Plate tectonic model for the oligo-miocene evolution of the western Mediterranean

This paper outlines a plate tectonic model for the Oligo-Miocene evolution of the western Mediterranean which incorporates recent data from several tectonic domains (Corsica, Sardinia, the Kabylies, Balearic promontory, Iberia, Algero-Provençal Basin and Tunisian Atlas). Following late Mesozoic anticlockwise rotation of the Iberian peninsula (including the Balearic promontory and Sardinia), late Eocene collision occurred between the Kabylies and Balearic promontory forming a NE-trending suture with NW-tectonic polarity. As a result of continued convergence between the African and European plates, a polarity flip occurred and a southward-facing trench formed south of the Kabylie—Balearic promontory suture. During late Oligocene time an E-W-trending arc and marginal basin developed behind the southward-facing trench in the area of the present-day Gulf of Lion. Opening of this basin moved the Corsica—Sardinia—Calabria—Petit Kabylie—Menorca plate southward, relative to the African plate. Early Miocene back-arc spreading in the area between the Balearic promontory and Grand Kabylie emplaced the latter in northern Algeria and formed the South Balearic Basin. Coeval with early Miocene back-arc basin development, the N-S-extension in the Gulf of Lion marginal basin changed to a more NW-SE direction causing short-lived extension in the area of the present-day Valencia trough and a 30° anticlockwise rotation of the Corsica-Sardinia-Calabria—Petit Kabylie plate away from the European plate. Early—middle Miocene deformation along the western Italian and northeastern African continental margins resulted from this rotation. During the early late Miocene (Tortonian), spreading within a sphenochasm to the southwest of Sardinia resulted in the emplacement of Petit Kabylie in northeastern Algeria.     

Tethyan evolution of Turkey: A plate tectonic approach

The Tethyan evolution of Turkey may be divided into two main phases, namely a Palaeo-Tethyan and a Neo-Tethyan, although they partly overlap in time. The Palaeo-Tethyan evolution was governed by the main south-dipping (present geographic orientation) subduction zone of Palaeo-Tethys beneath northern Turkey during the Permo-Liassic interval. During the Permian the entire present area of Turkey constituted a part of the northern margin of Gondwana-Land. A marginal basin opened above the subduction zone and disrupted this margin during the early Triassic. In this paper it is called the Karakaya marginal sea, which was already closed by earliest Jurassic times because early Jurassic sediments unconformably overlie its deformed lithologies. The present eastern Mediterranean and its easterly continuation into the Bitlis and Zagros oceans began opening mainly during the Carnian—Norian interval. This opening marked the birth of Neo-Tethys behind the Cimmerian continent which, at that time, started to separate from northern Gondwana-Land. During the early Jurassic the Cimmerian continent internally disintegrated behind the Palaeo-Tethyan arc constituting its northern margin and gave birth to the northern branch of Neo-Tethys. The northern branch of Neo-Tethys included the Intra-Pontide, Izmir—Ankara, and the Inner Tauride oceans. With the closure of Palaeo-Tethys during the medial Jurassic only two oceanic areas were left in Turkey: the multi-armed northern and the relatively simpler southern branches of Neo-Tethys. The northern branch separated the Anatolide—Tauride platform with its long appendage, the Bitlis—Pötürge fragment from Eurasia, whereas the southern one separated them from the main body of Gondwana-Land. The Intra-Pontide and the Izmir—Ankara oceans isolated a small Sakarya continent within the northern branch, which may represent an easterly continuation of the Paikon Ridge of the Vardar Zone in Macedonia. The Anatolide-Tauride platform itself constituted the easterly continuation of the Apulian platform that had remained attached to Africa through Sicily. The Neo-Tethyan oceans reached their maximum size during the early Cretaceous in Turkey and their contraction began during the early late Cretaceous. Both oceans were eliminated mainly by north-dipping subduction, beneath the Eurasian, Sakaryan, and the Anatolide- Tauride margins. Subduction beneath the Eurasian margin formed a marginal basin, the present Black Sea and its westerly prolongation into the Srednogorie province of the Balkanides, during the medial to late Cretaceous. This resulted in the isolation of a Rhodope—Pontide fragment (essentially an island arc) south of the southern margin of Eurasia. Late Cretaceous is also a time of widespread ophiolite obduction in Turkey, when the Bozkir ophiolite nappe was obducted onto the northern margin of the Anatolide—Tauride platform. Two other ophiolite nappes were emplaced onto the Bitlis—Pötürge fragment and onto the northern margin of the Arabian platform respectively. This last event occurred as a result of the collision of the Bitlis—Pötürge fragment with Arabia. Shortly after this collision during the Campanian—Maastrichtian, a subduction zone began consuming the floor of the Inner Tauride ocean just to the north of the Bitlis—Pötürge fragment producing the arc lithologies of the Yüksekova complex. During the Maastrichtian—Middle Eocene interval a marginal basin complex, the Maden and the Çüngüş basins began opening above this subduction zone, disrupting the ophiolite-laden Bitlis—Pötürge fragment. The Anatolide-Tauride platform collided with the Pontide arc system (Rhodope—Pontide fragment plus the Sakarya continent that collided with the former during the latest Cretaceous along the Intra Pontide suture) during the early to late Eocene interval. This collision resulted in the large-scale south-vergent internal imbrication of the platform that produced the far travelled nappe systems of the Taurides, and buried beneath these, the metamorphic axis of Anatolia, the Anatolides. The Maden basin closed during the early late Eocene by north-dipping subduction, synthetic to the Inner-Tauride subduction zone that had switched from south-dipping subduction beneath the Bitlis—Pötürge fragment to north dipping subduction beneath the Anatolide—Tauride platform during the later Palaeocene. Finally, the terminal collision of Arabia with Eurasia in eastern Turkey eliminated the Çüngüş basin as well and created the present tectonic regime of Turkey by pushing a considerable piece of it eastwards along the two newly-generated transform faults, namely those of North and East Anatolia. Much of the present eastern Anatolia is underlain by an extensive mélange prism that accumulated during the late Cretaceous—late Eocene interval north and east of the Bitlis—Pötürge fragment.     

Kurosegawa zone and its bearing on the development of the Japanese Islands

The Kurosegawa zone in southwest Japan is a 600 km long serpentinite mélange in the Chichibu terrains. It runs generally E-W but is slightly oblique to the subparallel arrangement of the Ryoke, Sanbagawa and Chichibu belts of Southwest Japan. A variety of geological units occurs in the Kurosegawa zone:

1. (1) granodiorite, gneiss and amphibolite of ca. 400 Ma,

2. (2) Siluro-Devonian formations,

3. (3) Upper Carboniferous to Jurassic formations,

4. (4) Upper Jurassic to Lower Cretaceous formations,

5. (5) serpentinite and

6. (6) low- to medium-grade metamorphic rocks of various baric types (ages, 220, 320, 360 and 420 Ma by K-Ar).

The most widespread is a high-pressure intermediate group of metamorphic rocks. Serpentinite is emplaced along the faults between and within the constituent units.Rocks of the Kurosegawa zone represent a mature orogenic belt along a continental margin or an island arc. Its original site as constrained by paleomagnetism was near the equatorial area. Here, 400 Ma old paired metamorphism and related magmatism took place. The island arc or microcontinent migrated northward to collide with the Eurasia plate during Late Jurassic, thus consuming the intervening ocean.     

Incipient subduction of the Ontong Java Plateau along the North Solomon trench

The Ontong Java Plateau (OJP) in the western central Pacific is the largest and thickest oceanic plateau and one of a few oceanic plateaus converging on an island arc (Solomon island arc—SIA). To better understand the evolution of the North Solomon trench (NST), active oblique convergence between the OJP and SIA, and late Neogene development of Malaita accretionary prism (MAP), we present 850 km of multichannel seismic reflection data integrated with 7832 km² of IZANAGI side-scan sonar coverage. We have focussed the study at the transition area between the well-defined northwestern end of the North Solomon trench and a diffusely deformed area where the trench is actively propagating in a northwestward direction. The deeper structure beneath the survey area is discussed by Phinney et al. [Oceanic plateau accretion in the Malaita accretionary prism inferred from multi-channel seismic reflection data, this issue] using deeper penetration, multichannel seismic reflection lines. The serial cross sections provided by multichannel seismic profiling combined with the IZANAGI backscattering imagery provides a time series evolution for the development of the North Solomon trench. The main evolutionary stages include (1) the incipient trench in the northern area marked by a diffuse zone of deformation above a broad arch in the crust. Deeper penetration profiles by Phinney et al. show the bulge is related to a deeper decollement fault that is propagating upward and seaward through the crust. (2) The formation of a continuous thrust front in the central area. Deeper penetration profiles by Phinney et al. show this thrust front is surface expression of the same decollement present at depth to the north. The boundary between the surface trace of the thrust and the diffuse area of deformation in the northern area is inferred as a vertical, high-angle tear fault with left-lateral offset. (3) The formation of a deep, elongate trench which controls gravitationally related slumping and sedimentation around the steep edges of the trench fill basin. The areas to the southeast are those that have undergone convergence for the longest period of time and therefore show better developed trench structures and a reduced width of the MAP. Areas to the northwest have undergone convergence for a shorter period of time and show less developed trench structures and a wide area of the MAP.     

Tectonic setting required for the preservation of sedimentary mélanges in Palaeozoic and Mesozoic accretionary complexes of southwest Japan

The Palaeozoic to Mesozoic accretionary complexes of southwest Japan include various types of mélange. Most mélanges are polygenetic in origin, being sedimentary or diapiric mélanges that were overprinted by tectonic deformation during subduction. Sedimentary mélanges, without a tectonic overprint, are present in the Permian accretionary complexes of the Akiyoshi and Kurosegawa belts and in the Early Cretaceous accretionary complex of the Chichibu Belt. These mélanges are characterized by dominant basalt and limestone clasts, within a mudstone matrix. The basalt and limestone clasts within the sedimentary mélanges were derived from ancient seamounts. Subduction of a seamount results in deformation of the pre-existing accretionary wedge, and it is difficult to incorporate a seamount into an accretionary wedge; therefore, preservation of seamount fragments requires a special tectonic setting. Oceanic plateau accretion might play an important role in interrupting the processes of subduction and accretion during the formation of accretionary complexes. Especially the Mikabu oceanic plateau might have caused the cessation of accretion during the Early Cretaceous. The subduction and accretion of volcanic arcs and oceanic plateaux helps to preserve sedimentary mélanges from tectonic overprinting by preventing further subduction.     

Bilelyeri Group, Antalya Complex: deposition on a Mesozoic passive continental margin, south-west Turkey

The Bilelyeri Group comprises complexly deformed Mesozoic sedimentary rocks of continental-margin affinities (Kumluca Zone). These are structurally intercalated between a coeval carbonate platform to the west (Bey Daǧlari Zone) and late Triassic ophiolitic rocks and sediments, interpreted as emplaced marginal oceanic crust, to the east (Gödene Zone). Four formations erected in the Bilelyeri Group record the later stages of continental rifting and the progressive development of part of a Mesozoic passive continental margin. The two late Triassic formations, the Telekta? Tepe and the Hatipalani Formations, are dominated by terrigenous clastic and calcareous clastic sediments, including large detached blocks of reef limestone. These rocks were laid down by mostly mass-flow and turbidity-flow into steep-sided rift depressions. Organic reefs were constructed in bordering shallow seas while terrigenous clastic sediment was shed from exposed basement horsts. Thick sequences of mafic lavas were extruded (Norian) in axial parts of the rift zones, followed by a regional change to deposition of pelagic Halobia-bearing limestone. This culminated in a major hiatus involving large-scale sliding of shallow-water limestones into deeper water. The Jurassic to early Cretaceous Dereköy Formation mostly consists of siltstones, radiolarian cherts and mudstones, intercalated with redeposited limestones and black shales. During this time parts of the margin were bordered by major offshore carbonate complexes constructed partly on basement fragments previously rifted off the parent continental areas. Black shales and reduced hemipelagic sediments were deposited in an elongate trough between the main platform and an offshore complex to the east. Some degree of margin reactivation in the early Cretaceous is indicated by renewed deposition of turbiditic sandstone and chloritic clays in some distal sequences. Strong relative enrichment of manganese in some horizons is attributed to offshore volcanic exhalations. Subsequent regional subsidence in the mid-to late Cretaceous is suggested by a switch to predominantly calcareous, pelagic sedimentation on the adjacent platform and the offshore massifs as well as on the Bilelyeri margin. Tectonic disruption of the platform edge during the late Cretaceous is implied by major redeposition of shallow-water shelf limestones in proximal Bilelyeri sequences. The Bilelyeri margin and the adjacent Gödene Zone were tectonically deformed in latest Cretaceous to early Tertiary time and were thrust over the adjacent Bey Daǧlari platform in the early Miocene. Viewed in an East Mediterranean perspective, the Bilelyeri sequences were part of a locally north-south trending segment of a regionally east-west margin to a substantial oceanic area further south. This segment apparently suffered significant strike-slip deformation both during its construction and its later emplacement. Instructive comparisons can be made with other areas of the East Mediterranean, especially south-west Cyprus.     

Neoproterozoic (800 Ma) orogeny in the Tuva-Mongolia Massif (Siberia): island arc–continent collision at the northeast Rodinia margin

The Tuva-Mongolia Massif is a composite Precambrian terrane incorporated into the Palaeozoic Sayany-Baikalian belt. Its Neoproterozoic amalgamation history involves early (800 Ma) and late Baikalian (600–550 Ma) orogenic phases. Two palaeogeographic elements are identified in the early Baikalian stage — the Gargan microcontinent and the Dunzhugur oceanic arc. They are represented by the Gargan Glyba (Block) and the island-arc ophiolites overthrusting it. The Gargan Glyba is a two-layer platform comprising an Early Precambrian crystalline basement and a Neoproterozoic passive-margin sedimentary cover. The upper part comprises olistostromes deposited in a foreland basin during the early Baikalian orogeny. The Dunzhugur arc ophiolite form klippen fringing the Gargan Glyba, and show a comprehensive oceanic-arc ophiolite succession. The Dunzhugur arc faced the microcontinent, as shown by the occurrence of forearc complexes. The arc–continent collision followed a pattern similar to Phanerozoic collisions. When the marginal basin lithosphere had been completely subducted, the microcontinental edge partially underthrust the arc, and the forearc ophiolite overrode it. Continued convergence caused a break of the arc lithosphere resulting in the uplift of the submerged microcontinental margin with the overthrust forearc ophiolites sliding into the foreland basin. Owing to the lithospheric break, a new subduction zone, inclined beneath the Gargan microcontinent, emerged. Initial melts of the newly-formed continental arc are represented by tonalites intruded into the Gargan microcontinent basement and its cover, and into the ophiolite nappe. The tonalite Rb–Sr mineral isochron age is 812±18 Ma, which is similar to a U–Pb zircon age of 785±11 Ma. A period of tonalite magmatism in Meso–Cenozoic orogenic belts is recognized some 1–10 m.y. after the collision. Accordingly, the Dunzhugur island arc–Gargan microcontinent collision is conventionally dated at around 800 Ma. It is highly probable that in the early Neoproterozoic, the Gargan continental block was part of the southern (in modern coordinates) margin of the Siberia craton. It is suggested that a chain of Precambrian massifs represents an elongate block separated from Siberia in the late Neoproterozoic. The Tuva-Mongolia Massif is situated in the northwest part of this chain. These events occurred on the NE Neoproterozoic margin of Rodinia, facing the World Ocean.     

Evolution of the Taupo-Hikurangi subduction system

The present day Taupo-Hikurangi subduction system is a southward extension of the Tonga-Kermadec Arc system into a sediment-rich continental margin environment. It consists of a shallow structural trench (the Hikurangi Trough), a 150 km wide, imbricate thrust controlled accretionary borderland (the continental slope, shelf, and coastal hills of eastern North Island), a frontal ridge (the main “greywacke” ranges of North Island), and a volcanic arc and marginal basin (the Taupo Volcanic Zone).Structural elements become progressively more elevated and subduction more oblique towards the south. The whole NNE-trending system is truncated at a largely strike-slip, transform boundary that extends along the southwestern part of the Hikurangi Trough and the Hope fault of South Island to the main Alpine Fault.The volcanic arc is 200–270 km from the structural trench and comprises a NNE trending chain of andesite-dacite volcanoes extending along the eastern side of the Taupo Volcanic Zone. Most of the andesites are olivine-bearing and have been erupted within the last 50,000 years.It is suggested the Taupo-Hikurangi margin has evolved by rotation of accretionary elements, from an original NW-trending subduction system north of New Zealand. The older elements of the prism were associated with subduction of a re-entrant of the Pacific Plate (and perhaps the South Fiji Basin) in Mid Tertiary times. They subsequently became separated from their NW-trending volcanic arc by dextral strike-slip movement along curved faults east of the main “greywacke” ranges. During the Plio-Pleistocene, oblique subduction and accretion intensified as the Taupo-Hikurangi margin rotated into line with the NNE-trending Kermadec system and a marginal basin was developed along a similar trend to form the Taupo Volcanic Zone. Within the last 50,000 years olivine-bearing andesite volcanism has commenced along the eastern side of the Taupo Volcanic Zone.     

Past and present geotectonic position of Sulawesi, Indonesia

Sulawesi with its peculiar K-shaped pattern is situated in an area where the Eurasian, Indian—Australian and Pacific plates interact and collide.Complex geological processess in this area resulted in the transformation of a normal island-arc structure into an inverted one, deformation of an already tectonized belt, sweeping of fragments against unrelated terrain, thrusting of oceanic and mantle material over the island arc, closing of deep-sea basins behind the arc, trapping of old oceanic crust caused by the rolling up of an island arc, formation of a marginal basin by the spreading of the sea floor behind the arc, development of small subduction zones with reverse polarities etc.Small deep-sea basins surrounding Sulawesi such as the Gulf of Bone and the Gulf of Gorontalo originally formed the arc—trench gap of the Sulawesi island arc.The Banda Sea is considered as an oceanic crust trapped by the bending of the east—west trending Banda arc due to the northward drift of Australia combined with the westward movement of the Pacific plate. Similarly the Sulawesi Sea consists of an old Pacific crust trapped by the westward bending of the Sulawesi island arc, caused by the spearheading westward thrust along the Sorong transform-fault system, in which later a minor spreading center became active in its central part. The Molucca Sea comprises tectonic mélange in which presumably a small spreading center developed between the two colliding arcs of northern Sulawesi and western Halmahera. While the Benioff zones dip under the northern Sulawesi and Halmahera arcs in normal fashion, the mélange thrusts over them. The Strait of Makassar is a marginal basin which was brought into existence by the spreading of the sea floor between Kalimantan and Sulawesi.The evolution of Sulawesi started in Miocene time or even earlier when 800 km east of Kalimantan a north—south trending east-facing island arc came into existence, originating from a spreading center located in the Pacific Ocean. Volcanism and plutonism accompanied this subduction process.Collision between Sulawesi and the Australian—New Guinea plate which occurred in early Pliocene time severely transformed Sulawesi into an island with its convex side turned towards the continent, at the same time causing obduction of ophiolite in the eastern arc of this island.The movement of the Pacific plate continued and gradually pushed Sulawesi towards the Asian continent, resulting in the closing of the sea between Kalimantan and Sulawesi islands separated by small straits and deep seas resembling the complicated pattern of the Philippine Archipelago, in which the original double island-arc structure can no longer be recognized.     

Mesozoic tectonothermal development of the Sambagawa, Mikabu and Chichibu belts, south-west Japan: evidence from 40Ar/39Ar whole-rock phyllite ages

Abstract Five whole-rock ⁴⁰Ar/³⁹Ar plateau ages from low-grade sectors of the Sambagawa belt (Besshi nappe complex) range between 87 and 97 Ma. Two whole-rock phyllite samples from the Mikabu greenstone belt record well-defined ⁴⁰Ar/³⁹Ar plateau ages of 96 and 98 Ma. Together these ages suggest that a high-pressure metamorphism occurred in both the Sambagawa and Mikabu belts at c. 90–100 Ma. The northern Chichibu sub-belt may consist of several distinct geochronological units because metamorphic ages increase systematically from north ( c. 110 Ma) to south ( c. 215 Ma). The northern Chichibu sub-belt is correlated with the Kuma nappe complex (Sambagawa belt). Two whole-rock phyllite samples from the Kurosegawa terrane display markedly older metamorphic ages than either the Sambagawa or the Chichibu belts.
Accretion of Sambagawa-Chichibu protoliths began prior to the middle Jurrasic. Depositional ages decrease from middle Jurassic (Kuma-Chichibu nappe complex) to c. 100 Ma (Oboke nappe complex) toward lower tectonostratigraphic units. The ages of metamorphic culmination also decrease from upper to lower tectonostratigraphic units. The Kurosegawa belt and the geological units to the south belong to distinctly different terrances than the Sambagawa-Chichibu belts. These have been juxtaposed as a result of transcurrent faulting during the Cretaceous.     

Structural zones and continental collision, Central Alps

The traverse of the Central Alps between Lake Constance and Lake Como (eastern Switzerland, northern Italy) allows the reconstruction of a cross-section through a collision belt some 140 km wide and 40 km deep. It can be described in terms of a series of structural zones (A–F), defined by the age and character of the latest phase of penetrative deformation affecting both basement and cover rocks, each zone showing a characteristic structural history. These zones do not coincide with the well-known tectono-stratigraphic Alpine subdivisions (Helvetic, Pennine, Austroalpine) which are based on gross geometry, facies and petrography. Zones A and B, in the north, developed during late Oligocene and Miocene times, affecting the Helvetic realm and the already overlying Pennine and Austroalpine units. Zone A is characterized by a steeply dipping penetrative cleavage S_A, zone B by the same cleavage later modified by nappe-forming movements. Zone F, in the south, also developed during the late Oligocene and Miocene, first as a monoclinal flexure, later as a steeply dipping zone of mylonitization and cataclasis (foliation S_f), affecting Pennine and Austroalpine units. The final manifestation of these movements was the Tonale line and their net result was the uplift of the region to the north by about 20 km. Between these two belts lay an area in which late Oligocene-Miocene movements had little effect — structural zones C (Pennine), D (Pennine-Austroalpine transition) and E (Austroalpine). In zones C and D, the latest phase of penetrative deformation, resulting in large recumbent fold structures and a penetrative foliation S_c zone C, can be dated as late Eocene-early Oligocene. This seems to be related to the overriding of the Austroalpine nappe complex (zone E), which already showed the effects of a late Cretaceous orogeny.Unravelling these events backwards, reveals, at the Eocene—Oligocene boundary, a southward dipping subduction zone in the process of locking. Its mouth is full of upper Cretaceous-Eocene flysch; its throat is choked by the Pennine nappe complex, undergoing the s_c ductile deformation. Before subduction, the Pennine nappe complex can best be described as a mega-mélange-a tectonic mixture of large fragments of continental basement, oceanic basement, trough-facies cover and platform-facies cover, already showing a complicated structural history. It is supposed that collision started in mid-Cretaceous times, not at a single subduction suture (trench), but by complicated surficial processes across a wide zone, as non-matching, rifted and thinned continental margins approached and small oceanic remnants were obducted. Post-mid-Oligocene events are essentially intra-plate compressional effects, combined with isostatic response.     

Cretaceous–Tertiary convergence and continental collision,Sanandaj–Sirjan Zone,western Iran

The Sanandaj–Sirjan Zone contains the metamorphic core of the Zagros continental collision zone in western Iran. The zone has been subdivided into the following from southwest to northeast: an outer belt of imbricate thrust slices (radiolarite, Bisotun, ophiolite and marginal sub-zones, which consist of Mesozoic deep-marine sediments, shallow-marine carbonates, oceanic crust and volcanic arc, respectively) and an inner complexly deformed sub-zone (late Palaeozoic–Mesozoic passive margin succession). Rifting and sea-floor spreading of Tethys occurred in the Permian to Triassic but in the Sanandaj–Sirjan Zone extension-related successions are mainly of Late Triassic age. Subduction of Tethyan sea floor in the Late Jurassic to Cretaceous produced deformation, metamorphism and unconformities in the marginal and complexly deformed sub-zones. Deformation climaxed in the Late Cretaceous when a major southwest-vergent fold belt formed associated with greenschist facies metamorphism and post-dated by abundant Palaeogene granitic plutons. In the southwest of the zone a Late Cretaceous island arc—passive margin collision occurred with ophiolite emplacement onto the northern Arabian margin similar to that in Oman. Final closure of Tethys was not completed until the Miocene when Central Iran collided with the northeast Arabian margin.     

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.     

Lithostratigraphy and biostratigraphy of the Campanian–Maastrichtian Toyajo Formation in Wakayama, southwestern Japan

The Upper Cretaceous Toyajo Formation is distributed around the Mt. Toyajo in the Aridagawa area, Wakayama, southwestern Japan. The formation is subdivided into three newly defined members, the Nakaibara Siltstone Member, Hasegawa Muddy Sandstone Member, and Buyo Sandstone Member, in ascending order. Close field observation elucidated the detailed biostratigraphy of the Toyajo Formation, and high-precision biostratigraphic correlation was made with the Yezo Group in Hokkaido (northern Japan) and Sakhalin and the Izumi Group in southwestern Japan.The Toyajo Formation contains diversified lower Campanian to upper Campanian heteromorph ammonoid assemblages, including Eubostrychoceras and Scaphites. Discovery of the heteromorph fauna demonstrates that scaphitid ammonoids survived until Campanian time in the northwestern Pacific region. Although Eubostrychoceras elongatum has been known in the northeastern Pacific region, the occurrence of this species in the northwestern Pacific region has been uncertain before. The rich occurrence of E. elongatum in the Aridagawa area indicates that this species was distributed widely in the northern Pacific realm.The Toyajo Formation is similar to the Izumi Group in various geologic features, and may indicate that the Toyajo Formation was deposited in a strike-slip basin along the Chichibu Belt formed by the movement along the Kurosegawa Tectonic Zone in the latest Cretaceous, like the Izumi Group, along the Median Tectonic Line.     

Restoration of Exotic Terranes Along the Median Tectonic Line, Japanese Islands: Overview

This paper reviews recent progress on the geotectonic evolution of exotic Paleozoic terranes in Southwest Japan, namely the Paleo-Ryoke and Kurosegawa terranes. The Paleo-Ryoke Terrane is composed mainly of Permian granitic rocks with hornfels, mid-Cretaceous high-grade metamorphic rocks associated with granitic rocks, and Upper Cretaceous sedimentary cover. They form nappe structures on the Sambagawa metamorphic rocks. The Permian granitic rocks are correlative with granitic clasts in Permian conglomerates in the South Kitakami Terrane, whereas the mid-Cretaceous rocks are correlative with those in the Abukuma Terrane. This correlation suggests that the elements of Northeast Japan to the northeast of the Tanakura Tectonic Line were connected in between the paired metamorphic belt along the Median Tectonic Line, Southwest Japan. The Kurosegawa Terrane is composed of various Paleozoic rocks with serpentinite and occurs as disrupted bodies bounded by faults in the middle part of the Jurassic Chichibu Terrane accretionary complex. It is correlated with the South Kitakami Terrane in Northeast Japan. The constituents of both terranes are considered to have been originally distributed more closely and overlay the Jurassic accretionary terrane as nappes. The current sporadic occurrence of these terranes can possibly be attributed to the difference in erosion level and later stage depression or transtension along strike-slip faults. The constituents of both exotic terranes, especially the Ordovician granite in the Kurosegawa-South Kitakami Terrane and the Permian granite in the Paleo-Ryoke Terrane provide a significant key to reconstructing these exotic terranes by correlating them with Paleozoic granitoids in the eastern Asia continent.     

Kurosegawa Terrane as a Transform Fault Zone in Southwest Japan

The Kurosegawa Terrane intervening in the Jurassic-Early Cretaceous accretionary complexes along the Pacific side of the SW Japanese Islands is a serpentinite mélange zone. It contains various kinds of exotic rocks, for example, granitoids, metamorphic rocks, Siluro-Devonian deposits and is intimately associated with Cretaceous forearc basin deposits. The terrane is regarded as a key to clarify the Mesozoic geotectonic history of the western circum-Pacific orogenic belts. The current model, in which the formation of the Kurosegawa Terrane is attributed to nappe-movement or sinistral strike-slip faulting, can explain neither the mode of occurrence of the Kurosegawa Terrane we observed in eastern Kii Peninsula nor the array of evidence obtained from the Ryoke Terrane southward to the Shimanto Terrane. We suggest a new hypothesis in which the Kurosegawa Terrane was a transform fault zone that originated because of oceanic ridge subduction along the southern margin of the coeval accretionary prism (Butsuzo T.L.) in the late Early Cretaceous. Our model is mainly based on new geological evidence from the Kurosegawa Terrane in eastern Kii Peninsula where the deepest erosion level is exposed due to neotectonic uplift.     

Oligocene to Recent tectonic history of the Central Solomon intra-arc basin as determined from marine seismic reflection data and compilation of onland geology

Systematic analysis of a grid of 3450 km of multichannel seismic reflection lines from the Solomon Islands constrains the late Tertiary sedimentary and tectonic history of the Solomon Island arc and its convergent interaction with the Cretaceous Ontong Java oceanic plateau (OJP). The OJP, the largest oceanic plateau on Earth, subducted beneath the northern edge of the Solomon arc in the late Neogene, but the timing and consequences of this obliquely convergent event and its role in the subduction polarity reversal process remain poorly constrained. The Central Solomon intra-arc basin (CSB), which developed in Oligocene to Recent time above the Solomon arc, provides a valuable record of the tectonic environment prior to and accompanying the OJP convergent event and the subsequent arc polarity reversal. Recognition of regionally extensive stratigraphic sequences—whose ages can be inferred from marine sedimentary sections exposed onland in the Solomon Islands—indicate four distinct tectonic phases affecting the Solomon Island arc. Phase 1: Late Oligocene–Late Miocene rifting of the northeast-facing Solomon Island arc produced basal, normal-fault-controlled, asymmetrical sequences of the CSB; the proto-North Solomon trench was probably much closer to the CSB and is inferred to coincide with the trace of the present-day Kia-Kaipito-Korigole (KKK) fault zone; this protracted period of intra-arc extension shows no evidence for interruption by an early Miocene period of convergent “soft docking” of the Ontong Java Plateau as proposed by previous workers. Phase 2: Late Miocene–Pliocene oblique convergence of the Ontong Java Plateau at the proto-North Solomon trench (KKK fault zone) and folding of the CSB and formation of the Malaita accretionary prism (MAP); the highly oblique and diachronous convergence between the Ontong Java plateau and the Solomon arc terminates intra-arc extension first in the southeast (Russell subbasin of the CSB) during the Late Miocene and later during the Pliocene in the northwest (Shortland subbasin of the CSB); folds in the CSB form by inversion of normal faults formed during Phase 1; Phinney et al. [Sequence stratigraphy, structural style, and age of deformation of the Malaita accretionary prism (Solomon arc-Ontong Java Plateau convergent zone)] show a coeval pattern of southeast to northwest younging in folding and faulting of the MAP. Phase 3: Late Pliocene–early Pleistocene arc polarity reversal and subduction initiation at the San Cristobal trench. Effects of this event in the CSB include the formation of a chain of volcanoes above the subducting Australia plate at the San Cristobal trench, the formation of the broad synclinal structure of the CSB with evidence for truncation at the uplifted flanks, and widespread occurrence of slides and “seismites” (deposits formed by seismic shaking). Phase 4: Pleistocene to Recent continued shortening and synclinal subsidence of the CSB. Continued Australia-Pacific oblique plate convergence has led to deepening of the submarine, elongate basin axis of the synclinal CSB and uplift of the dual chain of the islands on its flanks.     

Intraplate mountain building in response to continent–continent collision—the Ancestral Rocky Mountains (North America) and inferences drawn from the Tien Shan (Central Asia)

The intraplate Ancestral Rocky Mountains of western North America extend from British Columbia, Canada, to Chihuahua, Mexico, and formed during Early Carboniferous through Early Permian time in response to continent–continent collision of Laurentia with Gondwana—the conjoined masses of Africa and South America, including Yucatán and Florida. Uplifts and flanking basins also formed within the Laurentian Midcontinent. On the Gondwanan continent, well inboard from the marginal fold belts, a counterpart structural array developed during the same period. Intraplate deformation began when full collisional plate coupling had been achieved along the continental margin; the intervening ocean had been closed and subduction had ceased—that is, the distinction between upper versus lower plates became moot. Ancestral Rockies deformation was not accompanied by volcanism. Basement shear zones that formed during Mesoproterozoic rifting of Laurentia were reactivated and exerted significant control on the locations, orientations, and modes of displacement on late Paleozoic faults.Ancestral Rocky Mountain uplifts extend as far south as Chihuahua and west Texas (28° to 33°N, 102° to 109°W) and include the Florida-Moyotes, Placer de Guadalupe–Carrizalillo, Ojinaga–Tascotal and Hueco Mountain blocks, as well as the Diablo and Central Basin Platforms. All are cored with Laurentian Proterozoic crystalline basement rocks and host correlative Paleozoic stratigraphic successions. Pre-late Paleozoic deformational, thermal, and metamorphic histories are similar as well. Southern Ancestral Rocky Mountain structures terminate along a line that trends approximately N 40°E (present coordinates), a common orientation for Mesoproterozoic extensional structures throughout southern to central North America.Continuing Tien Shan intraplate deformation (Central Asia) has created an analogous array of uplifts and basins in response to the collision of India with Eurasia, beginning in late Miocene time when full coupling of the colliding plates had occurred. As in the Laurentia–Gondwana case, structures of similar magnitude and spacing to those in Eurasia have developed in the Indian plate. Within the present orogen two ancient suture zones have been reactivated—the early Paleozoic Terskey zone and the late Paleozoic Turkestan suture between the Siberian and East Gondwanan cratons. Inverted Proterozoic to early Paleozoic rift structures and passive-margin deposits are exposed north of the Terskey zone. In the Alay and Tarim complexes, Vendian to mid-Carboniferous passive-margin strata and the subjacent Proterozoic crystalline basement have been uplifted. Data on Tien Shan uplifts, basins, structural arrays, and deformation rates guide paleotectonic interpretations of ancient intraplate mountain belts. Similarly, exhumed deep crustal shear zones in the Ancestral Rockies offer insight into partitioning and reorientation of strain during contemporary intraplate deformation.     

Crustal scale balanced cross-sections of the Pyrenees; discussion

From surface and subsurface data, line-length and area balancing were used to construct four balanced and restored sections of the Pyrenees. In the Mesozoic cover, a thin-skinned tectonic model is used. In the basement an anticlinal stack geometry is applied for the foreland part of the thrust nappes. We present and discuss three possible models for the deep structures of the belt: a thin-skinned tectonic model, a thick-skinned tectonic model and an inhomogeneous strain model. The thrusts steepen downwards and the displacements die out in ductile deformation deep in the section. Therefore, we use the inhomogeneous strain model and we equal-area balance the surface of the continental crust.Hanging-wall sequence diagrams are constructed taking into account (1) the strong N-S thickness variations of the Mesozoic cover related to the Cretaceous drift of Spain and (2) the related crustal thinning of the North Pyrenean Zone superimposed upon a previous late Hercynian rise of the lower crust.The Moho step at the vertical of the North Pyrenean Fault results from the thinning of the North Pyrenean Zone. The thickening of both the Axial Zone and the North Pyrenean Zone during the Eocene compressional event preserved the step geometry.Calculated values of the minimum shortening range from 55 km in the western part of the belt to 80 km in the eastern part. Most of the shortening occurs south of the North Pyrenean Fault in the eastern part (Axial Zone) and north of the North Pyrenean Fault in the western part (Labourd thrust).     

山东郯城麦坡中更新世地震事件记录*

山东郯城麦坡被命名为典型地震活动断层遗址,其最醒目的标志是郯庐断裂带主干断层(F₂)东盘的紫灰色下白垩统逆冲到断层西盘的红棕色第四系之上且界线截然。野外调查和试验分析表明,郯城麦坡第四系于泉组中发育液化砂涌管、液化砂脉、震裂缝充填构造和同沉积断层等地震引发的软沉积物变形构造——地震事件记录。根据软沉积物变形构造的砂质黏土光释光测年分析,推断这些软沉积物变形构造所记录的地震事件属郯庐断裂带主干断层F₂在中更新世晚期发生的强构造与地震活动。这些地震事件记录为研究郯庐断裂带新构造运动与地震活动提供了新资料,也丰富了该地震活动断层遗址的内涵。