Arc Tectonics Research Articles

Edward Suess and Russian geologists

The publication is devoted to the history of one of the greatest concepts of tectonics of Asia, that has been widely accepted and yet obliterated with time, while the splendors of this concept are doubtful. Numerous citations in the Russian papers to «The Face of the Earth» by Edward Suess and the fact that he was elected a Corresponding Member of the Imperial Saint Petersburg Academy of Sciences clearly demonstrate how highly Suess’s contribution to studies of the structure and geological evolution of Asia was valued by the Russian geological community. Suess’s letters to Vladimir A. Obruchev give evidence how close and productive the relationship between Edward Suess and the Russian researchers was in the late 19th and the early 20th centuries and also illustrate how the great tectonic concept of Asia [Suess, 1908] was born and developed. The idea of centrifugal propagation of tectonic waves of the Altaids from a continental node located somewhere in Siberia was mainly inspired by Suess’s profound scientific intuition. The idea matured after Edward Suess got acquainted with Ivan D. Chersky’s paper [Черский, 1886] that greatly facilitated in shaping and improving this idea. It was mailed to Suess by Vladimir A. Obruchev who translated the paper, attached his own map and provided explanations to Chersky’s ideas. The available historical documents suggest that Vladimir A. Obruchev facilitated communication between the Russian geologists, on the one side, and Edward Suess and other Austrian geologists who conducted geological studies in Asia, on the other side. Being actively involved in exchange of publications and cooperation in field data processing, Edward Suess was aware of all the details of the Russian geological studies. In addition to the concept of tectonic arcs of the Altaids and descriptions of main geological structures located in Northern Asia and China, Edward Suess adopted a concept of disjunctive dislocations proposed by the Russian geologists. While interpreting the structure of huge territories of Asia that were poorly studied then, he stuck to the orometric geometry principles and thus, unfortunately, missed the Russian conclusions on disagreement between belts and mountain ridges, superimposed folding, mélanges etc., and such features were not incorporated in his concepts. Papers of Edward Suess, including «The Face of the Earth» which was very positively accepted by the geological scientific community, have never been translated into Russian, unlike other foreign publications. In 1930–1940, the name of Edward Suess gradually vanished from references in the Soviet scientific papers. Such a lapse seems to have resulted from the fact that Suess’s papers were misinterpreted and misunderstood by scientists who adhered to the geosyncline theory. Examples of such errors can be easily discovered by reviewing the history of development of concepts of the Siberian and Russian cratons. The truly gentlemanly behavior demonstrated by the key researchers of geology of Asia in the late 19th and the early 20th centuries is highly educative and commendable. It is worth studying the scientific reports of Edward Suess and his Russian colleagues with a more detail and comprehensive approach.

Eustatic and tectonic controls on Quaternary Ras Leona marine terraces (Strait of Gibraltar, northern Morocco)

Well-preserved Quaternary staircased marine terraces appear on Ras Leona limestone relief. This is a peculiar sector of the Betic-Rif Cordillera, lying in the four-way junction between the Atlantic and the Mediterranean, and Europe and Africa. The age and altitude correlation of the Ras Leona terraces with travertine-covered lateral equivalent terraces fashioned in the neighbouring Beni Younech area, and comparison with those along the Moroccan Atlantic coasts, would suggest that the Ras Leona terraces were mainly formed by eustatic factors. The importance of the eustasy is supported by further comparisons with Spanish and Moroccan Mediterranean terraces and with different marine terraces developed on passive-margin coasts around the world. A tectonic event occurred mainly during the period between the formation of the Maarifian and the Ouljian terraces (i.e., between 370 and 150 ka). The moderate Quaternary tectonic uplift deduced from the marine terraces and its comparison with uplifted marine terraces developed in active subduction setting disagrees with the model of an active eastwards subduction below the Gibraltar tectonic arc.

Evaluation of Recent Tectonomagmatic Discrimination Diagrams and their Application to the Origin of Basic Magmas in Southern Mexico and Central America

Discrimination diagrams to decipher tectonic settings have been in use for nearly 40 years. Although old diagrams have been extensively used, the recent ones based on discriminant functions of ratio variables, with or without log-transformation, proposed during 2004–2010 for the discrimination of four tectonic settings of island arc, continental rift, ocean-island and mid-ocean ridge, were newly evaluated to show their high success rates of 57.3–100% and 58.5–100% for major-element and immobile-element based diagrams, respectively. For the continental arc of the Andes evaluated for its similarity to island arc, these four sets of diagrams showed success rates of 62.1–83.8%. These four sets of five diagrams per set were therefore used to infer tectonic setting of the Mexican Volcanic Belt (MVB), Los Tuxtlas volcanic field (LTVF), and Central American Volcanic Arc (CAVA). Using this approach, the MVB, especially its western, central and eastern parts, and the LTVF of Southern Mexico show a dominantly continental rift setting and the CAVA shows an arc setting. The west-central part of the MVB is consistent with dual tectonics of arc and rift. These results confirm the application of an unusual mantle upwelling rift-model for the Mexican on-land volcanism, whereas the conventional plate tectonic subduction model seems to be applicable for the CAVA from Guatemala to north-western Costa Rica.

Structure of the Mountain Front Flexure along the Anaran anticline in the Pusht-e Kuh Arc (NW Zagros, Iran): insights from sand box models

Abstract The Mountain Frontal Flexure shows a single step along the front of the Pusht-e Kuh Arc with about 3 km of structural relief. This front has been interpreted as being formed by a basement monocline above a blind crustal-scale and low-angle thrust with a ramp–flat geometry (the ramp dips 12–15° towards the inner part of the orogen and cuts the entire crust). The Anaran anticline on top of the Mountain Frontal Flexure shows an irregular geometry in map view and consists of four segments with diverse directions of which the SE Anaran, the Central Anaran and the NW Dome are culminations. The North–South Anaran segment may form a linking zone developed during the rise and amplification of single culminations, the NW Dome and the Central Anaran, above the Mountain Frontal Flexure. The asymmetric Anaran anticline is characterized by the existence of multiple normal faults, some of them with significant dip-slip displacements of up to 1000 m. These faults limit grabens located along the crests of the anticline segments. Cross-cutting relationships show that the normal faults along the Central Anaran are older than along the North–South Anaran, reinforcing the temporal constraints on the later growth of this segment of the anticline. The geometry of the Anaran anticline is asymmetric with the subvertical forelimb very little exposed. This forelimb is cut above and below by a thrust system that seems to develop along the fold hinges. The lower thrust, with a ramp–flat geometry, carries the entire anticline towards the foreland on top of slightly deformed rocks in the footwall. The thrust flattens in the Gachsaran evaporitic level forming a typical triangular zone filled with evaporites, which produce a strong fold disharmony between the overburden (Passive Group) and the underlying rocks (Competent Group). The growth of the Anaran anticline lasted for about 6 Ma and was the consequence of detachment folding that was subsequently thrust, rotated and uplifted above the Mountain Frontal Flexure with coeval reactivation of earlier crestal layer-parallel extension normal faults to accommodate the large increase of structural relief between the foreland and the tectonic arc. Three main results from analogue modelling have been combined with field data to resolve the geometry of the Anaran anticline as well as its evolution: (1) a thickening of intermediate evaporites (Gachsaran Formation) is produced above the flat segment of the thrust carrying the anticline on top of foreland strata; (2) growth strata deposited in the adjacent syncline modify the geometry of the anticline by increasing the dip and the length of its forelimb; (3) coeval erosion to anticline growth, as well as thick growth strata deposition, increases fold amplification rather than foreland propagation of deformation. The proposed fold model may be applied to other anticlines on top of this major basement-related thrust, such as the Siah Kuh and Khaviz anticlines in the Pusht-e Kuh Arc and Dezful Embayment domains.

Overview of a Special Issue on “Geotectonic Evolution of the Japanese Islands under New Paradigms of the Next Generation (Part I-III)”

This article provides an overview of papers collected in the special issue titled “Geotectonic Evolution of the Japanese Islands under New Paradigms of the Next Generation (Part I-III)” for international readers. Research on the geologic evolution of the Japanese Islands started from the geotectonic subdivision with definitions of elements and boundaries in the late 19th century. Traditionally, some remarkable faults, such as Median Tectonic Line (MTL), Fossa magna, and Tanakura Tectonic Line (TTL), were regarded as the most important; however, these apparent features were in fact formed during the Miocene opening of the Japan Sea, whereas the major structures of Japan were made by the Pacific-type orogenies throughout the Phanerozoic after the initiation of subduction at 520 Ma (the Cambrian). Marine geophysicists have realized that the formation of accretionary complex (AC) along modern active subduction zones is rather exceptional; instead, tectonic erosion takes place dominantly. The estimated addition rate of juvenile arc crust (composed of tonalite–tronjemite–granodiorite suite; TTG) is negative at present for the entire globe, i.e., the total volume of the continental crust is currently decreasing. This indicates that tectonic erosion must also have occurred effectively in the past, and that its geological remnants need to be investigated carefully. A possible proxy geologic body for ancient tectonic erosion is a serpentinite melange, which often includes calc-alkaline volcanics and coeval high-P/T blueschists within serpentinite matrices as shown in the geology of Japan. The large-scale shortening of a fore-arc crust can occur solely due to tectonic erosion, which may involve more than 150 km-wide areas of missing rocks between volcanic arc and trench. The sporadic occurrence of tectonic blocks of older (520-150 Ma)TTG rocks in serpentinite melange indicates that four arc crusts, out of the five made by Pacific-type orogeny in Japan during the past 500 million years, have already been consumed. The Pacific-type orogeny was revisited after recent critical discoveries of; (1) orogenic core of high-P/T regional metamorphic belt as a thin (< 2 km) solid high-T intrusion, bounded on the top and bottom by a paired fault; (2) rapid increase of 200-300 km-wide TTG belt formed by slab-melting; (3) these culminated with an approaching mid-oceanic ridge, associated with large-scale tectonic erosion; and (4) formation of an AC after ridge subduction and doming up of a sandwiched set of high-P belt and accretionary belt below and above. The Pacific-type orogeny has been ignored since the “terrane plague”; however, Pacific-type orogeny dominated to increase the TTG crust leading to the births of continents and supercontinent by 1.8 Ga. The extensive tectonic erosion and arc subduction through time, suggesting 10 times more TTG materials, enriched with radiogenic elements, floated at the bottom of the mantle transition zone, which generated heat to warm up the mantle up to 200 K within 100 million years in the Archean, and to 100 K in the Phanerozoic. Subducted TTG and presence of second continents in the mid-mantle may explain more general aspects of mantle dynamics, and the concept may become a frontier of solid Earth science in the 21st century. From the viewpoint of the history of science, the previous geological studies in Japan since the Meiji Era are also summarized briefly, with special reference to the orogeny, tectonics, and geotectonic subdivision of the Japanese Islands.

Imaging the subducted slab under the Calabrian Arc, Italy, from receiver function analysis

We use data from the CAT/SCAN seismic deployment in southern Italy to reconstruct the crust and uppermost mantle structure above one of the narrowest active subduction zones worldwide, the Calabrian Arc, where the last fragment of the former Tethys ocean is being subducted. An E-W time-domain profile composed of teleseismic receiver functions shows the geometry of the main seismic discontinuities beneath Calabria. It provides a clear two-dimensional (2-D) image of the subducting Ionian plate at shallow depth where it bends and starts to descend into the mantle. In the profile the Moho of the subducting Ionian plate lies at ~35 km and gently dips westward beneath the eastern part of Calabria. Then the depth increases steeply to ~80 km below western Calabria. The locus where the Ionian plate changes its dip and starts to subduct corresponds to the transition from an uplifted plateau to an extensional basin at the surface. It suggests that the Ionian plate has not rolled back relative to Calabria since it slowed. This implies the uplift of the Sila Plateau is more likely due to under-plating than eastward growth of the mantle wedge. When projected on a map, the depths of the Ionian Moho beneath each seismic station reveal a more complex geometry for the subducting plate, including an unexpected deepening northward. This may be related to a tear in the plate or other tectonic motions between Calabria and the Southern Apennines. Thus the relationship between the imaged geometry of the subducting plate and structural elements at the surface provides new knowledge about the geodynamic evolution of the subduction system and the tectonics of the Calabrian Arc.

Block tilting of the North Provence early Cretaceous carbonate margin: stratigraphic, sedimentologic and tectonic data

AbstractIn the western part of the Castellane tectonic arc, the so-called “ Provence platform area “, corresponding to the foreland of the Alpine nappes (figs. 1–2), is marked by Tithonian-Berriasian shallow water carbonates capped by hemipelagic sediments deposited from the Valanginian up to the Aptian-Albian. A detailed biostratigraphic study of the Berriasian succession, based on calcareous algae and foraminifera, allows us to distinguish a Lower to Middle Berriasian, with Clypeina sulcata, Clypeina isabellae and Holosporella sarda, from an Upper Berriasian with Pfenderina neocomiensis, Danubiella cernavodensis, Falsolikanella campanensis and Macroporella praturloni (fig. 3). We performed a field survey of 30 sites located from Quinson to the west, and Escragnolles to the east (figs. 4–5) including the study of measured stratigraphic sections and the collection of samples for biostratigraphic interpretations. These stratigraphic investigations show that below the Valanginian beds, the Berriasian platfom carbonate succession, is locally incomplete, i.e. Upper Berriasian beds are frequently absent. During the Early and Middle Berriasian, depositional environments are marked by a strong bathymetric instability, with frequent subaerial exposure events, and a significant marine restriction; by contrast, during the Late Berriasian, the overall biological diversity increases and water agitation as well, which means a significant marine opening towards the basin. The Upper Berriasian hiatus is consequently regarded as the result of a Berriasian/Valanginian and/or a lowermost Valanginian erosion (fig. 6). The spatial distribution of complete or truncated Berriasian successions identifies east-west bands, in each band truncated series are located northward and complete series are located southward. Bands are limited by thrust or strip faults interpreted as palaeofaults reactivated during the Alpine orogeny (fig. 7). These fault-bounded blocks, 3 to 10 km in width, known as the Aiguine, La Palud-sur-Verdon, Carajuan-Audibergue and Peyroulles-La Foux blocks, are southerly rotated by 1 to 2o. We regard this structural architecture as the result of basinward tilting of blocks. Due to their rotation, the uplifted parts were eroded whereas the depressed parts were protected against erosion (fig. 8). Such a dynamic behavior reflects a distensive tectonic regime, which has been active at least during the Valanginian, that is after the drowning of the North-Provence carbonate platform. These structural events are considered as the regional expression of the Neocimmerian tectonic phase coupled with an enhancement of the Atlantic rifting. The orientation of the major Alpine structural elements (folds and faults) of the Castellane arc, is mostly inherited from these early Cretaceous tectonic events.

Evaluating the subsurface structures due to the Syrian Arc Tectonics at El-Hasana area, Sinai Peninsula, Egypt

Three major domal structures are trending northeast-southwest directions prevailed at El-Hasana area, northern part of Sinai Peninsula. These structures and their altered rock constituents attracted us to study these formations from geophysical view. The main target of this work is to establish the relation between these structures and Syrian Arc Tectonic System (SATS) prevailed at the studied area. The investigations were carried out using land magnetic survey, aeromagnetic, gravity and laboratory measurements.An intensive analysis using tectonic trend analysis and, 3D Euler deconvolution with different coefficients were applied to the potential field data. Also, cross-sections Euler deconvolution, Werner deconvolution and radial power spectrum have been applied. The results illustrate that most prevailed tectonics that could forming these anticlines are trending at N 35°–65° E direction which related to the Syrian tectonics (SATS) direction. The depth to the basement rocks ranges between 2 km at the southern parts to 2.3 km at the northern parts of the studied area.The magnetic susceptibility map was constructed using laboratory measurements for more than 120 collected rock samples along the surveyed area. The average values range between 0.7 × 10 −3 to 1 × 10 −3 SI for limestone samples, whereas the sandstone varies between 1 × 10 −3 to 1.5 × 10 −3 SI unit. The results show that the magnetic values are higher for the rock samples picked along the anticlinal structures.

Tomographic Determination of the Upper Crustal Structure in Jiashi Strong Earthquake Swarm Region‐Joint Inversion of Explosion and Earthquake Data

AbstractThe Jiashi strong earthquake swarm region is located at the juncture area of three tectonic units, i.e. the Tianshan fold zone, Pamir tectonic arc and Tarim block. Many strong earthquakes occurred recently in this area. In order to obtain the fine crustal velocity structures of this area, the data from a 3‐D seismic transmission experiment and a portable seismograph network acquired in the Jiashi strong earthquake swarm region in 1997 are processed. Joint inversion of explosion and earthquake data is carried out to reconstruct the 3‐D images of velocity perturbations of the upper crust under the seismic array. The temporary seismic array was used to record the seismic waves generated by 8 shots fired at different azimaths, and the additional 20 seismic stations were deployed in this area to record local earthquakes. The results show that the structure of the upper crust in the study area is characterized by obvious inhomogeneities both laterally and vertically. The low P wave velocity zone centered at Saruxi beneath the Jiashi earthquake swarm area is gradually replaced by high P wave velocity with the increase of depth. There is a high P wave velocity anomaly striking NNW at the depth of 12 km under the earthquake swarm area. It extends downwards to the depth of 16 km at least, and is surrounded by relatively low velocities. VP/VS ratios are also characterized by high values in this region. This kind of structural differences may be related to the occurrence of the Jiashi strong earthquake swarm. It can be found from the P wave velocity images at the depth of 16 km that the Jiashi strong earthquake swarm generally occurred in the regions with comparatively high P wave velocity perturbations or in the transition zones between high and low velocity perturbations. The existence of the high P wave velocity anomaly in the crust may play an important role in the seismogenic process.

Transpressional tectonics and Carboniferous magmatism in the Limousin, Massif Central, France: Structural and40Ar/39Ar investigations

New structural, microstructural, and40Ar/39Ar data from the NW Massif Central (France) provide additional constraints on the timing and tectonic setting of late Variscan granite magmatism. Previous studies had emphasized the role of late orogenic extension in the emplacement of granite plutons in the Limousin region. In contrast, the new data set is consistent with syntectonic emplacement of magma in a dextral simple shear active from 350 to 300 Ma in a transpressional regime. As an alternative hypothesis to late orogenic extension, we propose that magmas migrated into tensional bridges between active P shears associated with a lithospheric shear zone comparable to a pop‐up structure. The Galician region, in the western end of the Ibero‐Armorican tectonic arc, exhibits major left‐lateral ductile shear zones which can be interpreted as conjugate structures to the Limousin and Armorican shear zones.

Kaulonia, southern Italy: Calabrian Arc tectonics inducing Holocene coastline shifts

Kaulonia, an ancient Greek settlement (~700 to 389 B.C.), is positioned in a tectonically mobile setting on the Ionian coast of Calabria in southern peninsular Italy. Several extensive back-and-forth shoreline migrations have occurred along this coast since the mid-Holocene. These large-scale shifts, from 100 m landward of the present shoreline to 300m offshore, are recorded by tracing the lateral distribution of archaeological material and a distinct Holocene stratigraphic marker unit (beachrock-cobble sequence). Landward-directed (transgressive) shifts were induced during tectonically calmer periods when phases of relative sea-level rise prevailed. In contrast, a major seaward (regressive) coastal migration resulted primarily from an important ‘see-saw’ phase, with uplift landward and at the coast and a downward seafloor tilt. This Calabrian Arc tectonic event, leading to this offshore displacement in the mid- to late Holocene, is also recorded elsewhere in southern Italy. The approach used here, integrating geological and archaeological databases, serves to more precisely interpret settings where major nearshore changes have been driven largely by effects of structural mobility.

Review and New Insights on Foreland Tectonics in Western Taiwan

Taiwan is located on the boundary between the Eurasian and Philippine Sea plates. As a result, foreland tectonics in western Taiwan can be divided into two domains: pre-orogenic extensional structures and those of the outer part of the fold-and-thrust belt that mingled with syn-orogenic normal fault reactivation. This paper proposes a synthetic model for foreland tectonics in western Taiwan, and advances possible mechanisms by which pre-existing normal faults might have affected the evolving thrust tectonics in foreland areas of western Taiwan. The sedimentary basins of pre-orogenic extensional tectonics are of two types—Paleogene and Neogene—which reflect two stages of continental rifting. Results from several studies have been synthesized to provide a tectonic map displaying the regional distribution of tectonic settings at different stages and the trends of normal faults in the basins. The similarity of the en echelon patterns of arrangement for both the Neogene and Paleogene tectonic and structural settings, as shown by the tectonic map, strongly suggests that the entire foreland area was influenced by regional dextral shear. We also provide a detailed description of structures in each tectonic setting, and propose a tectonic evolution model for Cenozoic basin architecture in western Taiwan. Among the pre-orogenic sedimentary basins, the Neogene ones, in which normal faults extend to the frontal areas of the fold-and-thrust belt in western Taiwan, open northeastward. Structural analysis of the thrust fault geometry indicates that, during development of the fold-and-thrust belt on the rifted continental margin in western Taiwan, the pre-existing normal faults in northwestern Taiwan were reactivated to form inversion structures of various types on different scales, depending on the angle between the strike of the normal faults and the direction of maximum compressive stress field. In southwestern Taiwan, where normal fault reactivation is absent from the eastern part of the foreland areas, pre-existing normal faults interacted with developing low-angle thrusts in the inner part of the fold-and-thrust belt. Normal fault reactivation, regardless of how it occurs, thus plays an important role in forming the deformation front of the fold-and-thrust belt. Based on this view, we propose that the orocline or tectonic arc of the island has been influenced more by normal fault reactivation than by the morphology of basement highs.

The Waldemar Lindgren Award for 2005 Citation of Stephen Piercey

Mr. President, friends, and colleagues: It is my great pleasure to present to you Stephen J. Piercey as this year’s winner of the Waldemar Lindgren Award. This award is offered annually to a young scientist whose published research represents an outstanding contribution to economic geology. Steve’s nomination was based on a track record of exceptional achievement in the area of igneous geochemistry and economic geology since the completion of his Ph.D. research in 2001. Over the past few years he has authored 15 peer-reviewed papers (seven of these first-authored; plus two more accepted, pending revisions), 18 government publications, and 4 geologic maps. His publications have appeared in leading scientific journals, including Economic Geology, Geology, Geological Society of America Bulletin, and Canadian Journal of Earth Sciences . Steve has wisely chosen as his research area the topic of crustal growth and evolution of convergent margins through geologic time. His research theme has been the petrogenesis and tectonics of arc and back-arc volcanic and plutonic rocks in ancient orogenic and greenstone belts, and their metallogenic implications for the formation of volcanogenic massive sulfide (VMS) deposits. In short, he investigates the linkages and drivers between magmatism and ore deposits formation. Although Steve’s research achievements are in the broad field of economic geology and metallogeny, the significance of his findings extends into the areas of volcanology, igneous petrology, and tectonics. Steve received his Honors B.Sc. degree in 1996 at Memorial University of Newfoundland, completing a thesis on “The Geology and Geochemistry of the Southern Pacquet Harbour Group Volcanics, Baie Verte Peninsula, Newfoundland.” This work garnered the President’s Gold Medal of the Canadian Institute of Mining, Metallurgy and Petroleum for best undergraduate student thesis in Canada in 1997. He stayed on at Memorial for his M.Sc. thesis, entitled “An Integrated Study of Magmatism, Magmatic Ni-Cu-Co …

Active faults, seismicity and stresses in an internal boundary of a tectonic arc (Campo de Dalías and Níjar, southeastern Betic Cordilleras, Spain)

The Betic-Rif Cordilleras, formed by the interaction of NW–SE convergence between the Eurasian and African plates and the westward motion of their Internal Zones, provide a good example of an active tectonic arc. The Campo de Dalías and Campo de Níjar constitute outcropping sectors of Neogene and Quaternary rocks located in the southeastern border of the Betic Cordilleras and allow us to study the recent deformations developed in the internal border of this tectonic arc. The main active faults with related seismicity, representing a moderate seismic hazard, associated to the southeastern Betic Cordilleras boundary, include high-angle NW–SE-oriented normal faults that affect, at least, the upper part of the crust, a main detachment located at 10 km depth, and probably another detachment at 20 km as well. Seismite structures, recent fault scarps with associated colluvial wedges that deform the drainage network and the alignment of the coastline, indicate that the high-angle faults have been active at least since the Quaternary. Paleostresses determined from microfault analysis in Quaternary deposits generally show an ENE–WSW trend of extension. Present-day earthquake focal mechanisms include normal, strike-slip and reverse faulting. Normal and strike-slip focal mechanisms generally indicate ENE–WSW extension, and strike-slip and reverse focal mechanisms are related to NNW–SSE compression. The maximum horizontal compression has a consistently NNW–SSE trend. The deep activity of detachments and reverse faults determines the NNW–SSE crustal shortening related to the Eurasian–African plate convergence. At surface, however, the predominance of normal faults is probably produced by the increase in the relative weight of the vertical stress axis, which in turn may be related to relief uplift and subsequent horizontal spreading. The internal mountain front boundary of the Betic Cordilleras developed through the activity of a set of structures that is more complex than a typical external mountain front, probably as a consequence of a vertical variable stress field that acted on previously deformed rocks belonging to the Internal Zone of the cordilleras.

Terminal suturing of Gondwana and the onset of the Ross–Delamerian Orogeny: the cause and effect of an Early Cambrian reconfiguration of plate motions

A review of U–Pb age data from intrusive and volcanic rocks for Antarctica and Australia has highlighted the remarkable synchronicity in the shift from passive margin or continental arc tectonism, to convergent orogenesis along the Pacific margin of Gondwana. This event, the Ross–Delamerian Orogeny, marks a distinct and rapid shift in the style of deformation, rate of uplift, volume of magmatism and syn-orogenic sedimentation along the entire 4000+ km length of the orogen. The Ross–Delamerian Orogeny is constrained to have begun at 515±5 Ma, an interval that also encompasses the youngest collisional events that lead to the suturing of Gondwana. We propose that these geographically separate events are intimately linked and represent the cause and effect of a major tectonic reorganisation in the motion of the Earth’s plates. We suggest the mechanism controlling this Early to Middle Cambrian plate reorganisation was the rapid reduction in relative motion, driven by continental collision, between the final two components of Gondwana. Due to the need to maintain a zero sum for all plate motions, the change in the rate of convergence at this boundary resulted in compensatory changes elsewhere in the global plate circuit. One such change was an increase in the rate and perhaps direction of convergence along the Pacific margin of Gondwana.

特集「東北日本の地震活動とテクトニクス」

特集「東北日本の地震活動とテクトニクス」

Tectonics of the Jurassic‐Early Cretaceous magmatic arc of the north Chilean Coastal Cordillera (22°–26°S): A story of crustal deformation along a convergent plate boundary

The tectonic evolution of a continental magmatic arc that was active in the north Chilean Coastal Cordillera in Jurassic‐Early Cretaceous times is described in order to show the relationship between arc deformation and plate convergence. During stage I (circa 195–155 Ma) a variety of structures formed at deep to shallow crustal levels, indicating sinistral arc‐parallel strike‐slip movements. From deep crustal levels a sequence of structures is described, starting with the formation of a broad belt of plutonic rocks which were sheared under granulite to amphibolite facies conditions (Bolfin Complex). The high‐grade deformation was followed by the formation of two sets of conjugate greenschist facies shear zones showing strike‐slip and thrust kinematics with a NW–SE directed maximum horizontal shortening, i.e., parallel to the probable Late Jurassic vector of plate convergence. A kinematic pattern compatible to this plate convergence is displayed by nonmetamorphic folds, thrusts, and high‐angle normal faults which formed during the same time interval as the discrete shear zones. During stage II (160–150 Ma), strong arc‐normal extension is revealed by brittle low‐angle normal faults at shallow levels and some ductile normal faults and the intrusion of extended plutons at deeper levels. During stage III (155–147 Ma), two reversals in the stress regime took place indicated by two generations of dikes, an older one trending NE–SW and a younger one trending NW–SE. Sinistral strike‐slip movements also prevailed during stage IV (until ∼125 Ma) when the Atacama Fault Zone originated as a sinistral trench‐linked strike‐slip fault. The tectonic evolution of the magmatic arc is interpreted in terms of coupling and decoupling between the downgoing and overriding plates. The structures of stages I and IV suggest that stress transmission due to seismic coupling between the plates was probably responsible for these deformations. However, decoupling of the plates occurred possibly due to a decrease in convergence rate resulting in extension and the reversals of stages II and III.

Miocene intra‐arc bending at an arc–arc collision zone, central Japan: Reply

Abstract Recent paleomagnetic studies are reviewed in an effort to clarify the relationship between the intra-arc deformation of central Japan and the collision tectonics of the Izu-Bonin Arc. The cusp structure of the pre-Neogene terranes of central Japan, called the Kanto Syntaxis, suggests a collisional origin with the Izu-Bonin Arc. The paleomagnetic results and newly obtained radiometric ages of the Kanto Mountains revealed the Miocene rotational history of the east wing of the Kanto Syntaxis. More than 90° clockwise rotation of the Kanto Mountains took place after deposition of the Miocene Chichibu Basin (planktonic foraminiferal zone of N.8: 16.6–15.2 Ma). After synthesizing the paleomagnetic data of the Japanese Islands and collision tectonics of central Japan, it appears that approximately a half rotation (40–50°) probably occurred at ca 15 Ma in association with the rapid rotation of Southwest Japan. The remainder (50-40°) continued until 6 Ma, resulting in the sharp bent structure of the pre-Neogene accretionary complexes (Kanto Syntaxis). The latter rotation seems to have been caused by the collision of the Izu-Bonin Arc on the northwestward migrating Philippine Sea Plate.

Some outstanding issues in the geology of Lebanon and their importance in the tectonic evolution of the Levantine region

The concept of pervasive and disruptive late Neogene–Quaternary strike-slip tectonics in Lebanon has made establishing a satisfactory pre-Neogene palaeogeographic and palaeotectonic history for the area difficult. Research has allowed clarification of three fundamental issues that permit the setting of Lebanon against its adjacent regions to be more firmly assigned. The first issue is the identification of the significance of the 100-km-long NNE–SSW-trending monocline of the Western Lebanon Flexure. This feature is interpreted as a modified and rejuvenated early Mesozoic hinge line that is presumed to be the northern continuation of the Carmel–Sinai hinge line to the south. The second issue is the correlation of the Mount Lebanon, Bekaa and Anti-Lebanon complex with the adjacent intracontinental Palmyride Fold Belt of Syria through recognition in each of common divisions in structural style between a higher northern zone dominated by broadly spaced box folds and a lower southern zone with more numerous, closely spaced and more internally complex folds. The third issue involves the identification of major two-stage Late Cretaceous to Palaeogene Syrian arc tectonism within Lebanon. Resolution of these issues suggests a tectonic model for the region whereby the Levantine region formed in two main phases. The first of these was the multiphase late Palaeozoic–early Mesozoic rifting of the area, which imposed a fundamental extensional fabric over much of the region. The second tectonic phase was the subsequent inversion of these extensional structures in the two-stage Syrian Arc deformation of Senonian and late Palaeogene age which gave rise to the main Lebanon structures. It is this Syrian Arc tectonism, rather than the late Cenozoic strike-slip, that was probably the main structuring agent which affected the Lebanon area. The transpression from the braided Dead Sea fault motion, although locally complex, is an essentially neotectonic overprint on an already existing fabric. Offset of Syrian Arc structures in Lebanon suggests that the total sinistral motion of the Yammouneh Fault segment of the Dead Sea Fault is around 47 km.

Neotectonic intersection of the Aegean and Cyprus tectonic arcs: extensional and strike-slip faulting in the Isparta Angle, SW Turkey

The Isparta Angle and adjacent Antalya Bay areas constitute an important segment of the eastern Mediterranean region, located at the intersection of the southward-convex Aegean and Cyprus arcs. Some recent tectonic maps show the Isparta Angle as a NW–SE compressional lineament extending eastwards into the Kyrenia Range of northern Cyprus. However, fault data from the onshore Isparta Angle, together with offshore shallow seismic reflection data, show that the present morpho-tectonic setting is dominated by extension. The last phase of compression to affect the area studied in the Late Miocene, was accompanied by regional nappe emplacement (Lycian Nappes). Onshore, fault planes, measured from fault zones bounding both the limbs and the core of the Isparta Angle are oriented predominantly NE–SW, NW–SE and N–S. Superimposed slickenfibres show that reverse faults were succeeded, in turn, by right-lateral faults, then by normal faults. The fault phases are dated by stratigraphical and geomorphological evidence. Reverse faults date from the Late Miocene, or earlier compressional deformation, whereas the right-lateral faults mainly developed during latest Miocene–Early Pliocene. Normal faulting dominated from the Late Pliocene–Recent. An interpretation of shallow seismic reflection data shows that Antalya Bay is characterised by a NW–SE-trending asymmetrical graben system that has continued to be active. During the Late Miocene–Early Pliocene right-lateral strike-slip resulted from shear along the eastern termination of a zone of extension and rotation that characterises the western Aegean. This shear was focused in a N–S direction by inherited zones of structural weakness in the basement (Antalya Complex). The switch to NE–SW extension in the Late Pliocene–Quaternary relates to a regional change in stress direction throughout the Aegean region and was accompanied by strong uplift of the Bey Daǧları region of the Taurus Mountains, bordering the Isparta Angle in the west. The Isparta Angle is the link between: (a) the extensional province of western Turkey bounded to the south by the actively subducting Hellenic arc; and (b) the uplifted Anatolian plateau bounded to the south by the Cyprus subduction zone. Understanding the Miocene to Recent tectonic development helps elucidate the kinematics of the region. The new structural data presented lend no support for recent suggestions that the Isparta Angle and Antalya Bay represent parts of a regional compressional zone related to plate collision.