共查询到20条相似文献，搜索用时 703 毫秒
1.
Throughout the Early Tertiary the area of the Farallon oceanic plate was episodically diminished by detachment of large and small northern regions, which became independently moving plates and microplates. The nature and history of Farallon plate fragmentation has been inferred mainly from structural patterns on the western, Pacific-plate flank of the East Pacific Rise, because the fragmented eastern flank has been subducted. The final episode of plate fragmentation occurred at the beginning of the Miocene, when the Cocos plate was split off, leaving the much reduced Farallon plate to be renamed the Nazca plate, and initiating Cocos–Nazca spreading. Some Oligocene Farallon plate with rifted margins that are a direct record of this plate-splitting event has survived in the eastern tropical Pacific, most extensively off northern Peru and Ecuador. Small remnants of the conjugate northern rifted margin are exposed off Costa Rica, and perhaps south of Panama. Marine geophysical profiles (bathymetric, magnetic and seismic reflection) and multibeam sonar swaths across these rifted oceanic margins, combined with surveys of 30–20 Ma crust on the western rise-flank, indicate that (i) Localized lithospheric rupture to create a new plate boundary was preceded by plate stretching and fracturing in a belt several hundred km wide. Fissural volcanism along some of these fractures built volcanic ridges (e.g., Alvarado and Sarmiento Ridges) that are 1–2 km high and parallel to “absolute” Farallon plate motion; they closely resemble fissural ridges described from the young western flank of the present Pacific–Nazca rise. (ii) For 1–2 m.y. prior to final rupture of the Farallon plate, perhaps coinciding with the period of lithospheric stretching, the entire plate changed direction to a more easterly (“Nazca-like”) course; after the split the northern (Cocos) part reverted to a northeasterly absolute motion. (iii) The plate-splitting fracture that became the site of initial Cocos–Nazca spreading was a linear feature that, at least through the 680 km of ruptured Oligocene lithosphere known to have avoided subduction, did not follow any pre-existing feature on the Farallon plate, e.g., a “fracture zone” trail of a transform fault. (iv) The margins of surviving parts of the plate-splitting fracture have narrow shoulders raised by uplift of unloaded footwalls, and partially buried by fissural volcanism. (v) Cocos–Nazca spreading began at 23 Ma; reports of older Cocos–Nazca crust in the eastern Panama Basin were based on misidentified magnetic anomalies.There is increased evidence that the driving force for the 23 Ma fission of the Farallon plate was the divergence of slab-pull stresses at the Middle America and South America subduction zones. The timing and location of the split may have been influenced by (i) the increasingly divergent northeast slab pull at the Middle America subduction zone, which lengthened and reoriented because of motion between the North America and Caribbean plates; (ii) the slightly earlier detachment of a northern part of the plate that had been entering the California subduction zone, contributing a less divergent plate-driving stress; and (iii) weakening of older parts of the plate by the Galapagos hotspot, which had come to underlie the equatorial region, midway between the risecrest and the two subduction zones, by the Late Oligocene. 相似文献
2.
The Late Tertiary shallow subduction of the Cocos ridge under the Caribbean plate controlled the evolution of the Cordillera de Talamanca in southeast Costa Rica, which is a mountain range that consists mainly of granitoids formed in a volcanic arc setting. Fission track thermochronology using zircon and apatite, as well as 40Ar–39Ar and Rb–Sr age data of amphibole and biotite in granitoid rocks constrain the thermal history of the Cordillera de Talamanca and the age of onset of subduction of the Cocos ridge. Shallow intrusion of granitoid melts resulted in fast and isobaric cooling. A weighted mean zircon fission track age (13 analyses) and Rb–Sr biotite ages of about 10 Ma suggest rapid cooling and give minimum ages for granitoid emplacement. In some cases 40Ar–39Ar and Rb–Sr apparent ages of amphibole and biotite are younger than the zircon fission track ages, which can be attributed to partial resetting by hydrothermal alteration. Apatite fission track ages range from 4.8 to 1.7 Ma but show no correlation with the 3090-m elevation span over which they were sampled. The apatite ages seem to indicate rapid exhumation caused by tectonic and isostatic processes. The combination of the apatite fission track ages with subduction parameters of the Cocos plate such as subduction angle, plate convergence rate and distance of the Cordillera de Talamanca to the trench implies that the Cocos ridge entered the Middle America Trench between 5.5 and 3.5 Ma. 相似文献
3.
Ten new focal mechanisms are derived for earthquakes in southern Central America and its adjacent regions. These are combined with a study of seismicity and data of previous workers to delineate the position and nature of the plate boundaries in this complex region.The Middle America subduction zone may be divided into four or five distinct seismic segments. The plate boundary between North America and the Caribbean near the trench might be located more towards the south than previously suspected. Subduction has basically stopped south of the underthrusting Cocos Ridge. There is not much evidence for a seismically active strike-slip fault south of Panama, but its existence cannot be ruled out. More activity reveals the zone north of Panama which is identified as a subduction zone with normal fault events. Shallow seismicity induced by the interaction of the Nazca plate extends from the Colombia-Panama border south along the Pacific coast to meet a high-angle continental thrust fault system. Subduction with a pronounced slab starts only south of that point near a hot region which offsets the seismic trend at the trench. The Carnegie Ridge and/or the change of direction of subduction in Ecuador produce a highly active zone of seismicity mainly at the depth of 200 km. The area in the Pacific displays a termination of activity at a propagating rift west of the Galapagos Islands. The main eastern boundary of the Cocos plate, the Panama Fracture Zone, is offset towards the west at the southern end of the Malpelo Ridge. Its northern end consists of two active branches as defined by large earthquakes. A strike-slip mechanism near the southeastern flank of the Cocos Ridge was previously believed to be the site of an extended fracture zone. This paper proposes submarine volcanic activity as an alternative explanation. 相似文献
4.
Using recently acquired marine magnetic data and existing magnetic and bathymetric data sets together with ODP Leg 170 age determinations we present a revised plate tectonic model for the southern Cocos and northern Nazca plate area. According to this model the formation of the southern Cocos plate was governed by spreading at different ridge axes with alternations between spreading ridges producing a complex magnetic anomaly pattern. In the Cocos and Malpelo ridge area we have identified two precursors of the recently active Cocos–Nacza spreading system which were active from 22.8 to 14.7 Ma, with a change in spreading direction from NW–SE to ENE–WSW at 19.5 Ma. The oceanic crust of these abandoned spreading systems was subsequently thickened and overprinted by hotspot volcanism that formed the Cocos and Malpelo ridges. The centre of this hotspot volcanism is about 500 km away from, but most probably related to, the Galapagos hotspot. 相似文献
5.
The “subduction initiation rule” (SIR) (Whattam and Stern, 2011) advocates that proto-arc and forearc complexes preserved in ophiolites and forearcs follow a predictable chemotemporal and/or chemostratigraphic vertical progression. This chemotemporal evolution is defined by a progression from bottom to top, from less to more depleted and slab-metasomatized sources. This progression has been recently documented for other igneous suites associated with subduction initiation. The Sona-Azuero forearc complex of southern Panama represents the earliest magmatic arc activity at the Central American Volcanic Arc system. Comparison of new and existing geochemical data for the circa 82-40 Ma Sona-Azuero Proto-Arc/Arc, its underlying 89-85 Ma “oceanic plateau” of SW Panama and the 72-69 Ma Golfito Proto-Arc of southern Costa Rica with the 70-39 Ma Chagres-Bayano Arc of eastern Panama exhibits a chemotemporal progression as described above and which follows the SIR. Sona-Azuero lavas are predominantly MORB-like, whereas those of the younger Chagres-Bayano complex are mostly VAB-like; lavas of the Golfito Proto-Arc typically show characteristics intermediate to that of the Sona-Azuero and Chagres-Bayano proto-arc/arc complexes. On the basis of isotope evidence as shown in other studies, lava types of all three complexes are clearly derived from a source contaminated by the Caribbean Large Igneous Province plume; we term these “plume-contaminated” forearc basalts and volcanic arc basalts, respectively. Apart from a plume-induced subduction initiation origin for the Panamanian forearc, these insights suggest otherwise similar petrogenetic origins and tectonic setting to lavas comprising earliest-formed forearc crust, and most ophiolites, which follow the SIR. 相似文献
6.
The convergent plate margin off the Osa peninsula in southern Costa Rica is characterized by the indentation of the Cocos ridge at 4–5 Ma. The indentation causes the uplift of the Osa mélange which we interpret to represent an exhumed major channel for the transport of tectonically eroded material down into the subduction zone. We present evidence that, similar to the Nicoya segment of the Costa Rica convergent margin, subduction erosion rather than accretion has been the dominant process along the plate boundary. The composition of the Osa mélange is dominated by tectonized material of the upper-plate Nicoya ophiolite complex (basalt, radiolarite, limestone). Strong deformation is concentrated in numerous discrete shear zones and produced the layered fabric of large rock volumes, which partly experienced temperatures > 200°C. We thus interpret the Osa mélange to be a product of subduction erosion at the base of the outer arc wedge structure. 相似文献
7.
Volcanic gaps due to oblique consumption of aseismic ridges 总被引：1，自引：0，他引：1
The present-day consumption of oceanic ridges and other buoyant rises and fragments at circum-Pacific subduction zones, and presumably elsewhere, are closely related to existing gaps in volcanism. Examples are the gaps associated with the Nazca, Juan Fernandez, Cocos, Marcus-Necker and Louisville ridges. The buoyancy of these ridges breaks the continuity of the subducted plate, which may lead to reduced water supply required for melting of magma, and therefore create temporary volcanic gaps. The oblique consumption of these ridges causes the gap to migrate with time. This mechanism may be useful in interpreting time-space patterns of past volcanic chains associated with subduction in terms of the consumption of the disruptive oceanic plateaus and ridges. 相似文献
8.
The outer arc of the costa rican orogen (oceanic basement complexes of the Nicoya and Santa Elena Peninsulas) 总被引：1，自引：0，他引：1
J. de Boer 《Tectonophysics》1979,56(3-4)
The basement of the Costa Rican outer arc consists of two major complexes. The older is composed of peridotite-serpentinite, pillow lava and radiolarite; the younger is made up of gabbro-diorite, pillow lava, pyroclastic rocks and siliceous limestone.The observational data are interpreted as follows. The older pillow lavas are believed to be oceanic crust generated along the north-south-spreading Carnegie Ridge during the late Coniacian. The younger lavas flowed from fissures along a west-northwest-trending volcanic belt (Culebra arc) which developed in this crust during early to middle Campanian time, when it collided with the Chortis block. Paleomagnetic evidence suggests that the older sequence originated on the Southern Hemisphere, and the younger in the Northern.During the Paleocene, the crust fragmented and separated into the Caribbean and Cocos plates, probably as a result of the outer arc escaping the tectonic influence of the Carnegie Ridge and entering that of the ancestral East Pacific Rise. This fragmentation resulted in the formation of two parallel volcanic belts (San Antonio and Cachimbas arcs) in the inner deep (Tempisque Valley), which remained active throughout the Eocene. It is postulated that subduction of the Cocos beneath the Caribbean plate was initiated during Oligocene time and resulted in the formation of yet another volcanic belt (Tilarán-Talamanca arc). The outer arc was uplifted, folded, and thrust south westward. The resulting pattern shows a gradual clockwise rotation west to northwest, and north-astward migration of the volcanic arcs through time. Aeromagnetic and tectonic data indicate that differential uplift and later gravitational décollement of the sedimentary rock blanket characterize the tectonic deformation of singular volcanic belts, and that tectonic overprinting is usually restricted to one major phase. 相似文献
9.
菲律宾吕宋岛上约5Ma以来的斑岩铜金矿床主要集中在北部的Baguio和Mankayan地区,它们在时空上与黄岩海山链密切相关。1907~2013年间的地震数据表明,在吕宋岛中部(16°N)附近存在地震稀疏带。吕宋岛上的斑岩铜金矿床分布在该地震稀疏带的两侧。收集到的相应时期埃达克岩的Sr/Y-(La/Yb)N、Sr/Y-Y和La/Yb-Yb图解表明,这些埃达克岩几乎都是洋壳部分熔融形成的。与吕宋岛北部侵入型埃达克岩相比,位于16°N附近的埃达克岩具有更高的Sr含量,这可能与南海古扩张脊俯冲撕裂形成的板片窗有关。斜长石是辉长岩的主要矿物之一,因此,撕裂的洋壳边缘的辉长岩层部分熔融,形成具有更高Sr含量的埃达克质岩浆。而位于吕宋岛南部Bataan弧中的埃达克质火山岩,可能是在南海古扩张脊俯冲之前形成的。根据已发表的斑岩铜金矿床数据,Mankayan地区的成矿年龄在约3.5~1.4Ma,Baguio地区的成矿年龄在约3.1~0.5Ma之间,有从北向南变年轻的趋势,这与黄岩海山链沿马尼拉海沟向南迁移一致。此外,吕宋岛北部Mt.Cagua到Baguio之间存在一个延伸了220km的第四纪火山活动的空隙,该区域大部分火山已经在中新世停止活动。这可能是黄岩海山链的俯冲使得俯冲倾角逐渐变缓、挤压加强而导致的。同时期的斑岩铜矿床正好分布在这一火山空隙中,是俯冲洋壳部分熔融的产物。 相似文献
10.
Seth Sadofsky Kaj Hoernle Svend Duggen Folkmar Hauff Reinhard Werner Dieter Garbe-Schönberg 《International Journal of Earth Sciences》2009,98(4):901-913
New geochemical data from the Cocos Plate constrain the composition of the input into the Central American subduction zone
and demonstrate the extent of influence of the Galápagos Hotspot on the Cocos Plate. Samples include sediments and basalts
from Ocean Drilling Program (ODP) Site 1256 outboard of Nicaragua, gabbroic sills from ODP Sites 1039 and 1040, tholeiitic
glasses from the Fisher Ridge off northwest Costa Rica, and basalts from the Galápagos Hotspot Track outboard of Central Costa
Rica. Site 1256 basalts range from normal to enriched MORB in incompatible elements and have Pb and Nd isotopic compositions
within the East Pacific Rise MORB field. The sediments have similar 206Pb/204Pb and only slightly more radiogenic 207Pb/204Pb and 208Pb/204Pb isotope ratios than the basalts. Altered samples from the subducting Galápagos Hotspot Track have similar Nd and Pb isotopic
compositions to fresh Galápagos samples but have significantly higher Sr isotopic composition, indicating that the subduction
input will have a distinct geochemical signature from Galápagos-type mantle material that may be present in the wedge beneath
Costa Rica. Gabbroic sills from Sites 1039 and 1040 in East Pacific Rise (EPR) crust show evidence for influence of the Galápagos
Hotspot ∼100 km beyond the morphological hotspot track. 相似文献
11.
全球火山活动分布特征 总被引：14，自引：0，他引：14
根据全球活动火山目录 ,分析研究了全球火山分布的特征 ,描述了各区的火山活动分布 ,总结了火山活动强度的时、空分布特征。全球火山活动可分为三大区 ,西太平洋火山活动区 ,主要与太平洋板块向北西西方向的俯冲活动有关 ;东太平洋火山活动区 ,主要与太平洋东面的小板块 (胡安德富卡板块、科科斯、纳斯卡板块 )向美洲板块的俯冲有关 ;大西洋火山活动区 ,与大西洋和非洲的裂开 ,以及地中海带的活动有关。不同火山区带具有各自的最大喷发等级与相应的复发周期。一条火山弧上活动强度的分布往往是不对称的 ,意味着火山弧在整体上有其动力学的控制机理。火山活动显示了随纬度成带状分布。在 - 10～ 0° ,10 2 0° ,30 4 0°,5 0 6 0°分布有高值带。火山喷发活动还与当地的重力势有关 ,重力势正异常可能与高的正压力有关 ,有利于产生特大喷发。火山活动与大角度的正面俯冲带的弧后火山活动最强 ,当板块运动方向与板块边缘走向成小角度相交时 ,缺少正面俯冲的动力 ,火山活动相对平静。 相似文献
12.
Yasushi Watanabe 《Mineralium Deposita》2005,40(3):307-323
Kyushu Island, Japan, is located at the junction of the Southwest Japan arc and the Ryukyu arc. There are two major late Cenozoic
epithermal gold-silver provinces in Kyushu, which are termed the Northern and Southern provinces. The provinces are characterized
by: 1) Pliocene volcanism dominated by calc-alkaline andesite, followed by Quaternary volcanism including extrusion of both
calc-alkaline and tholeiitic magmas; 2) formation of extensional grabens; 3) Pliocene to Pleistocene mineralization, which
was dominated by abundant low sulfidation (LS) epithermal deposits with a few high sulfidation (HS) examples. The two epithermal
gold-silver provinces have evolved differently since about 5 Ma; the Northern province has exhibited diminished hydrothermal
activity from the Pliocene to Pleistocene, whereas the Southern province has witnessed increased hydrothermal activity mainly
in easterly and northerly directions. Changes of tectonic setting from the Pliocene to Pleistocene account for the variable
trends in epithermal gold deposit formation. Westward oblique subduction of the Philippine Sea plate beneath the Southwest
Japan arc caused development of the Hohi graben and arc-related volcanism at about 6 Ma. This was associated with widespread
LS mineralization in and surrounding the Hohi graben, as is represented by the Bajo and Taio deposits. The subduction of the
relatively buoyant Kyushu-Palau ridge during the early Pliocene strengthened the coupling between the slab and overriding
Ryukyu arc, leading to polygenetic andesite volcanism with associated HS (Kasuga, Iwato, and Akeshi) and LS (Kushikino) mineral
deposits forming in the Southern province. A change of the subduction direction of the Philippine Sea plate, from west to
north-northwest in the early Pliocene, increased the orthogonal convergence rate between the Southwest Japan arc and the Philippine
Sea plate, resulting in a decrease of volcanic and hydrothermal activity in the Hohi graben of the Northern province. The
more northerly subduction of the Philippine Sea plate shifted the locus of the Kyushu-Palau ridge subduction northward, resulting
in underplating of the older (85–60 Ma), negatively buoyant Amami basin oceanic slab in the Southern province, rather than
continued subduction of the young (27–15 Ma), buoyant Shikoku basin slab. This replacement caused steepening of the slab angle
and slab-rollback in the Southern province, which was associated with regional extension, an eastward shift of the Ryukyu
volcanic front, and development of the Kagoshima and Shimabara grabens, as well as the Okinawa trough. Rhyolite and basalt
volcanism, in addition to andesite volcanism, have occurred since 2 Ma in the area of the Ryukyu back arc; coincident LS mineralization
at Hishikari and Ohkuchi was affiliated with the rhyolite volcanism. Another change of the subduction direction of the Philippine
Sea plate to the northwest occurred at 2–1 Ma. The forearc sliver of the Southwest Japan arc shifted westward, in association
with right-lateral strike-slip faulting along the Median tectonic line, due to the increase of the westward convergence rate.
This shift resulted in shortening and cessation of graben development in the Hohi area, restricting the subsequent volcanism
and related hydrothermal activity to the central part of the graben. 相似文献
13.
Magmatism in NW Mexico records a Late Miocene transformation from convergence to extension in the Gulf of California rift system. Miocene calc-alkalic rocks in the Baja California peninsula are related to the final subduction of the Farallon plate system, but the heterogeneous nature of volcanism younger than 12.5 Ma has led to conflicting tectonic interpretations. Neogene volcanic rocks in the Sierra Santa Ursula, Sonora, were emplaced in three magma pulses, according to mapping, K–Ar geochronology, and geochemistry. From 23.5 to 15 and 14 to 11.4 Ma, calc-alkalic rocks show an arc-like signature. The 12–11 Ma calc-alkalic dacites, however, are characterized by higher K, Rb, 87Sr/86Sr, and light REE abundances than are the older rocks. The timing, petrography, and geochemistry of the 12–11 Ma rocks are interpreted to reflect postsubduction magmatism. A change in magma chemistry from predominantly calc-alkalic to tholeiitic rocks at 10.3 Ma corresponds to orthogonal extension during early Gulf of California evolution. Sr, Nd, and Pb radiogenic isotope signatures show minor changes over time. The volcanic record for 20–12.5 Ma at Sierra Santa Ursula and adjacent areas is consistent with the reconstructed history of the Guadalupe microplate. The interval of magmatism produced from 12 to 11 Ma appears to reflect changes in plate geometry during the transition from subduction to rifting. 相似文献
14.
Stephanie Ingle Paul A. Mueller Ann L. Heatherington Marianne Kozuch 《Tectonophysics》2003,371(1-4):187-211
Establishing the age and crustal nature of exotic terranes and their underlying basements helps to determine their paleogeographic origin and tectonic histories. We present U–Pb ages of zircons and Sm–Nd whole rock isotopic data for volcanic and plutonic rocks of the Carolina terrane, one of several peri-Gondwanan terranes that were accreted to the margins of the circum-Atlantic continents during the Paleozoic. Volcanism in this subduction-related arc culminated in the eruption of the Morrow Mountain rhyolite, at ca. 540 Ma; thus, magmatism in the Carolina terrane ceased at the beginning of the Cambrian. The presence of inherited zircons and non-juvenile depleted mantle model ages of Carolina slate belt rocks favor a basement that is, at least in part, composed of evolved continental crust. Ages of inherited xenocrystic zircons cluster at ca. 1000, 2100 and 2500 Ma. These ages, in addition to volcanism at ca. 618–540 Ma, correlate best with well-known tectonic events in present-day northern South America. Specifically, the Orinoquian-Sunsas, the Trans-Amazonian and the Central Amazonian orogenic zones are likely candidates for potential basement correlatives to the Carolina terrane. Sm–Nd isotopic signatures vary significantly, but permit assimilation of Orinoquian age (1000 Ma) crust by magmas derived from the depleted mantle in a subduction (arc-related) setting. Our findings are also consistent with proposed correlations between the Carolina terrane and Avalonia which is likewise believed to have formed along the northern margin of present-day South America. 相似文献
15.
Early Cretaceous rhyolitic tuffs, widely distributed on Port Island, provide insights into the volcanism and tectonic setting of Hong Kong. In this paper we present petrological, geochronological and geochemical data of the rhyolitic tuff to constrain the diagenesis age and petrogenesis of the rocks, tectonic setting and early Cretaceous volcanism of Hong Kong. The first geochronological data show that the zircons in the volcanic rocks have U-Pb age of 141.1–139.5 Ma, which reveals that the rhyolitic tuff on Port Island was formed in the early Cretaceous (K1). Geochemically, these acid rocks, which are enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depleted in high field strength elements (HFSEs), belong to the high K calc-alkaline to shoshonite series with strongly-peraluminous characteristic. The geochemical analyses suggest that the volcanic rocks were derived from deep melting in the continental crust caused by basaltic magma underplating. Based on the geochemical analysis and previous studies, we concluded that the rhyolitic tuffs on Port Island were formed in a back-arc extension setting in response to the subduction of the Paleo-Pacific Plate beneath the Eurasian Plate. 相似文献
16.
New radiometric dating of the dykes from the Hurd Peninsula, Livingston Island, South Shetland Islands 总被引：1，自引：0，他引：1
Fifteen new K–Ar ages in the range of 79–31 Ma are partially confirmed by three 40Ar/39Ar plateaus and isochron data of 64.9±0.4, 55.5±0.1 and 52.8±0.6 Ma. The new geochronological data reveal a much more detailed picture of the subduction imprint in the Hurd Peninsula. Using cutting relationships, the dyke emplacement history is divided into four episodes. The Late Cretaceous–Paleocene dykes in the range of 80–60 Ma are related to the main magmatism in Livingston Island and most likely reflect the final stages of subduction of the proto-Pacific oceanic crust. The Early Eocene dykes (56–52 Ma) fill the gap in volcanic activity 70–50 Ma ago. They are the only magmatic event manifested at this time in the region. The 45–42 Ma dykes may be related to the intrusion of the Barnard Point tonalite. Three samples of Oligocene age appear to represent the last igneous activities on the Hurd Peninsula prior to the opening of the Bransfield Strait. 相似文献
17.
F. L. Sutherland I. T. Graham S. Meffre H. Zwingmann R. E. Pogson 《Australian Journal of Earth Sciences》2013,60(7):983-1005
Prolonged intraplate volcanism along the 4000 km-long East Australian margin for ca 100 Ma raises many genetic questions. Studies of the age-progressive pulses embedded in general basaltic activity have spawned a host of models. Zircon U–Pb dating of inland Queensland central volcanoes gives a stronger database to consider the structure and origin of Australian age-progressive volcanic chains. This assists appraisal of this volcanism in relation to plate motion and plate margin tectonic models. Inland Queensland central volcanoes progressed south-southeast from 34 to 31 Ma (～5.4 cm/yr) until a surge in activity led to irregular southerly progression 31 to 28 Ma. A new inland southeastern Queensland central volcano line (25 to 22 Ma), from Bunya Mountains to North Main Range, followed 3 Ma behind the adjacent coastal progression. The Australian and Tasman Sea age-progressive chains are compared against recent plate motion modelling (Indian Ocean hotspots). The chain lines differ from general vector traces owing to west-facing swells and cessations in activity. Tectonic processes on the eastern plate margin may regulate these irregularities. These include subduction, rapid roll-back and progressive detachment of the Loyalty slab (43 to 15 Ma). West-flowing Pacific-type asthenosphere, related to perturbed mantle convection, may explain the west-facing volcanic surges. Such westward Pacific flow for over 28 Ma is known at the Australian–Antarctic Discordance, southeast of the present Australian plume sites under Bass Strait–West Tasman Sea. Most basaltic activity along eastern Australia marks asthenospheric melt injections into Tasman rift zone mantle and not lithospheric plate speed. The young (post-10 Ma) fields (Queensland, Victoria–South Australia) reflect new plate couplings, which altered mantle convection and stress regimes. These areas receive asthenospheric inputs from deep thermal zones off northeast Queensland and under Bass Strait. 相似文献
18.
《International Geology Review》2012,54(5):437-447
Subduction-zone magmatism became extensive along the west coast of South America during the Ordovician, soon after Gondwana was assembled. During the remainder of the Paleozoic and the early Mesozoic, eastward subduction of the Farallon plate led to emplacement of a succession of granitic and volcanic rocks. During the Cretaceous, when South America broke away from Africa and began moving independently toward the Pacific Basin, the resulting opposite motions of the South American and Farallon plates toward the subduction zone caused vigorous tectonic mountain building. But by the Oligocene, South America had advanced more than 2000 km beyond the position of the Cretaceous subduction zone's root in the lower mantle. The South American plate, moving westward over the subducting plate, pushed down and flattened the curved top of the subducting slab, as indicated by today's flattened earthquake zone under South America. I hypothesize that this flattening increased the subducting slab's resistance with the underlying lower mantle. Crustal deformation slowed, and the mountains built during the Cretaceous and later were eroded to a peneplane. During the Oligocene, about 25 Ma, the Farallon plate broke into the Cocos and Nazca plates, and I suggest that along the west coast of South America a shear at a slope of about 30° cut through the subducting slab. The oceanic (Nazca) part of the slab then entered the lower mantle below the Andes with a steeper dip than before. As the newly sheared obtuse upper corner of the Nazca plate pushed eastward and downward, it buckled the rigid edge of the continent and began the folding and thrusting of the Andean (Quechua) orogeny. The orogeny continues, but earthquake foci indicate that as South America continues to move westward, the subduction zone once again is flattening; in the future we can expect the Nazca slab to shear once more and its new wedge-shaped end to enter the lower mantle again. 相似文献
19.
《International Geology Review》2012,54(12):1116-1132
The Mexican Volcanic Belt (MVB) is a major linear belt of Miocene to present-day volcanism in southern Mexico. Its origin has been controversial, although the majority opinion views it as a volcanic arc related to the subduction of the Cocos plate under the North American plate. Both calc-alkaline and alkaline volcanism characterize the belt; the latter has been previously cited as indicative of the role of a mantle plume. Here we present objections to these explanations, and conclude on the basis of geological, geochemical, and geophysical data that the MVB is unrelated to subduction or to a mantle plume, and is instead a rift-like structure experiencing active extension. Calc-alkaline or alkaline geochemistry of magmas is not useful for inferring tectonic setting, but reflects source parameters and petrogenetic processes. For the MVB, calc-alkaline geochemistry suggests crustal contamination, and the OIB-like geochemistry suggests an enriched mantle source. Our proposal of a heterogeneous mantle beneath the MVB comprising “normal” mantle and metasomatic, enriched veins, can explain the close association in space and time of calc-alkaline and alkaline volcanism throughout the belt. 相似文献
20.
《International Geology Review》2012,54(2):131-151
Geologic mapping and new K-Ar and 40Ar/39Ar geochronology of the southeastern Sierra Madre Occidental (SMO), at its intersection with the northern margin of the Mexican Volcanic Belt (MVB), indicate the occurrence of three volcanic groups. The oldest group corresponds to the SMO, and includes 29 to 22 Ma voluminous ignimbrites and 30 Ma andesites and rhyolites. The youngest group includes widespread basaltic-andesitic lava plateaus that yielded ages from 14.6 to 8.8 Ma and are interpreted as the beginning of the MVB. From 22 to 14.6 Ma, volcanic activity in the area was significantly reduced, but did not cease entirely. We refer to the third group as transitional volcanism, which is dominated by andesitic and rhyolitic lava domes but also includes high-grade andesitic ignimbrites. We conclude that the change from volcanism proper of the SMO to that of the MVB was gradual with respect to age and drastic with respect to composition and style, from a voluminous-silicic-ignimbrite domain to a widespread basaltic-andesitic-lava plateau domain. This change may have been related to major plate tectonic reorganizations within the interval from 25 to 12 Ma that involved the waning of subduction of the Farallon plate west of northern Mexico and the associated southward migration of the triple junction of the Pacific-Farallon-North America plates, the subsequent break-up of the Farrallon plate into the Guadalupe and Cocos plates, and the counterclockwise and clockwise rapid rotation of the ridge between them around 16 to 12.5 Ma. 相似文献