South Island's east coast extending inland to at least Barefell Pass. This study attempts to document the coseismic slip distribution during the 1848 earthquake rupture. We examine a previously unmapped length of the fault trace in the upper Awatere Valley, along which we measured as many small (metre‐scale) geomorphic displacements as possible. Based on these new data, and previous observations of inferred 1848 slip on other parts of the Awatere Fault, we infer that the 1848 earthquake had a mean coseismic displacement of 5.3 ± 1.6 m, a surface rupture length of 100–110 km, and a moment magnitude of Mw = 7.4–7.7. This new moment magnitude estimate supports previous estimates based on regressions of geometrical rupture parameters inferred for the 1848 earthquake and attenuation models of felt intensities. The set of second‐smallest horizontal geomorphic displacements (c. 8–15 m) are approximately double those attributed to the 1848 earthquake, suggesting the penultimate event was of a similar magnitude to the 1848 earthquake.

Refinements to the paleoseismic chronology of the eastern Awatere Fault from trenches near Upcot Saddle, Marlborough, New Zealand

The Awatere Fault is one of four principal strike‐slip faults within the obliquely convergent plate boundary zone in northeastern South Island, New Zealand. The eastern section of the fault last ruptured in 1848, in the Mwc. 7.5 Marlborough earthquake, when surface rupture was traced by first‐hand accounts from the South Island's east coast inland to Barefell Pass (c. 110 km). We excavated two paleoseismic trenches near Upcot Saddle on the eastern section of the fault, c. 70 km inland from the coast. Interpretation of crosscutting relationships between sediments exposed in these trenches, in association with 21 radiocarbon dates, suggest at least 5, and possibly 6, events have ruptured this part of the Awatere Fault in the last 5600 yr. These events include a young rupture (<300 cal. yr BP) that is presumed to be the surface rupture of the 1848 Marlborough earthquake. The Upcot Saddle rupture history is combined with paleoseismic data from two previous trenches, located c. 55 km to the northeast at Lake Jasper, to infer a chronology of 9–10 events between AD 1848 and 8610 cal. yr BP on the eastern section. These 9–10 events define a mean recurrence interval of 820–950 cal. yr on that fault, which is within error of the mean recurrence interval calculated using available data on late Quaternary slip rates (6 ± 2 mm/yr) and mean coseismic slip during the 1848 earthquake (5.3 ±1.6 m). The intervals between individual paleoearthquakes on the eastern section are non‐uniform, and range from 220–620 to 1540–2120 yr.

Tertiary palaeogeography and tectonostratigraphic evolution of the Northern and Southern Peri-Tethys platforms and the intermediate domains of the African–Eurasian convergent plate boundary zone

The increasing effects of African–Eurasian convergence during the Tertiary resulted in the uplift and emergence of the Northern and Southern Peri-Tethys platforms. Palaeogeographic maps covering six selected time slices, including the Middle Eocene, late Early Oligocene, late Early Miocene, early Middle Miocene, early Late Miocene and Middle to Late Pliocene illustrate that environmental and depositional differentiations on the northern platform and along the bordering domains of the convergence zone were more pronounced than on the southern platform and its adjacent areas. The tectonic evolution of the northern platform included an overall eastward-directed trend in the onset of basin uplift and emergence, which started at about the Eocene–Oligocene transition, at 34 Ma. Tectonostratigraphic analyses indicate a striking contemporaneity of the events which defined the temporal and spatial development of both the domains of the convergent plate boundary zone and the bordering platforms. Five episodes of major regional change in palaeogeographic and tectonic setting are distinguished. They occurred in the Late Eocene (37–34 Ma), early Late Oligocene (30–27 Ma), latest Early to earliest Middle Miocene (17–15 Ma), early Late Miocene (9–8 Ma) and late Early to early Middle Pliocene (4–3 Ma). These episodes encompassed changes which were most probably induced by geodynamic events primarily related to the relative motions of the African/Arabian and Eurasian plates. In turn, the plate motions are assumed to have ‘triggered’ discrete steps in the regional kinematics and geodynamics that governed the palaeogeographic evolution of the Peri-Tethys platforms and the intermediate domains of the African–Eurasian plate boundary zone.

サハリン（樺太）島の地質と地形 サハリン最北部の地質構造からみた北米・ユーラシアプレート収束境界のテクトニクス

The region from Sakhalin in Russia to the eastern margin of the Japan Sea has been regarded as a convergent plate boundary zone between the Eurasia and the North America Plates because large earthquakes and active crustal movements are prominent in this zone. These activities along the eastern margin of the Japan Sea to southern Sakhalin are almost consistent with the expected relative plate motion deduced from the Euler pole which is estimated from the magnetic anomaly lineations in the Atlantic Ocean, but the modern tectonic aspect in northern Sakhalin is inconsistent; e.g., 1995 Neftegorsk Earthquake is one of the typical events.As a result of a structural and tectonic study in northern Sakhalin, the NE-SW compressive tectonic feature since the Late Miocene was clarified. In the northernmost area of the Schmidt Peninsula, the early Cretaceous ophiolite thrusts upon the Late Cretaceous sedimentary rocks. The ophiolite has an overturned sequence: serpentinites, gabbros, basaltic rocks, and hemipelagic sediments in descending order. The Cretaceous sediments form a map-scale and NW vergent synclinorium. All the deformation structures in outcrop scale, such as micro-folds, minor reverse and normal faults, axial plane cleavages and bedding slips, are consistent with a large scale folding. These structural relationships suggest only one event of deformation, which appears to be linked to overthrusting of the ophiolitic rocks.A structural investigation of the southeastern part of the Schmidt Peninsula revealed that the Middle Miocene sediments composed of siltstone and sandstone are folded with wavelength of several tens to several hundred meters. Their fold axes trend in the NW direction, which indicates that deformation under the NE compression is the same as the deformation in the northernmost area. The thickness of sedimentary layers are constant everywhere in folding. This fact indicates that the timing of the deformation is after sedimentation, that is, after the Middle Miocene. This deformation event is consistent with modern activity in northern Sakhalin. Therefore, the modern tectonic framework might have started in the Late Miocene time.A plate tectonic model indicates that the modern relative motion between the Eurasia and the North America Plate started at about 11 m. y. ago. The Late Miocene onset of the modern tectonic framework in northern Sakhalin occurred at almost the same time. The most reliable model to explain the discrepancy of sense of movement from the Eurasia-North America retative motion, may be the “extrusion” of the Okhotsk Block toward the Pacific Ocean. The dextral extrusion boundary in northern Sakhalin may be traced along theeastern coast of Sakhalin to the northern edge of the Kuril Basin, where active seismicityhas been observed although a detailed study of focal mechanisms and other tectonic aspects is needed in the future.

Cenozoic evolution of the Central Andes in Bolivia and northern Chile

Abstract The Central Andes in Bolivia and northern Chile form part of a wide and obliquely convergent plate-boundary zone where the oceanic Nazca plate is being subducted beneath the continental South American plate. In the latest Cretaceous and Palaeocene, this part of the Central Andes formed a volcanic arc along what is today the forearc region of northern Chile, with a wide zone of subsidence, as much as 400 km wide, at or close to sea level behind the arc. In the Eocene, the central part of the behind-arc basin was inverted to form a zone of uplift (proto-cordillera), about 100 km wide and along what is today the western margin of the Eastern Cordillera of Bolivia. The Altiplano basin and an early foreland basin were initiated at this time, receiving sediment from the Eocene proto-cordillera. Subsequently, the proto-cordillera widened, as the rate of deformation increased and deformation spread westwards into the early Altiplano basin, and also eastwards towards the Brazilian Shield. In the Late Miocene, deformation essentially ceased in the Altiplano and Eastern Cordillera. An intense zone of shortening was initiated in what is today the Subandean Zone on the eastern margin of the Central Andes, deforming the Oligo-Miocene foreland basin. Shortening in the Subandean Zone accommodated both underthrusting of the Brazilian Shield and also bending of the entire mountain belt about a vertical axis. It is suggested that much of the distinctive Cenozoic tectonic evolution of this part of the Andes is related to pre-Andean strength inhomogeneities in the South American lithosphere.

Large tectonic rotations since the Early Miocene in a convergent plate-boundary zone, South Island, New Zealand

A palaeomagnetic study in part of the New Zealand plate-boundary zone provides new constraints on the temporal and spatial distribution of Neogene and Quaternary tectonic rotations. Thermal demagnetization of samples from Cretaceous basaltic dykes, Palaeocene-Oligocene micritic limestone, and Miocene and Pliocene siltstones in the Marlborough region, South Island, have defined stable, high-temperature magnetic components, which are interpreted as the primary magnetization. Declination anomalies, after tectonic corrections, are interpreted as rigid body rotations about a vertical axis of sample sites relative to the Pacific plate. All palaeomagnetic data from Marlborough cluster into three main groups. A 60–100° clockwise rotation affected Palaeocene to Middle Miocene sedimentary sequences across Marlborough between ∼ 18 Ma and ∼ 8 Ma, coeval with a phase of low-angle thrusting. The absence of this rotation in a Late Cretaceous dyke swarm defines the present western limit of the early rotating zone. A regional ∼ 20° clockwise rotation occurred in the last 4 Ma during the development of the Marlborough Fault System in a zone of dextral transpression, although locally clockwise rotations ≤ 40° may have occurred near some of the major dextral strike-slip faults. However, a negligible rotation is observed in the same period in the region to the southeast of the major Kekerengu dextral strike-slip fault, which appears to have acted as a hinge zone, accommodating relative rotation by dextral strike-slip on an arcuate fault, bending, and internal deformation. The observed tectonic rotations record the overall clockwise rotation of the trend of the southern end of the Hikurangi margin from W to NW in the Early Miocene to ∼ NE today, determined independently from the long-term relative plate motion data for the Pacific and Australian plates.

Archean synsedimentary tectonic deformation—A comparison with the Quaternary

Observations from the 3.55–3.3 Ga Barberton greenstone belt indicate an important phase of shortening synchronous with the deposition of alluvial sediments. The structural style is remarkably similar in scale and geometry to that now occurring on land in the convergent plate-boundary zone in the North Island of New Zealand.

Neogene block tectonics of eastern Turkey and northern South America: Continental applications of the finite difference method

Continental plate boundary zones are, generally, wide diffuse zones of deformation within which, in the upper 5 to 15 km, may be recognized blocks, which are bounded by fault zones within which strain is highly concentrated and along which slip rates are fairly high (2–30 m Ma−1). These blocks may be irregular flakes, defined by older crustal inhomogeneities of an upper brittle crust below which strain occurs in a ductile and more homogeneous lower crust. With examples from the convergent plate boundary zone of eastern Turkey and the southern Caribbean plate boundary zone of northern South America, we use plate slip rates and trends and slip rates of faults to construct block vector diagrams to deduce the sums of relative motion among block mosaics, slip rates and trends of ill defined block boundaries, and the extent to which blocks are internally strained.