New seismic images of the Cascadia subduction zone from cruise SO108 — ORWELL
Ernst R. FluehMichael A. FisherJörg BialasJ. R. ChildsDirk KlaeschenNina KukowskiTom ParsonsDavid W. SchollUri S. ten BrinkA. M. TréhuN. Vidal
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Keywords:
Accretionary wedge
Seafloor Spreading
Basement
Convergent boundary
North American Plate
Continental Margin
The tectonics of the Pacific margin of North America between Vancouver Island and south-central Alaska are dominated by the northwest motion of the Pacific plate with respect to the North America plate at a velocity of approximately 50 mm/yr. In the south of this mapped region, convergence between the northern extent of the Juan de Fuca plate (also known as the Explorer microplate) and North America plate dominate. North from the Explorer, Pacific, and North America plate triple junction, Pacific:North America motion is accommodated along the ~650-km-long Queen Charlotte fault system. Offshore of Haida Gwaii and to the southwest, the obliquity of the Pacific:North America plate motion vector creates a transpressional regime, and a complex mixture of strike-slip and convergent (underthrusting) tectonics. North of the Haida Gwaii islands, plate motion is roughly parallel to the plate boundary, resulting in almost pure dextral strike-slip motion along the Queen Charlotte fault. To the north, the Queen Charlotte fault splits into multiple structures, continuing offshore of southwestern Alaska as the Fairweather fault, and branching east into the Chatham Strait and Denali faults through the interior of Alaska. The plate boundary north and west of the Fairweather fault ultimately continues as the Alaska-Aleutians subduction zone, where Pacific plate lithosphere subducts beneath the North America plate at the Aleutians Trench. The transition is complex, and involves intraplate structures such as the Transition fault. The Pacific margin offshore British Columbia is one of the most active seismic zones in North America and has hosted a number of large earthquakes historically.
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Convergent boundary
Transform fault
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The formation of a global network of plate boundaries surrounding a mosaic of lithospheric fragments was a key step in the emergence of Earth’s plate tectonics. So far, propositions for plate boundary formation are regional in nature but how plate boundaries are being created over 1000s of km in short periods of geological time remains elusive. Here, we show from geological observations that a >12,000 km long plate boundary formed between the Indian and African plates around 105 Ma with subduction segments from the eastern Mediterranean region to a newly established India-Africa rotation pole in the west-Indian ocean where it transitioned into a ridge between India and Madagascar. We find no plate tectonics-related potential triggers of this plate rotation and identify coeval mantle plume rise below Madagascar-India as the only viable driver. For this, we provide a proof of concept by torque balance modeling revealing that the Indian and African cratonic keels were important in determining plate rotation and subduction initiation in response to the spreading plume head. Our results show that plumes may provide a non-plate-tectonic mechanism for large plate rotation initiating divergent and convergent plate boundaries far away from the plume head that may even be an underlying cause of the emergence of modern plate tectonics.
Convergent boundary
Mantle plume
Seafloor Spreading
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Based on 13 new fault plane solutions and published seismological, geological, and geophysical data, we interpret the deformation along the Pacific‐North American plate margin in the eastern Gulf of Alaska. Three major tectonic units can be distinguished: (1) the North American plate, (2) the Pacific plate, and (3) a belt of mobile borderland terranes. The Pacific plate moves in a NNW direction at rates of about 6 cm/yr in relation to the North American plate. That motion results in mostly right‐lateral strike slip at the Queen Charlotte‐Fairweather fault system, a well‐known observation. A new finding,however, is that a small component (∼1 cm/yr) of convergence may also be present which results in minor subduction of the oceanic plate beneath portions of the continental margin. Heretofore the Queen Charlotte‐Fairweather fault zone and associated continental margin was interpreted as a classical, pure transform boundary. The Yakutat block, a borderland terrane about 400 km long and 100 to 200 km wide, is carried passively by the Pacific plate except that the block slowly overrides this plate at about 1 cm/yr. This motion is taken up by almost pure thrust faulting in a southwesterly direction along a 400‐km long SE striking shelf edge structure. At its NW edge the Yakutat block is in turn being thrust beneath the North American plate along the Pamplona zone‐Icy Bay lineament. The underthrusting of the Yakutat block results in a major orogeny, crustal shortening and uplift of the Chugach‐St. Elias range. The effects of this collision may extend as far as 500 km inland and cause some deformation at the Denali fault in the central Alaska Range. Subduction of the Pacific plate beneath the colliding margin appears responsible for development of an active volcanic arc up to 300 km inland which trends SE from the Wrangell Mountains to Yukon Territory, Canada, and perhaps to Mt. Edgecumbe volcano in southeast Alaska. The tectonic model proposed implies a high seismic hazard for the Queen Charlotte, Fairweather, and Chugach‐St. Elias fault systems. At these fault zones we estimate recurrence times for great events of about 100 years, but they may vary between 50 and 200 years. A temporarily very high potential for a great earthquake has been determined for the ‘Yakataga seismic gap’ located between Icy Bay and Kayak Island. Large or great thrust earthquakes on the detachment fault underlying the entire Yakutat wedge also appear possible but may only occur infrequently. Their recurrence times are estimated to be several
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Abstract The present‐day motions in and around the Arabian plate involve a broad spectrum of tectonic processes including plate subduction, continental collision, seafloor spreading, intraplate magmatism, and continental transform faulting. Therefore, good constraints on the relative plate rates and directions, and on possible intraplate deformation, are crucial to assess the seismic hazard at the boundaries of the Arabian plate and areas within it. Here we combine GNSS‐derived velocities from 168 stations located on the Arabian plate with a regional kinematic block model to provide updated estimates of the present‐day motion and internal deformation of the plate. A single Euler pole at 50.93 ± 0.15°N, 353.91 ± 0.25°E with a rotation rate of 0.524 ± 0.001°/Ma explains well almost all the GNSS station velocities relative to the ITRF14 reference frame, confirming the large‐scale rigidity of the plate. Internal strain rates at the plate‐wide scale (∼0.4 nanostrain/yr) fall within the limits for stable plate interiors, indicating that differential motions are compensated for internally, which further supports the coherent rigid motion of the Arabian plate at present. At a smaller scale, however, we identified several areas within the plate that accommodate strain rates of up to ∼8 nanostrain/yr. Anthropogenic activity and possible subsurface magmatic activity near the western margin of the Arabian plate are likely responsible for the observed local internal deformation. Put together, our results show a remarkable level of stability for the Arabian lithosphere, which can withstand the long‐term load forces associated with active continental collision in the northeast and breakup to the southwest with minimal internal deformation.
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Seafloor Spreading
Convergent boundary
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Eurasian Plate
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Hotspot (geology)
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Many hypothesis have been suggested in Japan, related to the plate-tectonic theory ; (1) “the high pressure metamorphic belts of the Sangun and the Sambagawa were produced within the subduction zones of oceanic plates”, (2) “the so-called ophiolites in the Sangun and the Mikabu zone are of the ancient oceanic crusts”, (3) “the Shimanto terrain was ancient trench area”, (4) “Hokkaido, which has its backbone range with a high pressure metamorphic belt on the continental side and a high temperature metamorphic belt on the oceanic side at present, has been rotated in a 180-degree”, and others. These hypothesis are not well fitted to the geology of Japan. In Honshu, there are no great fault which suggests the ancient boundary between the oceanic and continental plates, and no strata which have the characters of an ancient oceanic crust itself or of those made in the subduction zone of an oceanic plate.However, geological developments of Japan, in the Sambosan, the Shimanto and later stages, can well be explained by the movemnents of ancient oceanic plates, their positions and the changes of the positions, although many earth movements were produced by up-and-down movements probably related to granite intrusions.
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Convergent boundary
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<p>The formation of a global network of plate boundaries surrounding a mosaic of lithospheric fragments was a key step in the emergence of Earth&#8217;s plate tectonics. So far, propositions for plate boundary formation are regional in nature but how plate boundaries are being created over 1000s of km in short periods of geological time remains elusive. Here, we show from geological observations that a >12,000 km long plate boundary formed between the Indian and African plates around 105 Ma with subduction segments from the eastern Mediterranean region to a newly established India-Africa rotation pole in the west-Indian ocean where it transitioned into a ridge between India and Madagascar. We find no plate tectonics-related potential triggers of this plate rotation and identify coeval mantle plume rise below Madagascar-India as the only viable driver. For this, we provide a proof of concept by torque balance modeling revealing that the Indian and African cratonic keels were important in determining plate rotation and subduction initiation in response to the spreading plume head. Our results show that plumes may provide a non-plate-tectonic mechanism for large plate rotation initiating divergent and convergent plate boundaries far away from the plume head that may even be an underlying cause of the emergence of modern plate tectonics.</p>
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Seafloor Spreading
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The seismicity, structure and tectonics of the Indian/Pacific plate boundary in the North Island of New Zealand have been studied by means of a microearthquake traverse oriented in the direction of dip of the subducted Pacific plate and extending for about 210 km. The geometry of the top of the Pacific plate is inferred from a band of concentrated microearthquake activity approximately 10 km thick which is identified with the crust of the plate. The Pacific plate has two knee-like bends, one where the top of the plate is about 25 km deep, the other below the volcanic front, where the plate is about 70 km deep. The shallower bend and subsequent restraightening of the plate are related to phase changes in the plate, the deeper bend to volcanism. Composite focal mechanisms indicate that seaward of the shallower bend the Pacific plate is being loaded by the Indian plate, whereas landward of this bend the Pacific plate is sinking under its own weight.
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Composite plate
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