Extension Of Continental Crust Research Articles

The initiation of subduction by crustal extension at a continental margin

SUMMARY We investigate how subduction may be triggered by continental crust extension at a continental margin. The large topography contrast between continental and oceanic domains drives the spreading of continental crust over oceanic basement. Subduction requires the oceanic plate to get submerged in mantle, so that negative buoyancy forces may take over and drive further descent. This is promoted by two mechanisms. Loading by continental crust bends the oceanic plate downwards. Extension in the continental domain induces crustal thinning, which acts to raise mantle above the oceanic plate. In this model, the width of the continental region undergoing extension is an important control parameter. The main physical controls are illustrated by laboratory experiments and simple theory for elastic flexure coupled to viscous crustal spreading. Three governing dimensionless parameters are identified. One involves the poorly constrained oceanic plate buoyancy. We find that the oceanic plate can be thrust to depths larger than 40 km even if it is buoyant, enabling metamorphic reactions and density increase in the oceanic crust. Another parameter is the ratio between the width of the continental extension region and the flexural parameter for the oceanic plate. Initiating subduction is easier if the continent thins over a short lateral distance or if the oceanic plate is strong. The third important parameter is the ratio of oceanic plate thickness to initial continental crust thickness, such that a weak plate and a thick crust do not favour subduction. Thus, the change from a passive to an active margin depends on the local characteristics of the continental crust and is not determined solely by the age and properties of the oceanic lithosphere. It is shown that the spreading of continental crust induces uplift of the margin as the adjacent seafloor subsides. Evidence for the emplacement of continental crust over oceanic basement at passive margins is reviewed.

Post-folding magmatism (1.85–1.7 Ga) in the eastern part of the Baltic Shield: Correlation of its structural position and evolution of surrounding complexes

This work discusses the structural and compositional correlation of an unusual group of rocks that comprise post-folding massifs and dikes in the eastern Baltic Shield, which formed between 1.85 and 1.7 Ga. Occurring from the Barents Sea in the north to the Gulf of Finland in the south, these structures are associated with areas of granulite facies rocks that formed under conditions corresponding to the deep continental crust. Large-scale extension of continental crust, which led to the exhumation of the granulite complex, simultaneous with the formation of metasomatic formations and manifestations of post-folding magmatism, was confined to the periphery of a large circular structure, which is interpreted by us as the Baltic nucleus, or a tectonic portion of the continental crust at the end of early Precambrian. The formation of enriched mantle, a source of the Paleozoic alkaline melts, can also be associated with these processes of extension.

Chapter 26 Crustal structure of the East Siberian continental margin, Podvodnikov and Makarov basins, based on refraction seismic data (TransArctic 1989–1991)

Abstract The c . 1500 km-long refraction and shallow reflection seismic profile, TransArctic 1989–1991 from the East Siberian shelf northwards across the Podvodnikov and Makarov basins, provides a four-layer model of the crust: layer I ( V p =1.7–3.8 km s −1 ) of sedimentary formations of late Mesozoic and Cenozoic age; layer II ( V p =5.0–5.4 km s −1 ) of older sedimentary rocks on the shelf and possibly also mafic volcanics in the basins; layer III ( V p =5.9–6.5 km s −1 ); and layer IV ( V p =6.7–7.3 km s −1 ) of crystalline crust. The East Siberian margin has c . 40 km thick continental crust, mainly composed of layers III and IV, both c . 15 km thick. Beneath the Podvodnikov Basin, the Moho depth varies from c . 20 km bsl at southern and northern ends to c . 30 km bsl at the centre beneath the Arlis Gap; it was probably formed by longitudinal extension of continental crust during the late Mesozoic. The edge of the Alpha–Mendeleev Ridge, separating the Podvodnikov and Makarov basins, has a crustal thickness of c . 25 km, mainly composed of layers III and IV. The deep Makarov Basin is probably composed of oceanic crust, 8–12 km thick, but includes spurs of continental crust, rifted off the Lomonosov Ridge.

Quantifying extension of passive margins: Implications for sea level change

[1] Passive margins form by rifting and extension of continental crust. Extension of continental crust, associated with subduction elsewhere, decreases ocean basin volume, forcing sea level to rise. The amount that the continental crust has been extended can be quantified using total tectonic subsidence, which is determined using sediment thickness and water depth data coupled with a model of the rifting process. We calculate total tectonic subsidence, the amount of extension or stretching factor, and thickness of extended continental crust at a 5 min grid resolution for passive margins between 77°N and 70°S. Our modeled crustal thicknesses correlate well with seismic refraction data from North America and Australia and can be used to estimate the change in ocean volume and the impact of extension on sea level. The duration of extension is constrained by estimating the onset of rifting and the breakup age, which is the time when rifting ceased and seafloor spreading began. Our results indicate that extension of continental crust has increased sea level by 20.8 m since the start of Pangea rifting.

Structure and development of the southeast Newfoundland continental passive margin: derived from SCREECH Transect 3

New seismic reflection data from the Grand Banks of Newfoundland and the Newfoundland Basin add to the growing knowledge of the composition, structure and history of this nonvolcanic margin. Geophysical imaging is now approaching the extent of that done previously on the conjugate margin along Iberia, providing a valuable database for the development of rifting models. Two parallel profiles over the shelf platform image deep crustal fabric representing Precambrian or possibly Appalachian deformation as well asMesozoic extension. Progressively more intense extension of continental crust is imaged oceanwards without the highly reflective detachments frequently seen on profiles off Galicia. A landward-dipping event ‘L’ is imaged sporadically and appears to be analogous to a similar event on the Iberian IAM9 profile. The transition zone is probably exposed serpentinized mantle as interpreted off the Iberian margin although there appears to be a difference in the character of ridge development and reflectivity. The distinctive ‘U’ reflection identified previously at the base of the Newfoundland Basin deep water sedimentary section and recently identified as one or more thin basalt sills is imaged on newly presented profiles that connect previously published profiles SCR3 and SCR2 showing that ‘U’ is highly regular and continuous except where interrupted by basement highs. ‘U’ is also seen to have a major impact on the ability to image underlying basement. A full transect beginning over completely unextended continental crust through to oceanic crust has provided a data set from which estimates of extension and the pre-rifting location of the present continental edge can be made. Two estimates were obtained; 85 km based on faulting and 120 km based on crustal thickness.

East Greenland Ridge in the North Atlantic Ocean: An integrated geophysical study of a continental sliver in a boundary transform fault setting

The combined Greenland‐Senja Fracture Zones (GSFZ) represent a first‐order plate tectonic feature in the North Atlantic Ocean. The GSFZ defines an abrupt change in the character of magnetic anomalies with well‐defined seafloor spreading anomalies in the Greenland and Norwegian basins to the south but ambiguous and weak magnetic anomalies in the Boreas Basin to the north. Substantial uncertainty exists concerning the plate tectonic evolution of the latter area, including the role of the East Greenland Ridge, which is situated along the Greenland Fracture Zone. In 2002, a combined ocean‐bottom seismometer and multichannel seismic (MCS) survey acquired two intersecting wide‐angle reflection and coincident MCS profiles across and along the East Greenland Ridge. We present the results of integrated reflection seismic interpretation, first‐arrival tomography, 2D kinematic raytracing, full‐wave amplitude modeling, and gravity modeling of the intersecting profiles. The results show that (1) the Greenland Basin is characterized by a normal oceanic crustal velocity structure, (2) the velocity structure of the East Greenland Ridge is of overall continental type, and (3) a major faulted basin province above highly extended continental crust exists to the NE of the ridge. The results further suggest that a zone of extremely thin and faulted continental crust above partially serpentinized mantle peridotite defines the NW edge of the East Greenland Ridge and the transition to the NE Greenland margin.

Extension of continental crust at the margin of the eastern Grand Banks, Newfoundland

Seismic and gravity observations from the rifted margin of the eastern Grand Banks, Newfoundland, support a new model for extension of the continental crust from the shelf edge to ODP Site 1277, where mantle rocks are exhumed. We find that the largest decrease in crustal thickness, from about 28 km to 6 km, occurs beneath the continental slope of the Grand Banks over a distance of just 20 km. This rapid decrease in crustal thickness coincides with anomalously high seismic velocities (7.0–7.2 km·s − 1 ) in the lower crust of the shelf edge. The thin crust of the continent–ocean transition (COT) in this area has a smooth basement surface, void of upper crustal blocks and prerift sediments. We compare our geophysical results with a geodynamical model that represents rifting of a relatively hot continental lithosphere and with another numerical model that represents rifting of a cold lithosphere. Both geodynamic models suggest that crustal thinning beneath the continental slope was achieved by extensional faulting in the upper crust and ductile shear zones in the middle crust. The geodynamic models provide an explanation for the formation of distinct continental slopes at rifted margins: Beneath the continental shelf of the Grand Banks, the Moho and the strong lower crust rotated upwards toward to a 50° dip without visible internal deformation. The presence of these strong lower crustal rocks at shallow depth in the rift flank subsequently helped to localize the extension farther seaward. With ongoing extension, some high-angle normal faults may have rotated to a sub-horizontal orientation, which would explain the lack of brittle deformation visible in the seismic reflection data. The two geodynamic models produce different amounts of extension of continental crust in the distal margins. The hot rifting model localizes strain much more rapidly, leaving narrow zones of extended continental crust, and it produces a relatively large amount of melt (> 30%) in the final stages of rifting. Continental breakup may occur rapidly in hot lithosphere (< 5 Myr). On the other hand, a cold extension model extends the continental crust to a thickness smaller than 10 km over a width of 50 km in the distal margin, similar to what we inferred at the eastern Grand Banks. The cold lithospheric model requires about 23 Myr of extension before continental breakup, and it predicts much less melting in the mantle (13%). The long rift duration, wide zones of thinned continental crust, and small amount of magmatism make the cold rifting model the most applicable to Newfoundland–Iberia rift.

Quantitative tectonic reconstructions of Zealandia based on crustal thickness estimates

Zealandia is a key piece in the plate reconstruction of Gondwana. The positions of its submarine plateaus are major constraints on the best fit and breakup involving New Zealand, Australia, Antarctica, and associated microplates. As the submarine plateaus surrounding New Zealand consist of extended and highly extended continental crust, classic plate tectonic reconstructions assuming rigid plates and narrow plate boundaries fail to reconstruct these areas correctly. However, if the early breakup history shall be reconstructed, it is crucial to consider crustal stretching in a plate‐tectonic reconstruction. We present a reconstruction of the basins around New Zealand (Great South Basin, Bounty Trough, and New Caledonia Basin) based on crustal balancing, an approach that takes into account the rifting and thinning processes affecting continental crust. In a first step, we computed a crustal thickness map of Zealandia using seismic, seismological, and gravity data. The crustal thickness map shows the submarine plateaus to have a uniform crustal thickness of 20–24 km and the basins to have a thickness of 12–16 km. We assumed that a reconstruction of Zealandia should close the basins and lead to a most uniform crustal thickness. We used the standard deviation of the reconstructed crustal thickness as a measure of uniformity. The reconstruction of the Campbell Plateau area shows that the amount of extension in the Bounty Trough and the Great South Basin is far smaller than previously thought. Our results indicate that the extension of the Bounty Trough and Great South Basin occurred simultaneously.

Precise U–Pb ages of syn-extensional Miocene intrusions in the central Menderes Massif, western Turkey

Western Turkey is an area which has experienced large-scale extension of continental crust. Here we report precise crystallization ages of two intrusions in the central Menderes Massif, the Turgutlu and Salihli granodiorites, using U–Pb dating. Both intrusions occur in the southern footwall of the seismically active Alaşehir graben and were emplaced syntectonically in an extensional top-to-the-NNE shear zone which was active at retrograde greenschist-facies conditions. The U–Pb ages of 16.1 ± 0.2 Ma (monazite, Turgutlu granodiorite) and 15.0 ± 0.3 Ma (allanite, Salihli granodiorite) document that tectonic exhumation of middle-crustal rocks in the central Menderes Massif was already underway at the Early to Middle Miocene transition. Combined with published geochronological, structural and sedimentological data, the new U–Pb ages point to a continued extension since at least 16 Ma. There is no convincing evidence for a late Miocene/Pliocene phase of tectonic shortening.

Seismic velocity structure of the rifted margin of the eastern Grand Banks of Newfoundland, Canada

We present a compressional seismic velocity profile of the crust of the eastern margin of the Grand Banks of Newfoundland, Canada. This velocity model was obtained by a tomographic inversion of wide‐angle data recorded on a linear array of 24 ocean bottom seismometers (OBSs). At the landward side, we imaged a crustal thickness of 27 km in Flemish Pass and beneath Beothuk Knoll, which is thinner than the 35‐km‐thick crust of the central Grand Banks. We therefore assume that the eastern rim of the Grand Banks stretched uniformly by 25%. Farther seaward, the continental crust tapers rapidly beneath the continental slope to ∼6 km thickness. In the distal margin we find a 60‐km‐wide zone with seismic velocities between 5.0 and 6.5 km s−1 that thins to the southeast from 6 to 2 km, which we interpret as highly extended continental crust. Contrary to other seismic studies of the margins of the Grand Banks, we find seismic velocities of 8 km s−1 and higher beneath this thin crustal layer in the continent‐ocean transition. We conclude that mantle was locally emplaced at shallow levels without significant hydration from seawater or serpentinized mantle was removed along a décollement in the final stages of continental rifting. The outer edge of highly extended continental crust borders a 25‐km‐wide zone where seismic velocities increase gradually from 6.3 km s−1 just below the top of acoustic basement to 7.7 km s−1 at 5 km below basement. We interpret this area as a relatively narrow zone of exhumed and serpentinized continental mantle. Seaward, we imaged a thin and laterally heterogeneous layer with a seismic velocity that increases sharply from 5.0 km s−1 in basement ridges to 7.0 km s−1 at its base, overlying mantle velocities between 7.8 and 8.2 km s−1. We interpret this area as unroofed mantle and very thin oceanic crust that formed at an incipient, magma‐starved, ultraslow spreading ridge. A comparison of the conjugate rifted margins of the eastern Grand Banks and the Iberia Abyssal Plain show that they exhibit a similar seaward progression from continental crust to mantle to oceanic crust. This indicates that before continental breakup, rifting exhumed progressively deeper sections of the continental lithosphere on both conjugate margins. A comparison between the continent‐ocean transition of the Grand Banks and Flemish Cap shows that the final phase of continental rifting and the formation of the first oceanic crust required more time at the Grand Banks margin than at the southeastern margin of Flemish Cap.

Last stage of the Japan Sea back‐arc opening deduced from the seismic velocity structure using wide‐angle data

The Japan Sea is one of the most well studied back‐arc basins in the northwestern Pacific. The seismic crustal model, however, has been inadequate to elucidate the detailed opening model of the Japan Sea. In 2002, to clarify the late stage of the formation style of the Japan Sea opening, a seismic experiment using 35 ocean bottom seismographs (OBSs), an air gun array, and a multichannel hydrophone streamer was undertaken in the areas from the southwestern Yamato Basin, the Oki Ridge, and the southwestern Oki Trough to the coast of the southwestern Japan Island Arc. The crusts beneath the southwestern Yamato Basin and the Oki Ridge are estimated as having approximately 13 km and 19.5 km, respectively. The upper and lower crusts of the southwestern Yamato Basin are approximately 3.2 km and 8 km thick, respectively. Those of the Oki Ridge are approximately 8.2 km and 10.5 km thick, respectively. The upper crust of the Oki Ridge thickens more steeply than that of the southwestern Yamato Basin; however, the lower crust thickens more gently. The crustal structure of the southwestern Yamato Basin shows the extended continental crust accompanied with the opening of the Japan Sea. A remarkable structural characteristic, the upper crust being thinner than the lower crust, caused by listric or complicated normal faults developed in the upper crust of the southwestern Yamato Basin. This deformed upper crust is a common structural characteristic in the southern Japan Sea, which includes the Yamato Basin. The southern Yamato Basin, including the southwestern Yamato Basin, has the thinnest upper and lower crusts in the Japan Sea. For that reason, it is suggested that the southern Yamato Basin had the strongest deformation by a back‐arc opening and that the period of the opening in the southern Yamato Basin had been longest in the southern Japan Sea. The formation process of the southern Yamato Basin is inferred to have two stages: rifting and extension of continental crust separating the northeastern and southwestern Japan Island Arcs from the Asian continent and, further, the extension affected by the rotation of the southwestern Japan Island Arc.

Crustal thickness variations in the Aegean region and implications for the extension of continental crust

We installed 5 broadband and 45 short‐period temporary seismic stations, distributed partly as a dense, 100‐km‐long, N‐S linear array and partly as a regional network, throughout the Menderes Massif of western Turkey in order to study crust‐upper mantle structure and seismicity. In this study, we have combined teleseismic waveform data from these stations with data from several permanent seismic stations to determine crustal thickness variations in the Aegean region. Receiver function studies at seven broadband stations, using the H‐κ stacking method, have yielded crustal thicknesses and Vp/Vs ratios over a broad region of the Aegean. A more detailed crustal image was obtained in the central Menderes Massif, where we applied common conversion point stacking to receiver functions obtained from the N‐S linear array. The results show a general trend of westward crustal thinning from 36 km in central Anatolia to 28–30 km in the central Menderes Massif to 25 km beneath the Aegean Sea. The results also indicate that crustal thinning in the Aegean is not uniform in the N‐S extensional direction. The crust is thinner in the central Menderes Massif (28–30 km of crustal thicknesses) and the Cycladic Massif (25–26 km) than in surrounding regions where crustal thicknesses are 32–34 km. The long‐lived elevated Moho under the metamorphic core complexes suggests that the lower crust in the Aegean region is at least 3 times more viscous than that in the Basin and Range Province, where the Moho is much flatter.

The South China sea margins: Implications for rifting contrasts

Implications regarding spatially complex continental rifting, crustal extension, and the subsequent evolution to seafloor spreading are re-examined for the northern and southern-rifted margins of the South China Sea. Previous seismic studies have shown dramatic differences in the present-day crustal thicknesses as the manifestations of the strain experienced during the rifting of the margin of south China. Although the total crustal extension is presumed to be the same along the margin and adjacent ocean basin, the amount of continental crustal extension that occurred is much less along the east and central segments of the margin than along the western segment. This difference was accommodated by the early formation of oceanic crust (creating the present-day South China Sea basin) adjacent to the eastern margin segment while continued extension of continental crust was sustained to the west. Using the observed cross-sectional areas of extended continental crust derived from deep penetration seismics, two end-member models of varying rift zone widths and varying initial crustal thicknesses are qualitatively examined for three transects. Each model implies a time difference in the initiation of seafloor spreading inferred for different segments along the margin. The two models examined predict that the oceanic crust of the South China Sea basin toward the west did not begin forming until sometime between 6–12 my after its initial formation (∼32 Ma) toward the east. These results are compatible with crustal age interpretations of marine magnetic anomalies. Assuming rifting symmetry with conjugate margin segments now residing along the southern portions of the South China Sea basin implies that the total width of the zone of rifting in the west was greater than in the east by about a factor of two. We suggest the most likely causes of the rifting differences were east–west variations in the rheology of the pre-rift crust and associated east–west variations in the thermal structure of the pre-rift lithosphere. The calculated widths of rifted continental crust for the northern and southern margins, when combined with the differential widths of seafloor generated during the seafloor spreading phase, indicate the total crustal extension that occurred is about 1100 km and is remarkably consistent to within ∼10% for all three (eastern, central, western) segments examined.

Involvement of Crustal Material in Delamination of the Lithosphere after Continent-Continent Collision

Deep submersion of crustal material into the mantle, evidenced for the Variscan orogen at about 340 Ma, is explained by delamination of the continental lithosphere. The cause for such delamination is thickening of the continental crust and the underlying mantle lithosphere due to the collision of Gondwana and Laurussia. Scientific information on this continent-continent collision is scrutinized to test if the evolutionary model for the Variscan orogeny proposed here is compatible with the vast geoscientific data. It is then hypothesized that the evolution of an orogen, resulting from continent-continent collision since the late Neoproterozoic, can, in general, be described by six stages: (1) approach of continental plates; (2) beginning of continent-continent collision; (3) lateral extension of continental crust that was thickened to 60 km or more, for tens of million years; (4) delamination of the continental lithosphere involving material from the continental crust; (5) continuation of continent-continent collision without significant thickening of continental crust lasting again several tens of million years; and (6) cessation of continent-continent collision. These stages are characterized by the formation of different sedimentary, igneous, and metamorphic rock types. For instance, the origin and the beginning of exhumation of high- and ultrahigh-pressure rocks are related to stages 2 and 4. Thus, the corresponding rocks are good relative time markers. The above model was applied to the Cretaceous to Quaternary evolution of the orogenic chains in the Mediterranean region. It is concluded that this region underwent extended lithospheric delamination (stage 4) in the late Paleogene. Finally, the consequences of lithospheric delamination with continental crust involved are discussed. It is speculated that this process has resulted in an enormous loss of continental crust over the last 600 m.y. This loss could have also significantly influenced the climatic and biologic evolution on Earth.

Gulf of Mexico tectonic history: Hotspot tracks, crustal boundaries, and early salt distribution

A Late Jurassic mantle plume may have generated hotspot tracks on the North American plate and the Yucatan Penninsula tectonic block as the Gulf of Mexico opened (ca. 150 Ma). The tracks are identified from deep basement structural highs that have been mapped by integrating seismic refraction and gravity data. They are associated with high-amplitude, distinctive gravity anomalies that provide the basis for a kinematic reconstruction that restores the western ends of the hotspot tracks with a 20° clockwise rotation of the Yucatan block or almost one-half the total rotation required to open the Gulf of Mexico Basin. The duration of track generation is estimated to have been about 8–10 m.y. or almost one-half the total time required to open the Gulf of Mexico Basin. Prior to this rotation, extension of continental crust over a 10–12-m.y. interval was the result of approximately 22° of counterclockwise rotation and crustal thinning. Autochthonous salt appears to be confined to the continental flanks of the hotspot tracks, confirming that salt was deposited during continental extension and not after ocean floor had begun to form. A prominent gravity anomaly along the western boundary of the basin is interpreted to be produced by a marginal ridge, which was created along the ocean-continent transform boundary as the basin opened. The eastern flank of this marginal ridge and the northernmost, easternmost, and southernmost terminations of the hotspot tracks are interpreted to coincide with the oceanic-continental crustal boundary in the basin. Dale E. Bird consults in the oil- and gas-exploration industry as a potential fields specialist. Since 1981, his experience in the industry includes acquisition, processing, interpreting, and marketing geophysical data, including positions with Aerodat, World Geoscience, Marathon Oil Company, Digicon, and Aero Service. He earned a Ph.D. from the University of Houston in 2004, and his research interests are regional geology and tectonics and the integration and interpretation of gravity and magnetic data. He is a member of several local and international geological and geophysical societies.Kevin Burke has been a professor of geology in the University of Houston since 1983, having previously lived and worked in several parts of the world. His main research is in tectonics: The large-scale evolution of planetary lithospheres. His current interests include African and Asian geology and the derivation of large igneous provinces from the core-mantle boundary. Stuart A. Hall is a professor of geosciences specializing in the study of gravity and magnetic fields. He received a B.Sc.(honors) degree in physics from the University of Birmingham, United Kingdom (1968) and a Ph.D. in geophysics from the University of Newcastle-upon-Tyne, United Kingdom (1976). His current research interests are mainly focused on the use of geophysical data to investigate the development of small ocean basins and orogenic belts. He is a member of the Society of Exploration Geophysicists, American Geophysical Union, Royal Astronomical Society, and Sigma Xi Research Society. John F. Casey is a professor of geology and chair of the Department of Geosciences at the University of Houston specializing in structure, tectonics, and geochemisty. He received a B.A. degree in geology from the University of Pennsylvania in 1975 and a Ph.D. in geology with honors from the State University of New York, Albany, in 1980. His research has focused on the tectonics and geochemistry of midocean ridges, transform faults, and ophiolite-bearing orogenic belts.

Is small scale convection responsible for the formation of thick igneous crust along volcanic passive margins?

[1] Boutilier and Keen [1999] showed that small scale convection may produce the observed thick igneous crust along volcanic passive margins provided that a non-linear rheology is adopted and the viscosity of the upper mantle is increased suddenly after 132 km of continental crust extension. The sudden viscosity increase is justified by the melt induced dewatering of the upper mantle rocks. Through this comparative study, it is shown that inclusion of the pressure of the newly formed igneous crust into the melting model, and/or adopting a more realistic viscosity law, significantly reduces the thickness of igneous crust produced by Boutilier and Keen's models. Furthermore, it is shown that melt migration would narrow the width of the thick igneous crust. Results of this study show that small scale convection may not play a significant role in the formation of the observed thick igneous crust along volcanic passive margins.

A new magnetic map of the Weddell Sea and the Antarctic Peninsula

As part of the Antarctic Digital Magnetic Mapping Project (ADMAP) workers from VNIIOkeangeologia (Russia), the British Antarctic Survey (UK) and the Naval Research Laboratory (USA) have brought together almost all of the available magnetic data in the area 0–120°W, 60–90°S. The final map covers the whole Weddell Sea and adjacent land areas, the Antarctic Peninsula and the seas to the west, an area comparable in size with that of the USA. This paper describes the methods used during the compilation of the map and reviews briefly some of the main features shown on it. Distinct magnetic provinces are associated with Precambrian rocks of the East Antarctic craton, highly extended continental crust in the Weddell Sea embayment, the arc batholith of the Antarctic Peninsula, and oceanic crust of the northern Weddell Sea, which was created as a direct consequence of South America–Antarctica plate motion and oceanic crust generated at the Pacific–Antarctic ridge. The magnetic anomaly map thus provides an overview of the fragmentation of south-western Gondwana and the tectonic development of the Weddell Sea sector of Antarctica.

Regional implications of geochemistry and style of emplacement of Miocene I-type diorite and granite, Delos, Cyclades, Greece

The Miocene plutons of the Cyclades were emplaced in a subduction setting during regional back-arc extension of continental crust, that led to flat-lying mid-crustal detachment faulting. Mapping of the island of Delos shows that quartz diorite and tonalite were emplaced as dykes in country rock of schist and marble within shear zones parallel to the extension direction. Mafic magmas were followed by numerous small batches of felsic magma, with magmatic and ductile deformation synchronous with magma emplacement. Late granite dykes occupy brittle fractures in the more deformed rocks. Mafic and intermediate rocks show a bimodal distribution of incompatible trace elements, with one group of broadly tholeiitic character and the other with substantial enrichment in Sr, Nb, and HFSE, but low Th and Ba. These differences appear to be inherited from two distinct mafic sources that are different from the mafic source for the plutons of the eastern Cyclades. Voluminous granodiorite results from these mafic magmas fractionating and/or mixing with felsic crustal material, some of which was derived by anatexis of a sedimentary protolith, indicated by high B and Mn. Some late granites appear derived from partial melting of Hercynian paragneiss. Regionally, the shear zones appear to be feeders to more extensive granitic plutons located at space produced at ramps in detachment fault zones. The shear zones parallel the Mid-Cycladic Lineament, a broad zone of displacement between two crustal blocks rotating in opposing directions as rollback took place at the Hellenic subduction zone. Distinctive geochemical features in Miocene igneous rocks suggests that these two blocks had quite different geological histories. The localisation of plutonism and core complexes near the Mid Cycladic Lineament suggests that this crustal-scale shear played a role in bringing subduction-derived magmas to mid-crustal levels. The heat supplied by the mafic magmas promoted ductile deformation high in the crust, where extension was concentrated, leading to the formation of core complexes. The regional extension resulted in progressive shallowing of the position of the granite solidus within the crust, leading to welding of the Mid-Cycladic Lineament, which is no longer seismically active.

Strontium Isotopes, Age, and Tectonic Setting of Cambrian Salinas along the Rift and Transform Margins of the Argentine Precordillera and Southern Laurentia

The Argentine Precordillera generally is interpreted as a fragment of continental crust and sedimentary cover that was rifted from the Ouachita embayment of southern Laurentia in Cambrian time. Synrift sedimentary successions in the northern part of the Precordillera and in the Birmingham graben of southern Laurentia include thick evaporite deposits. Analyses of strontium isotopes indicate that the evaporites are of late Early Cambrian age, consistent with biostratigraphic correlations. Variations in strontium ratios indicate that these Early Cambrian salinas had a persistent base of marine water and that contributions from meteoric runoff varied through time and with location. The stratigraphic and isotopic data suggest a depositional setting of restricted circulation within a system of extensional fault blocks. The salinas occupied graben blocks that were connected to marine circulation and that intermittently received meteoric runoff carrying fine detritus and radiogenic strontium from horst blocks of continental crust. The evaporites in the Precordillera show less meteoric influence than the evaporites in the Birmingham graben, which was far inboard from the continental margin of southern Laurentia. These relationships are consistent with continental rifting in Early Cambrian time, extension of continental crust, and deposition of synrift sediment in graben blocks that were partially open to marine circulation.

Evidence for crustal extension and inversion in eastern Tasmania, Australia, during the Neoproterozoic and Early Palaeozoic

The island of Tasmania in southeast Australia consists of a number of stratotectonic elements. The relationships between these elements are largely obscured by younger cover of the Tasmania Basin, which contains extensive dolerite sills that limit the ability of potential field techniques to map basement. Therefore the development of a robust tectonic model for Tasmania has been inhibited. To assist in the development of a tectonic model, a deep seismic reflection program undertaken offshore around the entire island was designed to map the large-scale structures of Tasmania at depth. The airgun seismic energy was also recorded at a number of seismographs deployed across the island, allowing low resolution 3D tomographic imaging. Short reflection profiles were recorded onshore across structures which could not be imaged by the offshore profiling. This paper focuses on eastern Tasmania. In the seismic sections, the Proterozoic basement in the southeast is mostly featureless, except for large rotated blocks with weakly reflective boundary faults, indicating extension of the Tyennan Element by block faulting. The deposition of the sedimentary succession of the Adamsfield–Jubilee Element was related to this extensional event. In the northeast, a reflective lower crust is interpreted to represent thrust slices of previously highly extended continental crust and possibly fragments of oceanic crust. The Early Palaeozoic sedimentary succession of the Northeast Tasmania Element formed across the inverted margin. The apparently complex geology of eastern Tasmania therefore fits into an extensional model where continental extension eventually led to the formation of very thin continental crust and possibly oceanic crust to the east. The extension was probably related to Late Neoproterozoic extension recorded elsewhere in Australia. The region was subsequently shortened, probably in a northeast–southwest direction, with most shortening accommodated in the seismically reflective, probably oceanic part of the crust, and little or no shortening in the block-faulted, and extended continental crust.