Middle Miocene to present plate tectonic history of the southern Central American Volcanic Arc

New mid Miocene to present plate tectonic reconstructions of the southern Central American Volcanic Arc (CAVA) reveal that the inception of Cocos Ridge subduction began no earlier than 3 Ma, and possibly as late as 2 Ma. The Cocos Ridge has been displaced from the Malpelo Ridge to the southeast since 9 Ma along the Panama Fracture Zone (PFZ) system. Ambiguous PFZ and Coiba Fracture Zone (CFZ) interaction since 9 Ma precludes conclusively establishing the age of initial Cocos Ridge subduction. Detailed reconstructions based on magnetic anomalies offshore reveal several other variations in subduction parameters beneath southern Central America that preceded subduction of the Cocos Ridge, including southeastward migration of the Nazca–Cocos–Caribbean triple junction along the Middle America Trench (MAT) from 12 Ma to present, and subduction of ≤2 km high scarps both parallel and perpendicular to the trench from 6 to 1 Ma.The timing of changes in subduction processes has commonly been determined by (and correlated with) geologic changes in the upper plate. However, reliable ⁴⁰Ar/³⁹Ar dating of these events has become available only recently [Abstr. Programs-Geol. Soc. Am. (2002)]. These new dates better constrain the magmatic and structural history of southern Costa Rica. Observations from this data set include: a gap in the volcanic record from 11 to 6 Ma, which coincides temporally with emplacement of most plutons in southern Costa Rica, normal arc volcanism ceased after 3.5 Ma in southern Costa Rica, and Pliocene (mostly 1.5 Ma) adakite volcanism was widely distributed from central Panama to southern Costa Rica (though volumetrically insignificant).This new data reveals that many geologic phenomena, commonly attributed to subduction and underplating of the buoyant Cocos Ridge, in fact precede inception of Cocos Ridge subduction and seem to correlate more favorably in time with earlier tectonic events. Adakite volcanic activity corresponds in space and time with the subduction of a large scarp associated with a tectonic boundary off southern Panama. Regional unconformities and an 11–6 Ma gap in arc volcanism match temporally with oblique subduction of the Nazca plate beneath central and southern Costa Rica. Cessation of volcanic activity, low-temperature cooling of plutons in the Cordillera de Talamanca (CT), and rapid increases in sedimentation in the fore-arc and back-arc basins coincide with passage of the Nazca–Cocos–Caribbean triple junction and initiation of subduction of “rough” crust associated with Cocos–Nazca rifting 3.5 Ma, closely followed by initial subduction of the Cocos Ridge 2–3 Ma. None of the aforementioned geologic events occurred at a time that would allow for underplating by the Cocos Ridge. Rather they are probably related to complex interactions with subduction of complicated plates offshore. All of the aforementioned events indicate that the southern Central American subduction system has been in flux since at least 12 Ma.     

Seismicity of the Hellenic subduction zone in the area of western and central Crete observed by temporary local seismic networks

Temporary local seismic networks were installed in western Crete, in central Crete, and on the island Gavdos south of western Crete, respectively, in order to image shallow seismically active zones of the Hellenic subduction zone.More than 4000 events in the magnitude range between −0.5 and 4.8 were detected and localized. The resulting three-dimensional hypocenter distribution allows the localization of seismically active zones in the area of western and central Crete from the Mediterranean Ridge to the Cretan Sea. Furthermore, a three-dimensional structural model of the studied region was compiled based on results of wide-angle seismics, surface wave analysis and receiver function studies. The comparison of the hypocenter distribution and the structure has allowed intraplate and interplate seismicity to be distinguished.High interplate seismicity along the interface between the subducting African lithosphere and the Aegean lithosphere was found south of western Crete where the interface is located at about 20 to 40 km depth. An offset between the southern border of the Aegean lithosphere and the southern border of active interplate seismicity is observed. In the area of Crete, the offset varies laterally along the Hellenic arc between about 50 and 70 km.A southwards dipping zone of high seismicity within the Aegean lithosphere is found south of central Crete in the region of the Ptolemy trench. It reaches from the interface between the plates at about 30 km depth towards the surface. In comparison, the Aegean lithosphere south of western Crete is seismically much less active including the region of the Ionian trench. Intraplate seismicity within the Aegean plate beneath Crete and north of Crete is confined to the upper about 20 km. Between 20 and 40 km depth beneath Crete, the Aegean lithosphere appears to be seismically inactive. In western Crete, the southern and western borders of this aseismic zone correlate strongly with the coastline of Crete.     

Double-sided subduction with contrasting polarities beneath the Pamir-Hindu Kush: Evidence from focal mechanism solutions and stress field inversion

The Pamir-Hindu Kush region at the western end of the Himalayan-Tibet orogen is one of the most active regions on the globe with strong seismicity and deformation and provides a window to evaluate continental collision linked to two intra-continental subduction zones with different polarities. The seismicity and seismic tomography data show a steep northward subducting slab beneath the Hindu Kush and southward subducting slab under the Pamir. Here, we collect seismic catalogue with 3988 earthquake events to compute seismicity images and waveform data from 926 earthquake events to invert focal mechanism solutions and stress field with a view to characterize the subducting slabs under the Pamir-Hindu Kush region. Our results define two distinct seismic zones: a steep one beneath the Hindu Kush and a broad one beneath the Pamir. Deep and intermediate-depth earthquakes are mainly distributed in the Hindu Kush region which is controlled by thrust faulting, whereas the Pamir is dominated by strike-slip stress regime with shallow and intermediate-depth earthquakes. The area where the maximum principal stress axis is vertical in the southern Pamir corresponds to the location of a high-conductivity low-velocity region that contributes to the seismogenic processes in this region. We interpret the two distinct seismic zones to represent a double-sided subduction system where the Hindu Kush zone represents the northward subduction of the Indian plate, and the Pamir zone shows southward subduction of the Eurasian plate. A transition fault is inferred in the region between the Hindu Kush and the Pamir which regulates the opposing directions of motion of the Indian and Eurasian plates.     

Structural characteristics off Miyagi forearc region, the Japan Trench seismogenic zone, deduced from a wide-angle reflection and refraction study

The Japan Trench subduction zone, located east of NE Japan, has regional variation in seismicity. Many large earthquakes occurred in the northern part of Japan Trench, but few in the southern part. Off Miyagi region is in the middle of the Japan Trench, where the large earthquakes (M > 7) with thrust mechanisms have occurred at an interval of about 40 years in two parts: inner trench slope and near land. A seismic experiment using 36 ocean bottom seismographs (OBS) and a 12,000 cu. in. airgun array was conducted to determine a detailed, 2D velocity structure in the forearc region off Miyagi. The depth to the Moho is 21 km, at 115 km from the trench axis, and becomes progressively deeper landward. The P-wave velocity of the mantle wedge is 7.9–8.1 km/s, which is typical velocity for uppermost mantle without large serpentinization. The dip angle of oceanic crust is increased from 5–6° near the trench axis to 23° 150 km landward from the trench axis. The P-wave velocity of the oceanic uppermost mantle is as small as 7.7 km/s. This low-velocity oceanic mantle seems to be caused by not a lateral anisotropy but some subduction process. By comparison with the seismicity off Miyagi, the subduction zone can be divided into four parts: 1) Seaward of the trench axis, the seismicity is low and normal fault-type earthquakes occur associated with the destruction of oceanic lithosphere. 2) Beneath the deformed zone landward of the trench axis, the plate boundary is characterized as a stable sliding fault plain. In case of earthquakes, this zone may be tsunamigenic. 3) Below forearc crust where P-wave velocity is almost 6 km/s and larger: this zone is the seismogenic zone below inner trench slope, which is a plate boundary between the forearc and oceanic crusts. 4) Below mantle wedge: the rupture zones of thrust large earthquakes near land (e.g. 1978 off Miyagi earthquake) are located beneath the mantle wedge. The depth of the rupture zones is 30–50 km below sea level. From the comparison, the rupture zones of large earthquakes off Miyagi are limited in two parts: plate boundary between the forearc and oceanic crusts and below mantle wedge. This limitation is a rare case for subduction zone. Although the seismogenic process beneath the mantle wedge is not fully clarified, our observation suggests the two possibilities: earthquake generation at the plate boundary overridden by the mantle wedge without serpentinization or that in the subducting slab.     

郯庐断裂带渤南段构造特征及其控盆作用

渤南地区郯庐断裂带具有很好的油气勘探前景,但由于其构造特征复杂,目前对渤南地区油气成藏条件、主控因素及富集规律的认识尚不明晰.通过对三维地震和地质资料的分析解释,结合前人研究成果,探讨了渤南地区郯庐断裂带构造特征的时空差异及其对盆地结构的控制作用.研究表明,渤南地区郯庐断裂带具有3组分支断裂,每组分支断裂由2~4条断裂构成,均表现出了明显的走滑特征,整体由东向西、由深至浅走滑程度逐渐减弱.新生代古新世-早始新世郯庐断裂带渤南段左旋走滑,东部分带活动明显、强度大,中带和西带不活动或活动较弱,渤南地区中生代发育的NWW向伸展断裂系统复活,形成北断南超的复式半地堑或南北双断式结构;中始新世以来,渤南地区郯庐断裂带转为右旋走滑,3组分支断裂均开始活动,表现为强烈的走滑兼伸展运动,强度由东向西逐渐减弱,中带分支断裂形成的中央构造脊将黄河口凹陷分割成东、西两个次洼,并开始逐渐发育一系列次级断层,与主断裂构成帚状断裂组合;新近纪-第四纪郯庐断裂渤南段表现为右旋走滑兼挤压,主走滑断裂不连续,代之以大量规模较小的次级断裂系统.太平洋区板块俯冲方向、俯冲速率的变化以及深部动力背景的变迁共同造成了渤南地区郯庐断裂不同分支构造发育演化及其控盆作用的差异性,由于右旋走滑位移量小于先期的左旋走滑,现今渤南地区构造单元分布仍具左旋特征.      

Transtensional deformation in the Lake Tahoe region, California and Nevada, USA

Dextral transtensional deformation is occurring along the Sierra Nevada–Great Basin boundary zone (SNGBBZ) at the eastern edge of the Sierra Nevada microplate. In the Lake Tahoe region of the SNGBBZ, transtension is partitioned spatially and temporally into domains of north–south striking normal faults and transitional domains with conjugate strike-slip faults. The normal fault domains, which have had large Holocene earthquakes but account only for background seismicity in the historic period, primarily accommodate east–west extension, while the transitional domains, which have had moderate Holocene and historic earthquakes and are currently seismically active, primarily record north–south shortening. Through partitioned slip, the upper crust in this region undergoes overall constrictional strain.Major fault zones within the Lake Tahoe basin include two normal fault zones: the northwest-trending Tahoe–Sierra frontal fault zone (TSFFZ) and the north-trending West Tahoe–Dollar Point fault zone. Most faults in these zones show eastside down displacements. Both of these fault zones show evidence of Holocene earthquakes but are relatively quiet seismically through the historic record. The northeast-trending North Tahoe–Incline Village fault zone is a major normal to sinistral-oblique fault zone. This fault zone shows evidence for large Holocene earthquakes and based on the historic record is seismically active at the microearthquake level. The zone forms the boundary between the Lake Tahoe normal fault domain to the south and the Truckee transition zone to the north.Several lines of evidence, including both geology and historic seismicity, indicate that the seismically active Truckee and Gardnerville transition zones, north and southeast of Lake Tahoe basin, respectively, are undergoing north–south shortening. In addition, the central Carson Range, a major north-trending range block between two large normal fault zones, shows internal fault patterns that suggest the range is undergoing north–south shortening in addition to east–west extension.A model capable of explaining the spatial and temporal partitioning of slip suggests that seismic behavior in the region alternates between two modes, one mode characterized by an east–west minimum principal stress and a north–south maximum principal stress as at present. In this mode, seismicity and small-scale faulting reflecting north–south shortening concentrate in mechanically weak transition zones with primarily strike-slip faulting in relatively small-magnitude events, and domains with major normal faults are relatively quiet. A second mode occurs after sufficient north–south shortening reduces the north–south S_hmax in magnitude until it is less than S_v, at which point S_v becomes the maximum principal stress. This second mode is then characterized by large earthquakes on major normal faults in the large normal fault domains, which dominate the overall moment release in the region, producing significant east–west extension.     

Active right-lateral strike-slip fault zone along the southern margin of the Japan Sea

We describe an active right-lateral strike-slip fault zone along the southern margin of the Japan Sea, named the Southern Japan Sea Fault Zone (SJSFZ). Onshore segments of the fault zone are delineated on the basis of aerial photograph interpretations and field observations of tectonic geomorphic features, whereas the offshore parts are interpreted from single-/multichannel seismic data combined with borehole information. In an effort to evaluate late Quaternary activity along the fault zone, four active segments separated by uplifting structures are identified in this study. The east–northeast-trending SJSFZ constitutes paired arc-parallel strike-slip faults together with the Median Tectonic Line (MTL), both of which have been activated by oblique subduction of the Philippine Sea plate during the Quaternary. They act as the boundaries of three neotectonic stress domains around the eastern margin of the Eurasian plate: the near-trench Outer zone and NW–SE compressive Inner zone of southwest Japan arc, and the southern Japan Sea deformed under E–W compression from south to north.     

A Case Study of the Spatial Distribution of Seismic Hazard (El Salvador)

The republic of El Salvador in Central America is an area of high seismic hazard where at least twelve destructive earthquakes have occurred this century alone. The principal sources of seismic hazard are earthquakes associated with the subduction of the Cocos plate in the Middle America Trench and upper-crustal earthquakes in the chain of Quaternary volcanoes that runs across the country parallel to the subduction trench. Hazard assessments for Central America have suggested almost uniform distribution of hazard throughout El Salvador. Seismic zonations for three successive building codes in El Salvador simply divide the country into two regions, with the higher hazard zone containing the volcanoes and the coastal areas. Historical records suggest that the greatest hazard is posed by the upper-crustal earthquakes concentrated on the volcanic centres which, although of smaller magnitude than the subduction events, are generally of shallow focus and coincide with the main population centres. These earthquakes have repeatedly caused intense damage over small areas in the vicinity of some of the main volcanoes. This study focuses on El Salvador to explore the capability of different approaches to hazard assessment to reflect significant variations of seismic hazard within small geographical areas. In the study, three 'zone-free' methods are employed as well as the Cornell–McGuire approach. The results of the assessments are compared and their implications for seismic zoning for construction and insurance purposes are discussed.     

Structural characteristics controlling the seismicity crustal structure of southern Japan Trench fore-arc region, revealed by ocean bottom seismographic data

The Japan Trench is a plate convergent zone where the Pacific Plate is subducting below the Japanese islands. Many earthquakes occur associated with plate convergence, and the hypocenter distribution is variable along the Japan Trench. In order to investigate the detailed structure in the southern Japan Trench and to understand the variation of seismicity around the Japan Trench, a wide-angle seismic survey was conducted in the southern Japan Trench fore-arc region in 1998. Ocean bottom seismometers (15) were deployed on two seismic lines: one parallel to the trench axis and one perpendicular. Velocity structures along two seismic lines were determined by velocity modeling of travel time ray-tracing method. Results from the experiment show that the island arc Moho is 18–20 km in depth and consists of four layers: Tertiary and Cretaceous sedimentary rocks, island arc upper and lower crust. The uppermost mantle of the island arc (mantle wedge) extends to 110 km landward of the trench axis. The P-wave velocity of the mantle wedge is laterally heterogeneous: 7.4 km/s at the tip of the mantle wedge and 7.9 km/s below the coastline. An interplate layer is constrained in the subducting oceanic crust. The thickness of the interplate layer is about 1 km for a velocity of 4 km/s. Interplate layer at the plate boundary may cause weak interplate coupling and low seismicity near the trench axis. Low P-wave velocity mantle wedge is also consistent with weak interplate coupling. Thick interplate layer and heterogeneous P-wave velocity of mantle wedge may be associated with the variation of seismic activity.     

Seismicity distribution in the vicinity of the Chile Triple Junction,Aysén Region,southern Chile

The Aysén Region, southern Chile, is the area located at the southern end of the Nazca-South America subduction zone, to the east of the Chile Triple Junction. This region has historically presented low levels of seismicity mostly related to volcanism. Nonetheless, a seismic sequence occurred in 2007, related to the reactivation of the strike-slip Liquiñe-Ofqui Fault System (LOFS), confirmed that this region is not exempt from major seismic activity M ∼ 7. Here we present results from background local seismicity of two years (2004–2005) preceding the sequence of 2007. Event magnitudes range between 0.5 and 3.4 M_L and hypocenters occur at shallow depths, mostly within the upper 10 km of crust, in the overriding South American plate. No events were detected in the area locus of the 2007 sequence, and the Wadati–Benioff (WB) plane is not observable given the lack of subduction inter-plate seismicity in the area. A third of the seismicity is related to Hudson volcano activity, and sparse crustal events can be spatially associated with the trace of the Liquiñe-Ofqui fault, showing the largest detected magnitudes, in particular at the place where the two main branches of the LOFS meet. Other minor sources of seismicity correspond to glacial calving in the terminal zones of glaciers and mining explosions.     

基于鲜水河断裂带特征地震的地震动预测研究

鲜水河断裂带是中国西南地区一条活动强烈的大型左旋走滑断裂带,具有规模大、活动性强、地震频度高、潜在次生地震地质灾害严重等特点,其地震活动具有特征地震的典型特征,且活断层分段特征明显。本文建立活断层分段上的特征地震模型进行未来地震动预测研究,依据各分段的特征震级、特征破裂断面的几何尺寸、应力降和地震矩等重要震源参数,建立各分段上的随机有限断层震源模型,合成得到各分段观测点的地震动时程与峰值,最终得到整个断裂带的特征地震下的地震动场预测图。研究成果对中国的防震减灾工作具有重要的意义,技术方法可作为中国地震区划技术发展的有益探索。     

Subduction of the Woodlark Basin at New Britain Trench, Solomon Islands region

The Woodlark Basin, located south of the Solomon Islands arc region, is a young (5 Ma) oceanic basin that subducts beneath the New Britain Trench. This region is one of only a few subduction zones in the world where it is possible to study a young plate subduction of several Ma. To obtain the image of the subducting slab at the western side of the Woodlark Basin, a 40-day Ocean Bottom Seismometer (OBS) survey was conducted in 1998 to detect the micro-seismic activity. It was the first time such a survey had been performed in this location and over 600 hypocenters were located. The seismic activity is concentrated at the 10–60 km depth range along the plate boundary. The upper limit just about coincides with the leading edge of the accretionary wedge. The upper limit boundary was identified as the up-dip limit of the seismogenic zone, whereas the down-dip limit of the seismogenic zone was difficult to define. The dip angle of the plate at the high seismicity zone was found to average about 30°. Using the Cascadia subduction zone for comparison, which is a typical example of a young plate subduction, suggests that the subduction of the Woodlark Basin was differentiated by a high dip angle and rather landward location of the seismic front from the trench axis (30 km landward from the trench axis). Furthermore, as pointed out by previous researchers, the convergent margin of the Solomon Islands region is imposed with a high stress state, probably due to the collision of the Ontong Java Plateau and a rather rapid convergence rate (10 cm/year). The results of the high angle plate subduction and inner crust earthquakes beneath the Shortland Basin strongly support the high stress state. The collision of the Ontong Java Plateau, the relatively rapid convergence rate, and moderately cold slab as evidenced by low heat flow, rather than the plate age, may be dominantly responsible for the geometry of the seismogenic zone in the western part of the Woodlark Basin subduction zone.     

The Denali fault system and the tectonic development of Southern Alaska

The Denali fault system forms an arc, convex to the north, across southern Alaska. In the central Alaska Range, the system consists of a northern Hines Creek strand and a southern McKinley strand, up to 30 km apart. The Hines Creek fault may preserve a record of the early history of the fault system. Strong contrasts between juxtaposed lower Paleozoic rocks appear to require large dextral strike-slip or a combination of dipslip and strike-slip displacements along this fault. Thus the fault system may mark a reactivated suture zone between continental rocks to the north and a late Paleozoic island arc to the south, as suggested by Richter and Jones (1973). Major movements on the Hines Creek fault ceased by the Late Cretaceous, but local dip-slip movements continued into the Cenozoic.The McKinley fault is an active dextral strike-slip fault with a mean Holocene displacement rate of 1–2 cm/y. Post-Late Cretaceous dextral offset on this fault is probably at least 30 km and possibly as great as 400 km. Patterns of early Tertiary folding and reverse faulting indicate that the McKinley fault was active at that time and suggest that this fault developed shortly after strike-slip activity ceased on the Hines Creek fault. Oligocene — middle Miocene tectonic stability and late Miocene—Pliocene uplift of crustal blocks may reflect periods of quiescence and activity, on the McKinley fault.The two strands of the Denali fault divide the central Alaska Range into northern, central, and southern terranes. During the Paleozoic—Mesozoic there is evidence for at least two episodes of compressive deformation in the northern terrane, four in the central terrane, and two in the southern. During each, the axis of maximum compressive strain was subhorizontal and about north—south. This pattern suggests a Paleozoic and Mesozoic setting dominated by plate convergence, despite the possible large pre-Late Cretaceous lateral movement on the Hines Creek fault.The Cenozoic pattern of faulting and folding appears compatible with a plate tectonic model of (1) rapid northward movement of the Pacific plate relative to Alaska during the early Tertiary; (2) slow northwestward movement of the Pacific plate during the Oligicene and (3) rapid northwestward movement of the Pacific plate from the end of the Oligocene to the present.     

Intraplate deformation adjacent to the Macquarie Ridge south of New Zealand—The tectonic evolution of a complex plate boundary

The distribution of relocated seismicity and the evolving shape of fracture zones through time in the oceanic crust of the Australian Plate adjacent to the Australia:Pacific plate boundary south of New Zealand are used to constrain the deformation of this region of the Australian Plate, here called the Puysegur Block. Relocated seismicity reveals a broad distribution of earthquakes in the Puysegur Block on both inter- and intraplate structures, including two great (M8+) earthquakes in the region over the past twenty years, one of which occurred over 130 km from the plate boundary. Plate reconstructions from the Late Oligocene through Early–Mid Miocene allow us to determine the undeformed shape of fracture zones in the Puysegur Block, formed during the Paleogene when the plate boundary was dominantly a divergent mid-ocean ridge system. Comparing these reconstructions to the present-day shape of the fracture zones allows us to map the deformation that has occurred within the Puysegur Block since the fracture zones formed. These two sets of independent observations delineate a broad zone of deformation extending ~ 150 km into the plate interior from the Macquarie Ridge Complex, the modern plate boundary structure through the region. The persistence of this deformation through time indicates a link with the evolution of the plate boundary over the past ~ 25 Ma from divergence to translation and subduction of the Australian Plate further north at the Puysegur Trench. We infer that this deformation may be a result of stresses in the Puysegur Block resulting from the impingement of the subducting plate on the thickened lithosphere of southern New Zealand. Such a collision may resist subduction, and if resistance remains substantial, further deformation internal to the Puysegur Block may lead to a southward migration of the Australia:Pacific subduction interface and the capturing of this section of lithosphere onto the Pacific Plate.     

郯庐断裂带安徽段现代地震活动及应力场特征

分析了郯庐断裂带安徽段地震活动特征,利用近震直达波最大振幅比方法反演了99次中-小地震震源机制,并作了统计和聚类分析。现代地震活动整体上继承了历史地震分布格局,未显示增强趋势;断裂带及邻区应力场P轴和T轴优势方位大致呈近东西向和近南北向分布,现今承受的作用力以近水平或斜向为主,但也存在部分其它方向的应力场和近垂直方向的作用力。震源断层的破裂类型,带内以逆冲(或正断),或近逆冲(或正断)型为主。带附近较明显地呈现出走滑型或近走滑型破裂的优势;北、南亚段应力场有明显差异,即北压南张,其分界大约在北纬32°附近,北西西走向的桥头集—东关断裂是两者间的构造变换带。     

The westward lithospheric drift,its role on the subduction and transform zones surrounding Americas:Andean to cordilleran orogenic types cyclicity

We investigate the effect of the westerly rotation of the lithosphere on the active margins that surround the Americas and find good correlations between the inferred easterly-directed mantle counterflow and the main structural grain and kinematics of the Andes and Sandwich arc slabs.In the Andes,the subduction zone is shallow and with low dip,because the mantle flow sustains the slab;the subduction hinge converges relative to the upper plate and generates an uplifting doubly verging orogen.The Sandwich Arc is generated by a westerly-directed SAM(South American) plate subduction where the eastward mantle flow is steepening and retreating the subduction zone.In this context,the slab hinge is retreating relative to the upper plate,generating the backarc basin and a low bathymetry single-verging accretionary prism.In Central America,the Caribbean plate presents a more complex scenario:(a) To the East,the Antilles Arc is generated by westerly directed subduction of the SAM plate,where the eastward mantle flow is steepening and retreating the subduction zone.(b) To the West,the Middle America Trench and Arc are generated by the easterly-directed subduction of the Cocos plate,where the shallow subduction caused by eastward mantle flow in its northern segment gradually steepens to the southern segment as it is infered by the preexisting westerly-directed subduction of the Caribbean Plateau.In the frame of the westerly lithospheric flow,the subduction of a divergent active ridge plays the role of introducing a change in the oceanic/continental plate's convergence angle,such as in NAM(North American)plate with the collision with the Pacific/Farallon active ridge in the Neogene(Cordilleran orogenic type scenario).The easterly mantle drift sustains strong plate coupling along NAM,showing at Juan de Fuca easterly subducting microplate that the subduction hinge advances relative to the upper plate.This lower/upper plate convergence coupling also applies along strike to the neighbor continental strike slip fault systems where subduction was terminated(San Andreas and Queen Charlotte).The lower/upper plate convergence coupling enables the capture of the continental plate ribbons of Baja California and Yakutat terrane by the Pacific oceanic plate,transporting them along the strike slip fault systems as para-autochthonous terranes.This Cordilleran orogenic type scenario,is also recorded in SAM following the collision with the Aluk/Farallon active ridge in the Paleogene,segmenting SAM margin into the eastwardly subducting Tupac Amaru microplate intercalated between the proto-LiquineOfqui and Atacama strike slip fault systems,where subduction was terminated and para-autochthonous terranes transported.In the Neogene,the convergence of Nazca plate with respect to SAM reinstalls subduction and the present Andean orogenic type scenario.     

Plate subduction,and generation of earthquakes and magmas in Japan as inferred from seismic observations: An overview

A dense nationwide seismic network recently constructed in Japan has been yielding large volumes of high-quality data that have made it possible to investigate the seismic structure in the Japanese subduction zone with unprecedented resolution. In this article, recent studies on the subduction of the Philippine Sea and Pacific plates beneath the Japanese Islands and the mechanism of earthquake and magma generation associated with plate subduction are reviewed. Seismic tomographic studies have shown that the Philippine Sea plate subducting beneath southwest Japan is continuous throughout the entire region, from Kanto to Kyushu, without disruption or splitting even beneath the Izu Peninsula as suggested in the past. The contact of the Philippine Sea plate with the Pacific plate subducting below has been found to cause anomalously deep interplate and intraslab earthquake activity in Kanto. Detailed waveform inversion studies have revealed that the asperity model is applicable to interplate earthquakes. Analyses of dense seismic and GPS network data have confirmed the existence of episodic slow slip accompanied in many instances by low-frequency tremors/earthquakes on the plate interface, which are inferred to play an important role in stress loading at asperities. High-resolution studies of the spatial variation of intraslab seismicity and the seismic velocity structure of the slab crust strongly support the dehydration embrittlement hypothesis for the generation of intraslab earthquakes. Seismic tomography studies have shown that water released by dehydration of the slab and secondary convection in the mantle wedge, mechanically induced by slab subduction, are responsible for magma generation in the Japanese islands. Water of slab origin is also inferred to be responsible for large anelastic local deformation of the arc crust leading to inland crustal earthquakes that return the arc crust to a state of spatially uniform deformation.     

Deep roots of seismotectonics of the Far East: The Amur River and Primorye zones

The role of the lateral structure of the lithospheric mantle in the seismotectonics and seismicity of the southern part of the Russian Far East has been investigated. The positions of the epicenters of all the major earthquakes in Sakhalin (M ≥ 6.0), as well as in the Amur region and the Primorye zones (M ≥ 5.0), are defined by the boundaries of the Anyui block of highly ferruginous mantle, which lies at the base of the Sikhote-Alin area. Three cycles of large earthquakes are recognized in the region: the end of the 19th-beginning of the 20th century, the mid-20th century, and end of the 20th-beginning of the 21st century. In the seismic zone of the Amur region (hereafter, the Amur seismic zone), the epicenters of the large earthquakes in each cycle migrate from the SW to NE along the Tan-Lu fault megasystem at a rate of 30–60 km/yr. The specific features of the seismicity of the region are explained by the repeated arrival of strain waves from the west. The waves propagate in the upper part of the mantle and provoke the activation of the deep structure of the region. The detailed analysis of the earthquakes in the Sikhote-Alin area (M ≥ 4.0) in 1973–2009 confirmed the clockwise tectonic rotation of the mantle block. The characteristics of the Primorye zone of deep-focus seismicity at the Russia-China boundary are stated. Since 1973, 13 earthquakes with M ≥ 6.0 have been recorded in the zone at a depth of 300–500 km. This number of earthquakes is at least twice as many as the number of large deep-focus earthquakes elsewhere in the Sea of Japan-Sea of Okhotsk transition zone. The unique genesis of the Primorye seismic zone is related to the additional compression in the seismofocal area due to the creeping of the Anyui mantle block onto the subduction zone during its rotation. The geodynamic implications of the seismotectonic analysis are examined, and the necessity of division of the Amur plate into three geodynamically independent lithospheric blocks is substantiated.     

Creation of the Cocos and Nazca plates by fission of the Farallon plate

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.     

The tectonic regime along the Andes: Present‐day and Mesozoic regimes

The analyses of the main parameters controlling the present Chile‐type and Marianas‐type tectonic settings developed along the eastern Pacific region show four different tectonic regimes: (1) a nearly neutral regime in the Oregon subduction zone; (2) major extensional regimes as the Nicaragua subduction zone developed in continental crust; (3) a Marianas setting in the Sandwich subduction zone with ocean floored back‐arc basin with a unique west‐dipping subduction zone and (4) the classic and dominant Chile‐type under compression. The magmatic, structural and sedimentary behaviours of these four settings are discussed to understand the past tectonic regimes in the Mesozoic Andes based on their present geological and tectonic characteristics. The evaluation of the different parameters that governed the past and present tectonic regimes indicates that absolute motion of the upper plate relative to the hotspot frame and the consequent trench roll‐back velocity are the first order parameters that control the deformation. Locally, the influences of the trench fill, linked to the dominant climate in the forearc, and the age of the subducted oceanic crust, have secondary roles. Ridge collisions of seismic and seismic oceanic ridges as well as fracture zone collisions have also a local outcome, and may produce an increase in coupling that reinforces compressional deformation. Local strain variations in the past and present Andes are not related with changes in the relative convergence rate, which is less important than the absolute motion relative to the Pacific hotspot frame, or changes in the thermal state of the upper plate. Changes in the slab dip, mainly those linked to steepening subduction zones, produce significant variations in the thermal state, that are important to generate extreme deformation in the foreland. Copyright © 2009 John Wiley & Sons, Ltd.