bluuebunny/crossref_metadata_2025 huggingface dataset:
Application of net tectonic rotations and inclination analysis to a high-resolution palaeomagnetic study in the Betic Cordillera
Authors: Kirker A. , McClelland E.
Published: April 2008

Our initial structural investigations of the Sierra Endrinal thrust system in the western External Betic Cordillera suggested a differential block rotation associated with ramp and tipline structures in the thrust system. However, we present an analysis of new, high-resolution palaeomagnetic sampling which refutes this hypothesis and reveals a multicomponent remanence, including a syn-deformational component of magnetization. Due to the structural complications of the thrust system, our palaeomagnetic analysis required only the use of inclination data to determine the timing of magnetization, and comparison with expected inclinations to determine the degree of tilting prior to magnetization. In addition, a finite rotation study involving the combination of inclined axis rotations was preferred to simple bedding tilt corrections. This study emphasizes the usefulness of these techniques in structurally-complex regions, where there may be no valid basis for implementing a fold test, or applying the simple correction of untilting about strike. The study also demonstrates that parts of a thrust system may develop trends that are highly oblique to the regional norm without involving substantial differential rotation about a vertical axis.
New constraints on the bending of the Gibraltar Arc from palaeomagnetism of the Ronda peridotites (Betic Cordilleras, Spain)
Authors: Feinberg H. , Saddiqi O. , Michard A.
Published: April 2008

The study of 210 cores (15 sites) from the Ronda peridotites (Sierra Bermeja and Sierra Alpujata, Alpujarride nappe complex, Betic zone), and from the granites intruding these peridotites and their country rocks shows the occurrence of two stable antipodal directions of magnetization (
D
= 46°,
I
= 47°, α
95
= 6.6). The reverse polarity high-temperature component, only found in peridotites, is carried by hematite, while the normal polarity intermediate-temperature component is carried by magnetite in the peridotites, and by sulphides in the granites. Negative fold tests point to a late magnetization. The acquisition of remanence is attributed to the post-metamorphic cooling of the Alpujarrides, bracketed between 23 and 18 Ma by isotopic and stratigraphic data. Structural data and the homogeneity of the
in situ
mean palaeomagnetic directions preclude significant tilting of the massifs after their magnetization. The observed declination is interpreted as the result of a post-metamorphic, 46° ± 8° clockwise rotation of the Ronda massifs around a vertical axis. These results are compared with those from the Beni Bousera peridotites (southern branch of the Gibraltar Arc). In the latter massif, a
c.
74° ± 11° anticlockwise rotation has been documented, and dated from the time of cooling of the peridotite unit. Therefore the opposite rotations of the Spanish and Moroccan massifs occurred rapidly during Early Miocene. A tectonic model involving extensional collapse with preferential displacement towards the Atlantic free-margin is favoured.
Palaeomagnetism and magnetostratigraphy of the Middle Triassic in the Iberian Ranges (Central Spain)
Authors: Rey D. , Turner P. , Ramos A.
Published: April 2008

The study of the structure and origin of remanence in five sections comprising the Rillo Mudstones and Sandstones and Torete Multicoloured Mudstone and sandstone Formations (Middle Triassic, Central Spain) reveals the presence of several components of magnetization of different ages. Detrital hematite and ilmenohaematite carry a primary magnetization of Triassic origin in the form of a DRM or PDRM. In addition, it is likely that an early CRM also contributes to the primary remanence. Goethite and several textural phases of authigenic hematite are responsible for a recent overprint in the form of a late CRM. Thermal demagnetization allowed the isolation of a characteristic Triassic remanence on which a reliable magnetostratigraphy for Anisian-Ladinian times could be based, and correlated with sections of similar age in North America and Asia. However, it did not succeed in entirely removing the CRM associated with the latest component of the overprint which resulted in the primary remanence being biased by residual components of Recent origin.

Remanence acquisition was controlled by the sedimentary environment and the diagenetic processes that took place during burial and uplift. The sedimentary environment controlled the conditions of deposition and facies distribution which limited the occurrence of a particular diagenetic phase within the rock volume. Acquisition of secondary components of magnetization, and the consequent pervasive Holocene magnetization (in the form of a late CRM) is primarily related to coarser grained horizons. Sand body interconnectivity has also played an important role in the process. Primary remanence is better preserved in the finer-grained horizons. A PTRM associated with the uplift is easily removed during thermal demagnetization. Calculation of mean vectors by integrating data from adjacent magnetozones allows correction for the overprint bias. The corrected directions can then be closely compared with results from stable Europe.
Palaeomagnetism and palaeogeography of Adria
Authors: Channell J. E. T.
Published: April 2008

An ‘African’ Mesozoic apparent polar wander path (APWP) based on rotated North American data is compared with palaeomagnetic data from Adria and surrounding regions. The form of the APWPs from the Southern Alps, Umbria-Marche (Italy) and the Transdanubian Central Range (Hungary) is similar to that of the ‘African’ APWP. The Umbria-Marche and Transdanubian APWPs are rotated relative to the African APWP, and the rotation is best explained by local anticlockwise tectonic rotations of 20° and 30° for the NW Umbria-Marche and Transdanubian regions, respectively. Permian to Cretaceous palaeomagnetic poles from the Southern Alps are not distinguishable from the African APWP. The more recently derived Cretaceous poles from Istria, Cretaceous poles from Iblei (Sicily), Gargano and Apulia are all consistent, within the confidence limits of the data, with the African APWP. Rifting in the Ionian Sea/Eastern Mediterranean either predated the mainly Jurassic-Cretaceous palaeomagnetic dataset or did not produce recognizable rotation of Adria relative to Africa. The Northern Calcareous Alps are often considered to have been contiguous with the Southern Alps during the Mesozoic, however, the NCA (Liassic) poles are closer to coeval poles for Europe, and are rotated in excess of 80° clockwise relative to the African APWP. The Northern Calcareous Alps were probably detached from the Southern Alps during Late Triassic and Jurassic time, and did not undergo the ‘African’ rotation which affected Adria.
The pattern of crustal block rotations in the Italian region deduced from aeromagnetic anomalies
Authors: Fedi M. , Florio G. , Rapolla A.
Published: April 2008

In contrast to palaeomagnetic studies that consider outcropping rocks, the analysis of aeromagnetic anomalies can give averaged information about large volumes of deep-seated rocks. The recognition and analysis of anomalies having an ‘abnormal shape’ in the central Mediterranean area have yielded original and interesting results in recent years. Accurate interpretation of these anomalies gives information on the nature of magnetization of the deep crust and on the geodynamic history of this region. Results suggest that the southern Italian area can be divided into zones which experienced different tectonic rotations in both clockwise and anticlockwise senses. The pattern of rotations deduced in this way agrees well with that determined by standard palaeomagnetic analyses.
A review of palaeomagnetic research in the Troodos ophiolite, Cyprus
Authors: Morris Antony
Published: April 2008

A summary of the most significant results of palaeomagnetic research within the well-known Troodos ophiolite of Cyprus is presented. The studies are broadly divided into those relating to microplate rotation and those which shed light on processes operating during crustal genesis. Early recognition that the Troodos Complex as a whole experienced a 90° anticlockwise rotation generated several attempts to constrain the precise timing of rotation. A series of palaeomagnetic data obtained within the
in situ
Late Cretaceous to Recent sedimentary cover of the ophiolite demonstrate that rotation began in the Campanian, soon after Turonian crustal genesis, and was complete by the Early Eocene. Rotation occurred at an approximately constant rate of
c.
2° Ma
−1
. The most popular rotation mechanism is one involving a combination of oblique subduction to the south of Cyprus and collision of the trench with the Arabian continental margin to the east. A Troodos microplate became detached and rotated in response to the resultant anticlockwise torque exerted upon the supra-subduction zone crust. Detailed palaeomagnetic and structural studies have also revealed details of the Troodos spreading configuration, and specifically the relationships of horizontal and vertical axis rotations to ridge-parallel faulting and transform tectonism. Extension was achieved through a combination of magmatic accretion and amagmatic stretching, the latter producing significant rotations about ridge-parallel, sub-horizontal axes. Dykes and extrusives adjacent to the fossil Southern Troodos transform zone have been rotated clockwise in response to dextral shear along the transform, producing deviations away from the predominant N-S dyke trend within the Sheeted Dyke Complex of the ophiolite. The intersection of the transform with the principal Troodos spreading axis along the Solea graben has been identified, and transform-related rotations are now suggested to have occurred mainly in the active corner of the intersection. On-going palaeomagnetic research is now focused on the relationship between the Troodos Complex and the other major tectonostratigraphic terranes of Cyprus.
Palaeomagnetism of some Cretaceous Nubian Sandstones, Northern Sinai, Egypt
Authors: Kafafy A. M. , Tarling D. H. , El Gamili M. M. , Hamama H. H. , Ibrahim E. H.
Published: April 2008

A total of 23 sites have been sampled from the Nubian Sandstone of Gabal El-Minsherah and Gabal El-Halal, northern Sinai. Isothermal remanent magnetization acquisition showed that hematite is the predominant magnetic mineral carrying the remanence after the destruction of goethite at temperatures of 100–150°C. The
in situ
directions correspond to palaeomagnetic poles at 84°N 257°E (G. El Minsherah) and 78°N 288°E (G. El Halal). A fold-test shows that the magnetizations were acquired later than the Late Cretaceous (Laramide) tectonic disturbance. The presence of both normal and reversed polarities and
in situ
pole positions close to the expected Late Tertiary palaeomagnetic pole suggest that the magnetizations are of Miocene age. They are probably chemical magnetizations acquired during occasional fluid migrations through these permeable beds during arid Miocene climatic phases.
Emplacement temperatures of pyroclastic deposits on Santorini deduced from palaeomagnetic measurements: constraints on eruption mechanisms
Authors: Bardot Leon , Thomas Rick , McClelland Elizabeth
Published: April 2008

In pyroclastic eruptions, explosive fragmentation of magma occurs, breaking up some of the surrounding volcanic edifice, and a clastic deposit is formed containing fragments of juvenile magma and other rock debris. The temperature at which such a deposit is emplaced places important constraints on eruption mechanisms. The palaeomagnetic technique applied to lithic clasts which had cooled and solidified prior to the eruption can provide a quantitative estimate of emplacement temperature if there has been no modification of the magnetic mineralogy of the lithics during emplacement; thermal demagnetization is used to determine the temperature which removes all of the uniformly directed overprint component acquired during cooling in the deposit.

The volcano of Santorini (or Thera) in the Aegean has experienced a series of cataclysmic eruptions over the past 200 ka. We have determined emplacement temperatures for many of these tephra, from plinian airfalls, lag breccias, ignimbrites and phreatomagmatic deposits. The palaeomagnetic technique will give erroneous results if the magnetic mineralogy of a lithic clast altered during the eruption. This would give a CRM overprint which might completely or partially replace the primary remanence in blocking temperature intervals which extend above the actual emplacement temperature. We have carried out palaeointensity experiments on overprinted lithics to assess the magnetic origin of the overprint (pTRM, CRM or a combination) and hence the reliability of the temperature estimates. Results from the co-ignimbrite lithic breccias show they were emplaced hot. The Middle Pumice and Minoan plinian airfalls display equilibrium temperatures that decrease with increasing distance from vent, and provide constraints on the predictions concerning the cooling of tephra during fallout from eruption columns. Emplacement temperatures for the Minoan Phases 2 (phreatomagmatic deposits) and 3 (massive structureless white deposits) are low considering the deposits are principally composed of juvenile material. For Phase 2 this supports the conventional view of such deposits being the products of lateral explosions caused when water entered the plinian vent. The juvenile material of Phase 3 probably cooled as a result of interaction with water in the vent system, and we interpret them as very low grade ignimbrites.
Palaeomagnetic controls on the emplacement of the Neapolitan Yellow Tuff (Campi Flegrei, Southern Italy)
Authors: Gennaro Maurizio De’ , Gialanella Paola R. , Incoronato Alberto , Mastrolorenzo Giuseppe , Naimo Debora
Published: April 2008

Two outcrops of Neapolitan Yellow Tuff at Torregaveta and Posillipo have been investigated palaeomagnetically. Analyses of low blocking temperature components, in association with mineralogical, volcanological and magnetic anisotropy data, suggest the following sequence of events: (i) deposition of pyroclastics at temperatures not lower than 275°C, at least at Torregaveta; (ii) cooling associated with zeolitization and modification of the original positions of the clasts; (iii) further dislocation of the clasts after cooling; and (iv) lithification. Such a model reconciles otherwise conflicting volcanological and palaeomagnetic data.
Archaeomagnetic directions: the Hungarian calibration curve
Authors: Márton P.
Published: April 2008

The first archaeomagnetic dating in Hungary was made using a reference inclination curve derived by interpolation between three inclination curves then available from continental Europe, namely from France, Bulgaria and the Ukraine. As the corresponding declination curves could not be interpolated with confidence, all three declination curves were used only for an estimation of the time interval to which the measured declinations might be assigned. As data accumulated it became feasible for dating to be made exclusively on the basis of archaeomagnetic and direct observational results for Hungary. The results now cover the last 2000 years with relatively short gaps (sixth and thirteenth centuries
ad
), for which data interpolation is plausible. The variation of inclination is fairly sinusoidal and well resolved. It exhibits two maxima (
c.
70°) at approximately
ad
800 and
ad
1500 and three minima at about
ad
300 (
c.
58°),
ad
1300 (
c.
55°) and in the twentieth century (
c.
62°). Although the variation of the declination is twice as large as that of the inclination, the declination record has inherently less resolution (d
D
= d
I
/cos
I
> d
I
). Approximately zero declination values during the first half of the first millennium
ad
were followed by westerly values during the sixth to eighth centuries (
c.
10°). It appears that by
ad
900 the declination was already easterly and rapidly increasing until
ad
1000 when it peaked with a value over 20°E. A newly discovered feature is a short oscillation beginning with 16° at
ad
1300 and ending with 16.5° at
ad
1600 during which the declination became westerly with a minimum of −10° in the first half of the fifteenth century. From
ad
1600 on the declination rapidly decreased and described a negative half circle between about
ad
1620–30 and 1950 with a minimum of −18° at
ad
1800. The general pattern of the directional secular variation for Hungary is in agreement with that for France, Sicily, Britain, the Ukraine and the Balkans.
Continental collision in the Banda arc
Authors: Richardson A. N. , Blundell D. J. , Richardson A. N.
Published: April 2008

Two deep seismic reflection profiles across the zone of convergence between Australia and the Banda arc east of Timor reveal the structures formed during lithosphere deformation in response to continental collision. Australian continental crust is bent down to the north to form the lower lithospheric plate. The immediately overlying upper plate is made up of a former outlier of the Australian continental shelf, now squeezed between the Australian continent to the south and the Banda arc to the north. The ages of metamorphism and of a regional unconformity in Timor indicate that the outlier began to accrete to the upper plate in the late Miocene. Near the Timor trough, at the junction between the two plates, seismic reflection data show the upper plate to be cut by north-dipping structures interpreted as thrusts in an accretionary prism formed since the arrival of the Australian continental shelf at the collision zone at about 2.5 Ma. The northern part of the collision zone is dominated by structures dipping southwards, antithetic to subduction, which penetrate the lithosphere to depths of at least 50 km. Earthquake data indicate that these are active thrust faults which divide the upper plate into imbricate slices. The seismic reflection sections, supported by gravity data and seismicity patterns, have been used to develop a model of plate convergence and continental collision which now consists of a 200 km wide zone in which both oceanic and continental material is being shortened, thickened and uplifted.
Contrasting tectonic styles in the Neogene orogenic belts of Indonesia
Authors: Simandjuntak T. O. , Barber A. J.
Published: April 2008

The recent compilation of a new tectonic map of Indonesia as part of the Geotectonic Map Project of East Asia has prompted a reassessment of the contrasted tectonic styles represented by currently developing orogenic belts. These orogenic styles provide a range of models illustrating the diversity and complexity of tectonic processes which may provide the key to the interpretation of other orogenic belts elsewhere in the world. The following distinctive types of orogenies have been recognized within Indonesia. 1. Sunda Orogeny in Java and Nusa Tenggara: involving subduction of oceanic crust with normal convergence, producing an orogenic belt of Andean type with trench, accretionary complex, forearc basin and Quaternary magmatic arc with active volcanoes built on the margin of the Sundaland continent. 2. Barisan Orogeny in Sumatra: with strongly oblique convergence and major strike slip transcurrent fault movement within the magmatic arc, along which a segment of continental crust is being displaced northwards along the western margin of Sundaland. 3. Talaud Orogeny in the northern Molucca Sea: convergence of the Sangihe and Halmahera oceanic magmatic arcs as the Molucca Sea Plate subsides beneath them. 4. Sulawesi Orogeny in eastern Sulawesi: collision of microcontinental blocks with subduction systems along the eastern margin of Sundaland. 5. Banda Orogeny in the southern Banda Arc between Sumba and Tanimbar: collision of the northern margin of the Australian continent with the subduction system along the southern segment of the Banda Arc. 6. Melanesian Orogeny in Irian Jaya and Papua New Guinea: a more advanced stage in the collision of the northern margin of the Australian continent with a magmatic arc on the Philippine Sea plate, commencing in the early Miocene with a partial reversal of polarity and the subduction of the Caroline plate beneath the collision zone.

The present phase of orogenic activity in most of these occurrences commenced in mid-Miocene times and orogenic processes are still in progress.
Palaeomagnetism of the Sibumasu and Indochina blocks: implications for the extrusion tectonic model
Authors: Richter Bryan , Fuller Michael
Published: April 2008

The Jurassic-Cretaceous Kalaw redbeds of Myanmar yield a prefolding Late Cretaceous to Early Palaeogene magnetization which records c. 25–30° of clockwise (CW) rotation relative to the South China block. This corresponds to 10–15° CW relative to the Lower Cretaceous Khorat Plateau VGP. The data also show 5° of northward transport relative to the 100 Ma South China VGP or 12° relative to the Khorat Lower Cretaceous VGP. Similar CW rotations are measured in remagnetized Palaeozoic carbonates in Peninsular Thailand and Langkawi Island, Malaysia. These block motions most likely took place between the Late Cretaceous and the Late Oligocene. These and other recently published data have several implications for the extrusion tectonic model: (i) Sundaland has only rotated 25–30° CW relative to South China during the Tertiary; (ii) southeastward translation is only 300–500 km; and (iii) Sundaland is composed of smaller sub-blocks, some of which have moved northward. This is interpreted to indicate that deformation of the Sibumasu block is dominated by the oblique subduction of the Indian Ocean Plate while deformation of the Indochina block is dominated by extrusion, in turn driven by convergence between the Indian Craton and Eurasia.
Changes in tectonic stress field in northern Sunda Shelf basins
Authors: Tjia H. D. , Liew K. K.
Published: April 2008

The Tertiary basins of the northern Sunda Shelf are underlain by normal and attenuated continental crust that is characterized by moderate to high average geothermal gradients in excess of 5°C/100 m. In the Malay basin, upper Oligocene and younger sediments are more than 12 km thick; in the other basins, such sediments are between 4 and 8 km thick. The Malay, Penyu and West Natuna basins are aulacogens meeting at a triple junction that marks a Late Cretaceous hot spot in the centre of the Malay Dome. Sub-basins (commonly half-graben) developed as pull-apart basins within regional, north to northwest-striking, wrench fault zones. The NW-striking Three Pagodas fault most probably extends farther southeast as the Axial Malay fault in the basement along the length of the Malay basin. Pre-late Oligocene sinistral slip along this fault developed east-west half-graben that fixed the position and orientation of inverted anticlines that later developed in the basin-filling sediments through strike-slip reversal along the fault. Initial basin subsidence took place during Eocene-Oligocene time. Regional tensional conditions prevailed until the early Miocene. During the middle to late Miocene, regional compressional stresses caused reversals of the sense of motion on the major wrench faults, and structural inversion of the basin-filling sediments. On some of the north-striking wrench faults there are indications of up to 45 km of right-lateral displacement which is possibly post-Miocene. The regional wrench faults have acted as domain boundaries with each tectonic domain characterized by different stress fields. The stress systems evolving during the Cenozoic are attributed to varying degrees of interference of plates coupled with changes in convergent directions and/or rates of motion of the Pacific plate, the Indian Ocean-Australian plate, and continued, differential extrusion of SE Asian crust following the collision of the Indian plate with the Eurasian plate.
Far-field and gravity tectonics in Miocene basins of Sabah, Malaysia
Authors: Clennell Ben
Published: April 2008

At the end of the early Miocene, at
c.
17 Ma, oceanic spreading ceased in the South China Sea Basin, as a series of collisions were initiated between continental blocks derived from the Asian mainland and the northwestern margins of Borneo and Palawan. In the NW Sabah Basin this tectonic event caused a major period of uplift and erosion which produced the so-called ‘Deep Regional Unconformity’. While these mainly compressional events were occurring at the southern margin of the South China Sea, the Sulu Sea was undergoing extension, with rifting in the northwest and oceanic spreading in the southeast.

Recent seismic surveys in the Sabah offshore area show that the Deep Regional Unconformity can be traced through from the South China Sea into the Sulu Sea as a boundary between two contrasting depositional systems that are both represented in sedimentary sections onshore eastern Sabah. Eastern Sabah changed from an environment of deep marine clastic deposition in the Oligocene and early Miocene, to shallow marine and terrestrial sedimentation in the mid to late Miocene, with a major period of sedimentary mélange formation occurring at the time of the Deep Regional Unconformity.

Inversion of the Miocene sequence in eastern Sabah appears to be limited to the edges of basement blocks, which were moved by far-field tectonic stresses. Post Middle-Miocene basin evolution in the onshore (Central Sabah Basin) and adjacent parts of the Sulu Sea offshore (Sandakan Basin) has been strongly influenced by mud diapirism and gravitational sagging of progradational sand-rich sediments into underlying muds and mélange units. These sequences appear to have been decoupled from later tectonic events by the great thickness of underlying muds and mélanges.
Mesozoic and Cenozoic plutonic evolution of SE Asia: evidence from Sumatra, Indonesia
Authors: McCourt W. J. , Crow M. J. , Cobbing E. J. , Amin T. C.
Published: April 2008

New K/Ar mineral ages from the Barisan Mountains of southern Sumatra suggest four main periods of plutonic activity: Miocene-Pliocene (20–5 Ma), Early Eocene (60–50 Ma), Mid-Late Cretaceous (117–80 Ma) and Jurassic-Early Cretaceous (203–130 Ma). These and all other published ages from exposed plutons in western Sumatra indicate a further period of plutonic activity in the Permian (287–256 Ma). They also suggest either that Early Mesozoic activity began in the Late Triassic, or that there were two distinct magmatic cycles, one in the Late Triassic to Early Jurassic (220–190 Ma) and one extending from the Mid-Jurassic to Early Cretaceous (170–130 Ma). In addition, poorly controlled ages from eastern Sumatra indicate that the important Triassic to Early Jurassic (240–195 Ma) tin-belt magmatism of the peninsular Malaysia Main Range Province extends into that area.

Preliminary geochemical studies on the Mesozoic granitoids of the Barisan Range of southern Sumatra confirm that they are calc-alkaline, I-type, metaluminous, subduction-related volcanic arc granites (VAG). They broadly correspond to the southerly extension of a combination of the Central Valley and Western Granite Provinces of Thailand and Burma, and underline the fact that there has been a history of subduction-related magmatism along the southwestern edge of Sundaland since earliest Mesozoic times. The plutonic suites are crudely arranged in subparallel, locally overlapping, NW-SE trending belts, focused along deep-seated faults that have acted as magmatic conduits. It is proposed as a preliminary model that breaks in plutonic activity broadly correspond to changes in approach angle and/or rate of subduction, and that in some instances at least they relate to periods of collision and accretion of allochthonous material (terranes, slivers or blocks) of both oceanic and continental character. At least two such events seem to have occurred during the Mesozoic-Cenozoic tectono-plutonic evolution of Sumatra. One in the early Middle Cretaceous reflects collision and accretion of the oceanic Woyla terranes, and one in the latest Cretaceous is possibly related to collision of a continental sliver/block, the West Sumatra terrane, to the Sundaland margin.
The Mentawai fault zone and deformation of the Sumatran Forearc in the Nias area
Authors: Samuel M. A. , Harbury N. A.
Published: April 2008

The Sumatran Forearc is the site of oblique plate convergence between the Indo-Australian and SE Asian-Eurasian Plates. It is generally accepted that the forearc sliver is not behaving as a rigid plate and that the rate of slip increases along the right-lateral Sumatran Fault System from southeast to northwest. Two contrasting hypotheses have been invoked to explain this pattern of decoupling: by using slip vectors to suggest arc-parallel stretching; and a second major right-lateral strike slip zone, parallel to the Sumatran Fault System, to accommodate the oblique subduction. This zone, termed the Mentawai fault zone, lies just to the east of the outer-arc ridge and is indicated as intersecting with other faults on Nias Island. In this paper data are presented from Nias where a thick forearc succession is excellently exposed in three subbasins with half-graben geometries and broadly comparable Palaeogene-Recent histories. Mud diapirism has generated melanges which cut the Oligocene to Recent strata. Outcrop data, LANDSAT, Synthetic Aperture Radar and aerial photographs were used to determine fault offset patterns, and marked extension of the island along north-striking right lateral faults and ESE striking left-lateral faults is revealed. Extension was further accommodated along a set of ENE striking normal faults. In the region where the proposed Mentawai fault zone comes onto Nias there is an important, long-lived, basin-bounding fault. This structure had in excess of 5 km normal throw during the Oligocene-Miocene whilst minor contractional reactivation of the fault occurred during the Pliocene phase of uplift and deformation which affected all of Nias. Data from onshore and offshore the Nias region suggest that all the features visible from seismic sections collected over the Mentawai fault zone to the south of Nias can be explained in terms of inversion of originally extensional structures and mud diapirism. It is suggested that strike-slip motion is of limited importance along the 600 km long Mentawai fault zone. Rather this highly structured zone represents a deformation front whose origin can be explained by Pliocene to Recent subduction-driven inversion at the outer margin of the forearc.
Pb and Nd isotope constraints on the provenance of tectonically dispersed continental fragments in east Indonesia
Authors: Vroon P Z. , Van Bergen M. J. , Forde E. J.
Published: April 2008

Microcontinental fragments, tectonically dispersed at the triple junction of the converging SE Asian, Indo-Australian and Pacific plates, are a widespread phenomenon in the east Indonesian region. This paper investigates the potential of Pb-Nd isotope characteristics of igneous and (meta) sedimentary rocks from these fragments as indicators of their provenance. From the correspondence of isotopic signatures, it appears possible to identify provenance areas of four selected microcontinents, which were emplaced in different settings and regions of east Indonesia since late Mesozoic times. According to their isotopic affinities, the blocks of Ambon-Seram, Bacan, the Banda Ridges and Sumba originally formed part of southern New Guinea, north Australia, ‘Pacific’ New Guinea and Sundaland respectively. These conclusions are in line with geological constraints on the palaeopositions of the fragments. The results thus indicate that isotopic signatures are potentially a powerful tool in reconstructions of the tectonic history of east Indonesia.
Dating of Neogene igneous rocks in the Halmahera region: arc initiation and development
Authors: Baker Simon , Malaihollo Jeffrey
Published: April 2008

Potassium-argon ages of Neogene to Recent igneous rocks from the Halmahera region record a history of intra-oceanic arc development since the late Middle Miocene following an earlier phase of collisional plutonism. Arc formation from the Middle Miocene onwards was due to the east-directed subduction of the Molucca Sea plate beneath the Philippine Sea plate as it arrived at the Eurasian margin. The distribution of ages within the Neogene arc indicates a northward migration of volcanic activity during the Late Miocene to Pliocene. Results of the dating work show that after collision with the Australian margin at
c.
22 Ma there was a period of volcanic quiescence and limestone deposition before a new arc formed. This arc began erupting at around 11 Ma on Obi as a result of subduction of the Molucca Sea plate. Initiation of subduction is thought to have occurred around 15–17 Ma and may have been responsible for disturbing potassium-argon ages of pre-Neogene rocks. Dates from fresh rocks show that the volcanic front migrated northwards through Bacan and Halmahera throughout the Late Miocene to Early Pliocene. Limestone deposition was curtailed as arc activity migrated north while volcanism died out from the south. No Neogene volcanism younger than 8 Ma is observed in the Obi area while on Bacan subduction-related volcanism ceased at
c.
2 Ma. Late Pliocene crustal deformation caused a 30–40 km westward shift of the volcanic front. Quaternary volcanic rocks exposed in Bacan and the extreme south of Halmahera are not direct products of subduction but, rather, display geochemical characteristics of both subduction and fault-related magmatism. These volcanic rocks are distributed along splays of the Sorong fault system. The formation and propagation of the Halmahera arc is a consequence of the clockwise rotation of the Philippine Sea plate as the southern edge moved across the northern Australian margin and impinged on the east Eurasian margin. The ages of initiation of volcanism and subduction track the developing plate boundary as subduction propagated northwards.
Thermochronological and geochemical constraints on the tectonic evolution of northern Papua New Guinea
Authors: Crowhurst P. V. , Hill K. C. , Foster D. A. , Bennett A. P.
Published: April 2008

The Bewani-Torricelli-Prince Alexander Mountains, along the northern margin of Papua New Guinea, probably formed as a tholeiitic island arc in the Late Eocene-Early Oligocene. The arc is interpreted to have accreted to the margin by the Late Oligocene, but may have formed on a ribbon of extended continental crust along the New Guinea margin. Inferred roll-back of the subducting slab beneath New Guinea in the Early Miocene placed the margin into extension, creating starved graben in northern New Guinea and causing regional subsidence. Near the graben, metamorphic core complexes resulted as the upper crust was pulled off the lower crust along low angle detachments such that lower crustal rocks cooled rapidly from temperatures >500°C. The two inferred core complexes that have been dated show rapid cooling from 27-23 Ma and 20-18 Ma. Continued subduction beneath New Guinea resulted in formation of the Maramuni arc in the Middle Miocene and the end of extension. In the Late Miocene, collision of the Melanesian arc caused regional uplift of all basement of northern Papua New Guinea, mainly from 8-5 Ma, causing at least 3–4 km of denudation. The compressional deformation propagated south causing uplift, denudation and cooling in the Papuan Fold Belt at
c.
4 Ma, but is continuing at the present.
Geological controls on mineralization and dolomitization of the Lisheen Zn-Pb-Ag deposit, Co. Tipperary, Ireland
Authors: Shearley E. , Redmond P. , King M. , Goodman R.
Published: April 2008

The Lisheen deposit is a stratabound Zn-Pb-Ag deposit hosted in dolomitized Waulsortian limestone of Courceyan age. The deposit was discovered in April 1990 and the current resource is estimated to be 22 million tonnes at 11.5% Zn, 2% Pb and 16% Fe, with 26 g/t Ag. Lisheen is located along the Rathdowney Trend, a NE-SW trending, structurally controlled belt of carbonate rocks. Sulphides are present in the hanging wall of two major E to NE-trending normal faults, the Killoran and Derryville faults. These structures are large, northerly-dipping normal faults with displacements of approximately 200 m. The Killoran and Derryville structures form short fault segments of a major relay fault system that extends along the Rathdowney Trend. Faults are observed to have influenced sedimentation, and exercized a major control on the distribution of mineralization, hydrothermal alteration and dolomitization. Several phases of dolomitization, which affect Chadian rocks and temporally overlapped mineralization, help to constrain the timing of this mineralization to no earlier than the late Chadian.
The use of burial diagenetic calcite cements to determine the controls upon hydrocarbon emplacement and mineralization on a carbonate platform, Derbyshire, England
Authors: Hollis Cathy , Walkden Gordon , Hollis Cathy
Published: April 2008

Late diagenetic calcite cements in the Upper Dinantian limestones of the Derbyshire Platform are contemporaneous with both hydrocarbon emplacement and Mississippi Valley-type (MVT) mineralization. Calcite cementation began during the progressive burial of the Derbyshire Platform and the surrounding basins, principally within fractures generated during the waning effects of Upper Carboniferous extension. Six burial calcite cements can be recognized in dilational vein systems. Successive veins contain progressively more mature hydrocarbon inclusions, and calcite cements are intergrown with fluorite, baryte, galena and sphalerite in increasing quantities. Compacting Dinantian-Namurian shales in basins adjacent to the platform offer the most likely sources of fluids, trace elements and hydrocarbons. Fluids entered the platform along major fault systems, and circulated using smaller fracture systems, precipitating calcite. The final phases of calcite cementation and the main phase of MVT mineralization coincided with the onset of the Variscan Orogeny. A model is now established relating fluid flow to Variscan tectonic events in northern Britain.
Development of coral reef-facies during the Viséan at Los Santos de Maimona, SW Spain
Authors: Rodríguez Sergio
Published: April 2008

The
Siphonodendron
Limestone at Los Santos de Maimona Basin is regarded as a reef structure built mainly by rugose corals. This unit is composed of biogenic marls and biostromal limestones containing abundant rugose corals and brachiopods, and frequent calcareous algae, tabulate corals, foraminiferans, bryozoans, echinoderms, ostracodes and molluscs. It is present throughout the basin, which is 12 km long and 3 km wide, but the thickness, development of the framework and distribution of organic components varies from SE (seaward) to NW (landward). It is 40 m thick in the SE, but its thickness reaches only 6 m in the NW. The unit shows a vertical evolution of lithological facies, from biogenic marls at the bottom up to biostromal limestones at the top. The main environmental factors controlling the development of the organic framework are the tidal regime, minor subsidence pulses and periodic storms.

Arguments in favour of the reefal origin are structural (distribution, development and relationships between different facies of the
Siphonodendron
Limestone) and ecological (distribution and relationships of building organisms, environmental indicators, etc.). Objections to the reefal hypothesis (absence of reef crest and talus, low diversity, problematical wave resistance, biostromal nature of the upper beds, storm layers) are discussed. Building structures by corals, brachiopods and calcareous algae are briefly described.
Dinantian river systems and coastal zone sedimentation in northwest Ireland
Authors: Graham John R.
Published: April 2008

Northwest Ireland either formed or lay close to the coastal zone and represented the northernmost extent of marine sedimentation for large parts of the late Dinantian. The succession deposited during the initial early Viséan transgression in Donegal starts with northerly-derived fluvial gravels which pass upwards into coarse-grained fluvial sandstones and associated siltstones. Channel cross-sections are relatively small, suggesting stream widths of a few tens of metres and depths of up to 4 m. These data suggest small rivers with limited drainage basins and are inconsistent with a single drainage basin >95 000 km
2
, which had been proposed previously. Depositional style in the coastal zone was dominated by laterally migrating tidal creeks, initially containing siliciclastic detrita but with the proportion of carbonate increasing upwards. The succession in North Mayo is similar to that in Donegal but generally lacks the fluvial gravel unit.

Widespread deposition of limestones and shales was interrupted in late Arundian times by a southerly progradation of first-cycle feldspathic sandstones, the Mullaghmore Sandstone Formation and its correlatives. In the more proximal environments this is represented by fluvial sediments cutting down into the pre-existing carbonate shelf. This has the basic character of a Type I sequence boundary and represents a major lowering of sea level in northwest Ireland. During this clastic influx, large tidally-influenced fluvial channels developed, with channel widths up to 200 m and depths >5 m. This indicates an increase in river size in this area during the Dinantian which may have been due to river capture during the evolution of a relatively low-relief landscape. The regression that led to the southerly progradation of clastics is of similar age to fault movements documented elsewhere in Ireland, and may have a regional tectonic cause.
Initiation, growth and decline of a tectonically controlled Asbian carbonate ramp: Cuilcagh Mountain area, NW Ireland
Authors: Kelly John G.
Published: April 2008

The Lough MacNean Valley and the surrounding Cuilcagh and Belmore Mountains expose carbonates and subordinate shales of late Arundian to Asbian age. The Benbulben Shale, of late Arundian to Asbian age, is composed of fossiliferous shales with limestone bands. This is succeeded by the Glencar Limestone, composed predominantly of argillaceous micritic wackestones and packstones, and fossiliferous shales. This is overlain by the Asbian Dartry Limestone, which exhibits extreme lateral and vertical facies variations; it is quite different on east, north and west Cuilcagh, which are described in turn.

Lateral variations in facies and thickness in the Dartry Limestone are attributed to carbonate ramp development on a differentially subsiding block, initiated by sinistral strikeslip movement on the Castle Archdale-Belhavel and Curlew-Clogher Valley fault systems. The sediments exposed on east Cuilcagh represent the proximal portion of the the ramp, whilst the Knockmore and Cloghan Hill Members represent the mid and distal ramp. The chertyfacies limestones mark widespread open shelf conditions at the end of subsidence. Inversion is marked by the shallow-water Cloghany Limestone, the Carn Limestone and the top-Dartry disconformity prior to deposition of the Meenymore Formation.
The influence of climatic change on exposure surface development: a case study from the Late Dinantian of England and Wales
Authors: Vanstone Simon
Published: April 2008

Exposure surfaces represent an integral part of Asbian-Brigantian cyclothemic platform carbonates in England and Wales. These are characterized by the association of clay palaeosols, calcrete and palaeokarst and in most instances would appear to have been polygenetic. Alternating calcrete-karst stratigraphies associated with individual exposure surfaces indicate that the climate changed from semi-arid to humid to semi-arid conditions during each sea-level fall/rise cycle. Lowstand intervals were humid and resulted in karstification of the cyclothem-top sediments and the formation of a mineral soil. In contrast, regressive/transgressive phases were semi-arid and resulted in calcretization of the emergent platform carbonates.

The influence that climatic cyclicity had upon exposure surface development was modulated by variations in platform bathymetry, subsidence and spatial climatic variation, and platforms exhibit their own individual record of what was essentially an idealized sequence of events. As with the sea-level oscillations responsible for cyclothemic sedimentation, the climatic cyclicity is thought to be the product of orbital forcing and probably reflects either eccentricity-driven shifts in the locus of monsoonal precipitation, or precession-driven variations in monsoonal intensity. If precessional in origin, exposure surface development represents a single minimum to minimum excursion, some 20 ka in duration, whereas if eccentricity-driven this may have been appreciably longer. Nevertheless, the immature nature of the exposure surfaces suggests that emergence was probably only of the order of a few tens of thousands of years.
Contemporaneous erosion and reworking within the Dinantian of the South Munster Basin
Authors: Naylor D. , Sevastopulo G. D. , Sleeman A. G.
Published: April 2008

Contemporaneous erosion and reworking of sediment is now known from a wide scattering of locations within the Dinantian rocks of the South Munster Basin. The phenomenon is most marked close to the northern margin of the basin, which lay within the Cloyne Syncline from Cork Harbour westwards to Ballygarvan. Exploration drilling further west in this syncline, near Inishannon, has found Dinantian/Namurian basinal sequences containing thicknesses of breccias with shelf carbonate clasts. The northern basin margin continues obliquely across strike westwards to cross the extension of the Cork Syncline near Crookstown, where recent drilling has located the basin margin. There is no evidence of reworking in the basin during the latest Devonian-early Tournaisian. A widespread sequence boundary is recognized in the middle Tournaisian (base of Courtmacsherry-Reenydonagan Formations). Within the Kinsale sub-basin, Member 2 Courtmacsherry Formation is a silt-sand unit within a mudstone-carbonate sequence; sourcing of the sand demands considerable intrabasinal or basin margin erosion. In the Bantry sub-basin, reworked conodonts occur at different levels within Member 3, Reenydonagan Formation (late Tournaisian to Arundian). There is indirect evidence for a carbonate shelf or intrabasinal high of Tournaisian to Asbian age in this western region.
The Apedale tuffs, North Staffordshire: probable remnants of a late Asbian/Brigantian (P1a) volcanic centre
Authors: Rees J. G. , Cornwell J. D. , Dabek Z. K. , Merriman R. J.
Published: April 2008

A borehole sunk in 1920–21 at Apedale, North Staffordshire, unexpectedly proved a tuff-dominated sequence, at least 840m thick, below a thin cover of Silesian sedimentary rocks. The tuffs were initially interpreted as Dinantian in age. However, there are similarities of form and lineament between an aeromagnetic anomaly centred upon Apedale and anomalies in the Welsh Marches associated with Neoproterozoic (Uriconian) rocks, suggesting that Neoproterozoic rocks occur at Apedale and that the tuffs in Apedale Borehole might also be of this age. Recent modelling of geophysical data shows that Apedale is underlain by two (magnetic) volcanic bodies — a deeper ridge of probable Uriconian rock, and a shallower, broadly stratiform body which includes the tuffs in the Apedale Borehole. Comparison of the tuffs at Apedale with others in the region suggests that they are of P1a (late Asbian to early Brigantian) age. Although our knowledge of the extent of the Apedale tuffs is poor, their thickness shows that they erupted from one of the largest centres of Dinantian volcanism in central England. The trace element geochemistry is very similar to that of Dinantian volcanic rocks in southwest England.