Consuming Plate Boundaries Research Articles

Opening of Japan Sea and Major Tectonic Lines of Japan

Median Tectonic Line (MTL) and Fossa Magna (Itoigawa-Shizuoka Tectonic Line) had long been considered to be the most critical fault boundaries controlling development of the Japanese Islands since Naumann (1885) and Kobayashi (1941). After the appearance of plate tectonics, several new interpretations emerged, e.g., sub-surface Benioff plane for the MTL. In this paper, we propose that those tectonic lines, major faults, and Tanakura Tectonic Line (TTL) were formed through a process at micro-plate boundaries during the opening of the Japan Sea in the Miocene. MTL could have been formed along the consuming boundary between the PHS plate and Japan Sea microplate, which has shifted southward to the Nankai trough, accompanying large-scale tectonic erosion. Fossa Magna was formed as a gigantic transform fault with a transtension component in the Medial-Japan Sea when opening was initiated. The eastern and western boundaries of the Japan Sea must be a strike-slip fault, corresponding to TTL to the east, and a newly proposed strike-slip fault called the West Kyushu Tectonic Line, respectively. Fossa Magna, a medial region defined by two NS-trending Miocene parallel faults in central Honshu, defined by Nauman (1885) could be interpreted to be the largest transform fault in the Medial-Japan Sea to offset the spreading axis when the Japan Sea opened. It should be emphasized that large-scale tectonic erosion occurred in front of consuming plate boundaries facing the PHS and PAC plates oceanward during the opening of the Japan Sea. The volume of tectonic erosion is calculated to be 17,581,500 km3, which is equivalent to 2/3 of the present-day Japan arc crust, which is sufficient to reach the depth of the megalith between the upper and lower mantle boundary, even with 10 km thickness of materials eroded and transported along the Benioff zone. Although MTL, Fossa Magna, and TTL are remarkable in the geology of Japan, these young faults never affected the orogeneses of Japan back to 520 Ma, which grew the continental crust of Japan. We propose that microplate boundary processes decreased the volume of the Japan crust.

CO 2 windows from mantle to atmosphere: Models on ultrahigh-temperature metamorphism and speculations on the link with melting of snowball Earth

We attempt here to correlate the melting phase of major snowball Earth events in the planet with the processes associated with extreme crustal metamorphism and formation of ultrahigh-temperature (UHT) granulite facies rocks. While the dry mineral assemblages that characterize UHT granulites can result from different mechanisms, the direct evidence for the involvement of CO 2-rich fluids in generating diagnostic UHT assemblages has been recorded from the common occurrence of pure CO 2 fluid inclusions in several terranes. Here we evaluate the tectonic settings under which UHT rocks are generated using modern analogues and show that divergent tectonics—both post-collisional extension and rifting—play a crucial role. In an attempt to speculate the link among CO 2 liberation from the carbonated tectosphere, UHT metamorphism and major earth processes, we address some of the important issues such as: (a) how the subcontinental mantle i.e., the tectosphere, had become carbonated; (b) how and when the tectosphere degassed; and (c) what is the difference between Proterozoic orogens and those of the present day. The fate of the Earth as a habitable planet was possibly dictated by a reversal of the fundamental process of formation of oceans through the selective removal of CO 2 into mantle in the Hadean times, carbonation of the Archean mantle wedge, and subsequent decarbonation of the carbonated mantle through divergent metamorphism and water infiltration since the Late Proterozoic. The abundant CO 2 liberated by subsolidus decarbonation along consuming plate boundaries was probably one of the factors that contributed to the greenhouse effect thereby triggering the deglaciation of snowball Earth. Based on an evaluation of the distribution of carbonated subcontinental mantle in global reconstructions of the Proterozoic supercontinent assembly, and their link with crustal domains that have undergone CO 2-aided dry metamorphism at extreme conditions, we speculate that the UHT rocks might represent windows for the transfer of CO 2 from the mantle into the mid crust and ultimately to the atmosphere.

From Snowball to Phaneorozic Earth

The so-called Late Proterozoic snowball Earth disappeared at the beginning of Phanerozoic time: the surface of the Phanerozoic Earth was enveloped by an ocean and oxygen-rich atmosphere, and the planet had a warm climate. We propose that this dramatic change resulted from the return flow of seawater into the mantle beginning at 750 Ma, as a consequence of the cooling of the Earth and start of the main glacial epoch. Phase relations for minerals in the system MORB + H2O and peridotite + H2O suggest that old, cold subducting oceanic slabs would transport surface water through ultrahigh-P hydrous silicates to the mantle transition zone at depths of 410-660 km. This zone could store abundant H2O, inasmuch as hydrous β and γ phases of the olivine composition may contain up to 2 to 3 wt% water. Geotherms computed for rocks of ancient subduction zones from regional metamorphic belts around the world record such a warm-to-cold transition in and below the lithosphere at 750 Ma. The runaway cooling of the Earth's interior along consuming plate boundaries started at 750 Ma by propagation of a hydration front into the mantle from 70 km depth to 410-660 km through a narrow channel above the Wadati-Benioff plane, or directly by the descent of hydrated slab peridotites. Resultant hydration of the mantle wedge and adjacent regions caused a rapid sea-level drop during the interval of 750-600 Ma, and large landmasses appeared. This change was due to the increase of water content from 0.5 for amphibole peridotite to 6.5 wt% for antigorite peridotite in the wedge mantle, and to the popcorn effect to push up continental margins. The total seawater lost since then is estimated to have lowered sea level about 600 m. Introduction of seawater into the mantle drastically lowered the melting temperature and viscosity of mantle materials, both of which reactivated the plate and plume tectonic processes. Surface volcanism released mantle CO2, causing the greenhouse effect to melt global glaciers of the snowball Earth since about 750 Ma. The emergence of huge continental landmasses diversified surface environments; erosion and deposition of sediments became significant as in the present-day Earth. Extensive formation of sedimentary basins and accretionary complexes along consuming plate boundaries drastically increased free oxygen in the atmosphere by the burial of organic matter in those sediments, which was critical to the evolution of large multicellular organisms.

The emplacement of ophiolites by collision

Ophiolites, recognized in most of the world's orogenic belts, are generally interpreted to be oceanic crust and upper mantle (lithosphere) fragments that have been incorporated into continental margins at consuming plate boundaries. We suggest that the mechanism for ophiolite emplacement is the same in both the Alpine and Andean‐type orogenes. In both geological settings, obduction of oceanic lithosphere onto the continental lithosphere is caused by the convergence of light, buoyant bodies such as oceanic plateaus, continental slivers, island arcs, or old hot spot traces. For example, the Troodos ophiolite complex, previously interpreted by some workers as resulting from continental collision, may have been emplaced by the collision of Cyprus with the Eratosthenes Plateau embedded in the oceanic eastern Mediterranean crust. On the other hand, the Upper Jurassic Coast Range Ophiolites of California, previously interpreted as resulting from typical oceanic subduction, may be the result of a continuous injection of thick nonsubductable packages of light, continentally derived sedimentary rocks, seamounts, and plateaus into the subduction zones. Many other ophiolite complexes may be similarly related to accreted terranes.

On the geometry of convergent plate boundaries

Previous studies of plate dynamics suggest that, at consuming plate boundaries, driving forces (negative buoyancy) acting on the slab are counterbalanced by viscous forces, proportional to the consumption velocity and resisting the downgoing motion into the mantle. New observations on the geometry of subduction zones may help test this equilibrium theory. A condition for local equilibrium of the driving and resistive forces is that the consumption velocity ( V ⊥ ) remains constant along a given trench. From this condition, a geometrical property of the shape of trenches and island arcs on the sphere is inferred: the consuming margin of two plates should follow segments of helices about the axis of rotation of their relative motion. A comparison of the shape of island arcs and trenches with helices supports this inference.

A mechanical model for plate deformation associated with aseismic ridge subduction in the new hebrides arc

Tectonic features associated with a subducting fracture zone-aseismic ridge system in the New Hebrides island arc are investigated. Several notable features including a discontinuity of the trench, peculiar locations of two major islands (Santo and Malekula), regional uplift, and the formation of a basin are interpreted as a result of the subduction of a buoyant ridge system. The islands of Santo and Malekula are probably formed from an uplifted mid-slope basement high while the interarc basin of this particular arc is probably a subsiding basin instead of a basin formed by backarc opening. The situation can be modeled by using a thin elastic half plate overlying a quarter fluid space with a vertical upward loading applied at the plate edge. This model is consistent with topographic and geophysical data. This study suggests that subduction of aseismic ridges can have significant effects on tectonic features at consuming plate boundaries.

もぐり込むプレート境界付近の陸側プレートの変形

もぐり込むプレート境界付近の陸側プレートの変形

Submersible observations of the New England Seamounts

Plate tectonics has established the relationship of volcanism to constructional plate margins such as the Mid-Atlantic Ridge, to consuming plate boundaries such as the subduction zones comprising the Pacific “ring of fire,” and to leakage of magma along transform faults like St. Paul's Fracture Zone; however, mid-plate volcanism, which produces many oceanic islands and seamounts, is largely unexplained by plate tectonics. Studies of mid-plate volcanism have been mainly confined to oceanic islands which may be generically different from seamounts. To extend our knowledge of global tectonics and the nature of the underlying mantle source of mid-plate volcanism, we must direct our most advanced techniques to the study of seamounts and their lineaments. Herein we describe the first reported visual observations of the morphology and lithology of volcanoes comprising one such chain—the New England Seamounts.

Ophiolitic mélange and its significance in the Fleur de Lys Supergroup, northern Appalachians

Ophiolitic mélange consists of a chaotic mixture of sedimentary rocks and igneous rocks derived from the ophiolite suite of rock units. Its formation involves surficial mass wastage, gravity sliding, and tectonism at consuming plate boundaries. Most worldwide examples relate to ophiolite obduction and the destruction of stable continental margins.The Fleur de Lys Supergroup of the northern Appalachians consists of polydeformed and metamorphosed, mainly clastic sedimentary rocks that accumulated at the Hadrynian to early Paleozoic stable continental margin of eastern North America. Greenschists at or near the top of the succession in the east (Birchy Complex) contain zones of typical ophiolitic mélange. These contain large blocks of serpentinized ultramafic rocks, actinolite–fuchsite alterations of smaller ultramafic blocks, altered gabbro, a variety of clastic sedimentary blocks, and marble. All of the rocks are multideformed and metamorphosed so that the mélange was subjected to the full range of Fleur de Lys deformations.The presence of polydeformed and metamorphosed ophiolitic mélange in the Fleur de Lys Supergroup implies an early disruption or imbrication of its rock units. This disruption and mélange formation are interpreted to be related to transport of ophiolite complexes such as the Bay of Islands Complex across an initially undisturbed continental rise prism, the Fleur de Lys Supergroup. Later polyphase deformation and metamorphism of the Fleur de Lys Supergroup are possible results of continued structural submergence and telescoping of the continental margin beneath a thick cover of transported ophiolite rock units.

Displacements Across, and Strain in, Orogenic Belts: ABSTRACT

The production of accurate palinspastic maps demands knowledge of displacements across orogenic belts. Traditionally this has involved attempts to measure linear crustal strains (^egr) across orogenic belts by fabric studies, unraveling folds, unscrambling thrusted superposed facies, etc. The inherent problems of this approach (e.g., distinguishing between stratal and crustal shortening) and the consequent difficulties of making meaningful strain measurements are minor compared with the complexities imposed by relating orogenic strain to displacements across consuming plate boundaries. It is possible in a general way to convert relative plate displacements (D) and displacement rates (D) directly into gross shortening and shortening rate values across a particular Mesozoic Tertiary orogen (e.g., Mediterranean fold belt). This has little meaning, however, for ^egr and ^egr· values in orogens for the following reasons. 1. Relative plate displacement vectors change with time. 2. Most of D is not converted into orogenic crustal strain, but is lost by subduction. Only where continental collision has occurred is there a chance that ^egr and ^egr· are direct functions of D and D·. 3. ^egr and ^egr· may be related to second-order consequences of plate motion; for example, high-level spreading and gliding of marginal nappes. 4. Mechanically significant rates depend upon determining instantaneous ^egr· and this in turn depends on the width of a zone deforming homogeneously at an instant f time. Using Le Pichon's D· values across the Alpine Himalayan fold belt, ^egr· values vary from 1.27 × 10-13 (^egr concentrated in Indus suture) to 1.59 × 10-15 (^egr across 900 km wide seismic belt from the Zagros crush zone to the Caspian Sea). These rates are far slower than rates (5 × 10-2 -10-8) at which ductile strains have been achieved in laboratory experiments. Brittle and semi-brittle structural behavior is common in orogenic belts and suggests that natural instantaneous ^egr· values are much higher than those calculated from D·. This may be a function of ^egr being concentrated instantaneously in narrow fault zones or, by incremental strain propagation, cross a particular zone. Even these factors, though, do not seem to modify ^egr· enough (e.g., in a 1-cm wide thrust zone in the Himalayas where D· = 5.6 cm/yr., ^egr· = 1.8 × 10-7). Yet, brittle deformation is evidenced by shallow earthquakes suggesting either that ^egr is concentrated along hairline fractures or that large strain accumulations precede rupture. Orogenic strains, however, are small compared with displacements across orogenic belts. The displacements can only be calculated from oceanic magnetic anomaly fitting and, less accurately, from paleomagnetic data from stable forelands and cratons. End_of_Article - Last_Page 614------------