Mesozoic exhumation of the southern Cape, South Africa, quantified using apatite fission track thermochronology

Southern Africa's topography is distinctive. An inland plateau of low relief and high average elevation is separated from a coastal plane of high relief and low average elevation by a steeply dipping escarpment. The origin and evolution of this topography is poorly understood because, unlike high plateaus elsewhere, its development cannot be easily linked to present day compressional plate boundary processes. Understanding the development of this regional landscape since the break-up of Gondwana is a first order step towards resolving regional epeirogenesis. We present data that quantifies the timing and extent of exhumation across the southern Cape escarpment and coastal plane, using apatite fission track analysis (AFTA) of 25 outcrop samples and 31 samples from three deep boreholes (KW1/67, SA1/66, CR1/68). Outcrop fission track (AFT) ages are Cretaceous and are significantly younger than the stratigraphic ages of their host rocks, indicating that the samples have experienced elevated paleotemperatures. Mean track lengths vary from 11.86 to 14.23 μm. The lack of Cenozoic apatite ages suggests that major cooling was over by the end Cretaceous. The results for three boreholes, situated seaward (south) of the escarpment, indicate an episode of increased denudation in the mid-late Cretaceous (100–80 Ma). An earlier episode of increased denudation (140–120 Ma) is identified from a borehole north of the escarpment. Thermal modelling indicates a history involving 2.5–3.5 km of denudation in the mid-late Cretaceous (100–80 Ma) at a rate of 175 to 125 m/Ma. The AFT data suggest that less than 1 km of overburden has been eroded regionally since the late Cretaceous (< 80 Ma) at a rate of 10 to 15 m/Ma, but do not discount the possibility of minor (in relative amplitude) episodes of uplift and river incision through the Cenozoic. The reasons for rapid denudation in these early and mid-Cretaceous episodes are less clear, but may be related to epeirogenic uplift associated with an increase in mantle buoyancy as reflected in two punctuated episodes of alkaline intrusions (e.g. kimberlites) across southern Africa and contemporaneous formation of two large mafic igneous provinces (~ 130 and 90 Ma) flanking its continental margins. Because Cenozoic denudation rates are relatively minimal, epeirogenic uplift of southern Africa and its distinct topography cannot be primarily related to Cenozoic mantle processes, consistent with the lack of any significant igneous activity across this region during that time.     

Shape and amount of the Quaternary uplift of the western Rhenish shield and the Ardennes (western Europe)

A good evaluation of the Quaternary uplift of the Rhenish shield is a key element for the understanding of the Cenozoic geodynamics of the western European platform in front of the alpine arc. Previous maps of the massif uplift relied on fluvial incision data since the time of the rivers' Younger Main Terrace to infer a maximum post-0.73 Ma uplift of ~ 290 m in the SE Eifel. Here, we propose a new interpretation of the incision data of the intra-massif streams, where anomalies in the terrace profiles would result from knickpoint retreat in the tributaries of the main rivers rather than from tectonic deformation. We also use additional geomorphological data referring to (1) deformed Tertiary planation surfaces, (2) the history of stream piracy that severely affected the Meuse basin in the last 1 Ma, and (3) incision data outside the Rhenish shield. A new map of the post-0.73 Ma uplift of the Rhenish shield is drawn on the basis of this enlarged dataset. It reduces the maximum amount of tectonic uplift in the SE Eifel to ~ 140 m and modifies the general shape of the uplift, namely straightening its E–W profile. It is also suggested that an uplift wave migrated across the massif, starting from its southern margin in the early Pleistocene and currently showing the highest intensity of uplift in the northern Ardennes and Eifel. These features seem to favour an uplift mechanism chiefly related to lithospheric folding and minimize the impact on the topography of a more local Eifel plume.     

Global Cenozoic Paleobathymetry with a focus on the Northern Hemisphere Oceanic Gateways

The evolution of the Northern Hemisphere oceanic gateways has facilitated ocean circulation changes and may have influenced climatic variations in the Cenozoic time (66 Ma–0 Ma). However, the timing of these oceanic gateway events is poorly constrained and is often neglected in global paleobathymetric reconstructions. We have therefore re-evaluated the evolution of the Northern hemisphere oceanic gateways (i.e. the Fram Strait, Greenland–Scotland Ridge, the Central American Seaway, and the Tethys Seaway) and embedded their tectonic histories in a new global paleobathymetry and topography model for the Cenozoic time. Our new paleobathymetry model incorporates Northeast Atlantic paleobathymetric variations due to Iceland mantle plume activity, updated regional plate kinematics, and models for the oceanic lithospheric age, sediment thickness, and reconstructed oceanic plateaus and microcontinents. We also provide a global paleotopography model based on new and previously published regional models. In particular, the new model documents important bathymetric changes in the Northeast Atlantic and in the Tethys Seaway near the Eocene–Oligocene transition (~34 Ma), the time of the first glaciations of Antarctica, believed to be triggered by the opening of the Southern Ocean gateways (i.e. the Drake Passage and the Tasman Gateway) and subsequent Antarctic Circumpolar Current initiation. Our new model can be used to test whether the Northern Hemisphere gateways could have also played an important role modulating ocean circulation and climate at that time. In addition, we provide a set of realistic global bathymetric and topographic reconstructions for the Cenozoic time at one million-year interval for further use in paleo-ocean circulation and climate models.     

The Cenozoic evolution of the Roer Valley Rift System integrated at a European scale

The Roer Valley Rift System (RVRS) is located between the West European rift and the North Sea rift system. During the Cenozoic, the RVRS was characterized by several periods of subsidence and inversion, which are linked to the evolution of the adjacent rift systems. Combination of subsidence analysis and results from the analysis of thickness distributions and fault systems allows the determination of the Cenozoic evolution and quantification of the subsidence. During the Early Paleocene, the RVRS was inverted (Laramide phase). The backstripping method shows that the RVRS was subsequently mainly affected by two periods of subsidence, during the Late Paleocene and the Oligocene–Quaternary time intervals, separated by an inversion phase during the Late Eocene. During the Oligocene and Miocene periods, the thickness of the sediments and the distribution of the active faults reveal a radical rotation of the direction of extension by about 70–80° (counter clockwise). Integration of these results at a European scale indicates that the Late Paleocene subsidence was related to the evolution of the North Sea basins, whereas the Oligocene–Quaternary subsidence is connected to the West European rift evolution. The distribution of the inverted provinces also shows that the Early Paleocene inversion (Laramide phase) has affected the whole European crust, whereas the Late Eocene inversion was restricted to the southern North Sea basins and the Channel area. Finally, comparison of these deformations in the European crust with the evolution of the Alpine chain suggests that the formation of the Alps has controlled the evolution of the European crust since the beginning of the Cenozoic.     

Effect of local variations of vertical and horizontal stresses on the Cenozoic structuring of the mid-Norwegian shelf

The mid-Norwegian margin has a complex history and has experienced several phases of changing horizontal and vertical stresses on regional and local scale during the Cenozoic time. In addition to regional stresses related to the opening of the North Atlantic (i.e. ridge push), local variations in stress history may be important for development, distribution and reactivation of structures in the Vøring area in Cenozoic time. Presence and stability of flexural hinge zones between areas of relative uplift and subsidence have played an important role for focusing shallow horizontal stresses within the basins. Emplacement of lower crustal bodies during break-up will, whatever the nature of these bodies, have substantial isostatic effects, and modelling show that this could cause many hundred meters of temporal uplift above the lower crustal bodies, locally exceeding 1300 m of surface uplift. Effects of intra plate stress (IPS) are modelled along three 2D transects across the Vøring Basin. Modelling shows that IPS may have given substantial vertical motions in certain areas of the mid-Norwegian shelf, both with extensional IPS at the time of break-up, and later with compressive IPS during Tertiary time. The modelling assumes a strongly reduced effective elastic thickness (EET) due to lithospheric heating at break-up and later increasing EET as the lithosphere cooled towards present time. Our modelling takes into account the tectonic and isostatic effects of loading faulting and lithospheric thinning throughout the geological history, including several phases of extension prior to the Cenozoic compression. This approach emphasizes the importance of the deformation history of the lithosphere compared to other studies that only take into account the effects of Cenozoic processes of compression and loading on the sedimentary units. We do not state that isostatic uplift or intra plate stress are the most important causes for Cenozoic uplift and compressional deformation in this area, but point to the fact that these factors locally may have played an important role in focusing deformation caused by an interplay of different mechanisms.     

Ages and geodynamic settings of Xilamulun Mo–Cu metallogenic belt in the northern part of the North China Craton

The Xilamulun belt along the northern part of the North China Craton is located in eastern segment of the Central Asian Orogenic Belt and has great economic potential for Mo–Cu mineralization. More than ten medium to large ore deposits have been discovered in this region in the recent years. The major types of mineralization type include porphyry (Chehugou Mo–Cu, Kulitou Mo–Cu, Xiaodonggou Mo and Jiguanshan Mo), quartz vein (Nianzigou Mo, Xinjing Mo), epithermal (Hongshanzi Mo–U) and alteration assemblage (Liulingou Mo). The timing of mineralization was previously thought to be Yanshanian (208–290 Ma), however, Indosinian (260–208 Ma) ages for intrusions and mineralization have been recognized in recent years. Based on geochronologic data and regional geological evidence, it is suggested that the mineralization in the Xilamulun belt was formed during multiple events. The mineralization processes are related to a post-collisional extension stage (~ 258–210 Ma) with the generation of the porphyry molybdenum–copper deposit, a tectonic stress transformation from NS to EW (~ 185–150 Ma) that gave rise to vein or porphyry molybdenum deposit, and a lithospheric thinning stage (~ 140–110 Ma) with porphyry molybdenum deposit.     

The Paleoproterozoic North Hebei Orogen: North China craton's collisional suture with the Columbia supercontinent

Understanding the geologic history and position of the North China craton in the Paleoproterozoic Columbia supercontinent has proven elusive. Paleoproterozoic orogenic episodes (2.00–1.85 Ga) are temporally associated with ultimate stabilization of the North China craton (NCC), followed by the development of extensive craton-wide rift systems at 1.85–1.80 Ga. The age difference between the sedimentary cover and the metamorphic basement is up to 500–700 Ma, suggesting that uplift and doming of cratonic basement occurred in the latest Paleoproterozoic. Mafic dike swarms (1.80–1.77 Ga) and anorogenic magmatism (1.80–1.70 Ga) record the extensional breakup and dispersal of the North China craton during this stage. The late Paleoproterozoic tectonic framework and geological events documented provide important constraints for reconstruction of the NCC within the Late Paleoproterozoic supercontinent of Columbia.An east-west striking thousand kilometer long belt of khondalites (granulite facies metapelites) stretches along the northern margin of the North China craton, on the cratonward side of the Northern Hebei orogenic belt. This granulite belt includes Mg–Al (sapphirine bearing) granulites that reached ultrahigh-temperature “peak” metamorphic conditions of  1000 °C at 10 kbars at 1927 ± 11 Ma. Following peak ultrahigh-temperature conditions, the rocks underwent initial isobaric cooling and subsequent isothermal decompression, and these trajectories are interpreted to be part of an overall anti-clockwise P-T evolution indicating that the northern margin of the craton experienced continental collision at 1.93–1.92 Ga. The position of the khondalite belt south of the Northern Hebei orogenic belt makes it analogous to Tibet, a continental collision-related plateau characterized by double crustal thicknesses and granulite facies metamorphism at depth. We suggest that the tectonic evolution of the NCC during this period was closely related to the assembly and break-up of the Columbia supercontinent, and that the NCC was adjacent to the Baltic and Amazonian cratons in the period 2.00–1.70 Ga. Craton-wide extension occurred within 100–150 Ma of collision along the northern margin of the craton at 1.93–1.92 Ga. It is concluded that mantle upwellings are chiefly responsible for the breakup of the NCC from the Paleoproterozoic supercontinent.     

Ar–Ar ages of intrusions in East Greenland: Rift-to-drift transition over the Iceland hotspot

Sixteen ⁴⁰Ar–³⁹Ar ages are presented for alkaline intrusions to appraise prolonged post-breakup magmatism of the central East Greenland rifted margin, the chronology of rift-to-drift transition, and the asymmetry of magmatic activity in the Northeast Atlantic Igneous Province. The alkaline intrusions mainly crop out in tectonic and magmatic lineaments orthogonal to the rifted margin and occur up to 100 km inland. The area south of the Kangerlussuaq Fjord includes at least four tectonic lineaments and the intrusions are confined to three time windows at 56–54 Ma, 50–47 Ma and 37–35 Ma. In the Kangerlussuaq Fjord, which coincides with a major tectonic lineament possibly the failed arm of a triple junction, the alkaline plutons span from 56 to 40 Ma. To the north and within the continental flood basalt succession, alkaline intrusions of the north–south trending Wiedemann Fjord–Kronborg Gletscher lineament range from 52 to 36 Ma.

We show that post-breakup magmatism of the East Greenland rifted margin can be linked to reconfiguration of spreading ridges in the Northeast Atlantic. Northwards propagation of the proto-Kolbeinsey ridge rifted the Jan Mayen micro-continent away from central East Greenland and resulted in protracted rift-to-drift transition. The intrusions of the Wiedemann Fjord–Kronborg Gletscher lineament are interpreted as a failed continental rift system and the intrusions of the Kangerlussuaq Fjord as off-axis magmatism. The post-breakup intrusions south of Kangerlussuaq Fjord occur landward of the Greenland–Iceland Rise and are explained by mantle melting caused first by the crossing of the central East Greenland rifted margin over the axis of the Iceland mantle plume (50–47 Ma) and later by uplift associated with regional plate-tectonic reorganization (37–35 Ma). The Iceland mantle plume was instrumental in causing protracted rift-to-drift transition and post-breakup tholeiitic and alkaline magmatism on the East Greenland rifted margin, and asymmetry in the magmatic history of the conjugate margins of the central Northeast Atlantic.     

Estimates of heat flow from Cenozoic seafloor using global depth and age data

The total heat output of the Earth constrains models of mantle and core dynamics. Previously published estimates (42–44 TW) have recently been questioned because the measured conductive heat flow on young oceanic lithosphere is about a factor of 2 less than the expected heat flow based on half-space cooling models. Taking the conductive ocean heat flow values at face value reduces the global heat flow from 44 to 31 TW, which has major implications for geodynamics and Earth history. To help resolve this issue, we develop a new method of estimating total oceanic heat flow from depth and age data. The overall elevation of the global ridge system, relative to the deep ocean basins, provides an independent estimate of the total heat content of the lithosphere. Heat flow is proportional to the measured subsidence rate times the heat capacity divided by the thermal expansion coefficient. The largest uncertainty in this method is due to uncertainties in the thermal expansion coefficient and heat capacity. Scalar subsidence rate is computed from gradients of depth and age grids. The method cannot be applied over very young seafloor (< 3 Ma) where age gradient is discontinuous and the assumption of isostasy is invalid. Between 3 and 66 Ma, the new estimates are in agreement with half-space cooling model. Our model-independent estimate of the total heat output of Cenozoic seafloor is 18.6 to 20.5 TW, which leads to a global output of 42 to 44 TW in agreement with previous studies.     

A fission-track and (U–Th)/He thermochronometric study of the northern margin of the South China Sea: An example of a complex passive margin

Zircon fission track (ZFT), apatite fission track (AFT) and (U–Th)/He thermochronometric data are used to reconstruct the Cenozoic exhumation history of the South China continental margin. A south to north sample transect from coast to continental interior yielded ZFT ages between 116.6 ± 4.7 Ma and 87.3 ± 4.0, indicating that by the Late Cretaceous samples were at depths of 5–6 km in the upper crust. Apatite FT ages range between 60.9 ± 3.6 and 37.3 ± 2.3 Ma with mean track lengths between 13.26 ± 0.16 µm and 13.95 ± 0.19 µm whilst AHe ages are marginally younger 47.5 ± 1.9–15.3 ± 0.5 Ma. These results show the sampled rocks resided in the top 1–1.5 km of the crust for most of the Cenozoic. Thermal history modeling of the combined FT and (U–Th)/He datasets reveal a common three stage cooling history which differed systematically in timing inland away from the rifted margin. 1) Initial phase of rapid cooling that youngs to the north, 2) a period of relative (but not perfect) thermal stasis at ~ 70–60 °C which increases in duration from the south to the north; 3) final-stage cooling to surface temperatures that initiated in all samples between 15 and 10 Ma. The timing and pattern of rock uplift and erosion does not fit with conventional passive margin landscape models that require youngest exhumation ages to be concentrated at or close to the rifted margin. The history of South China margin is more complex aided by weakened crust from the active margin period that immediately preceded rifting and opening of the South China Sea. This rheological inheritance created a transition zone of steeply thinned crust that served as a flexural filter disconnecting the northern margin of the South China block and site of active rifting to the south. Consequently whilst the South China margin displays many features of a rifted continental margin its exhumation history does not conform to conventional images of a passive margin.     

Apatite fission track thermochronology of the Precambrian Aksu blueschist, NW China: Implications for thermo–tectonic evolution of the north Tarim basement

The Precambrian Aksu blueschist is located in the northwestern margin of the Tarim Block, NW China. In recent decades, many studies were carried out with focus on the metamorphic age. However, a complete understanding of the evolution of the Tarim Block requires the cooling history of the Precambrian metamorphic rocks and the time–temperature paths as determined by low-temperature thermochronometry. Therefore, apatite fission track (AFT) technique was applied on the Precambrian Aksu blueschist to reveal the thermo–tectonic evolution of the north Tarim basement. All of the six blueschist samples analysed in this study yielded AFT ages spanning 107.5–62.5 Ma, much younger than the blueschist facies metamorphic age of Neoproterozoic, and confined track lengths are between 10.46 and 12.12 µm. Based on regional stratigraphic sequences, the AFT thermal history modeling as well as previous chronological results, the thermo–tectonic evolution of the Aksu blueschist can be roughly reconstructed with four stages: (1) the Precambrain Aksu blueschist exhumed to the surface soon after its formation. Erosion during the Early Sinian is indicated by the lack of sedimentation until the Late Sinian; (2) the Late Sinian strata are continuous, while the Middle–Upper Silurian and the Lower–Middle Carboniferous strata are absent. The total thickness of the Late Sinian and Paleozoic strata probably reached 10,000 m and resulted in the total annealing and thermal resetting of AFT ages; (3) the AFT ages in the Cretaceous are related with the widespread uplift in Tian Shan and its adjacent regions that restarted the AFT clock during the Late Mesozoic. These reflect a distant effect of the collision of the Lhasa terrane with Eurasia in the Late Jurassic–Early Cretaceous; and (4) sediments of Cenozoic are documented in the Aksu area. The Aksu blueschist was heated to partial annealing zone with the overlying Cenozoic sediments. During Miocene time, the Aksu blueschist was re-exhumed which was probably a distant response to the ongoing India–Eurasia convergence.     

Evidence for coupled reverse and normal active faulting in W Iberia: The Vidigueira–Moura and Alqueva faults (SE Portugal)

The Vidigueira–Moura fault (VMF) is a 65 km long, E–W trending, N dipping reverse left-lateral late Variscan structure located in SE Portugal (W Iberia), which has been reactivated during the Cenozoic with reverse right-lateral slip. It is intersected by, and interferes with the NE–SW trending Alentejo–Plasencia fault. East of this intersection, for a length of 40 km the VMF borders an intracratonic tectonic basin on its northern side, thrusting Paleozoic schists, meta-volcanics and granites, on the north, over Cenozoic continental sediments preserved in the basin, on the south. West of the faults intersection, evidence of Cenozoic reactivation is scarce. In the eastern sector, Plio-Quaternary VMF reactivation is indicated by geomorphologic, stratigraphic, and structural data, showing reverse movement with a right-lateral strike-slip component, in response to a NW–SE trending compressive stress. An average vertical displacement rate of 0.06 to 0.08 mm/yr since late Pliocene (roughly the last 2.5 Ma) is estimated. The Alqueva fault (AF) is a subparallel, northward dipping, 7.5 km long anastomosing fault zone that affects Palaeozoic basement rocks, and is located 2.5 km north and on the hanging block of the VMF. The AF is also a reverse left-lateral late Variscan structure, which has been reactivated during the Tertiary with reverse right-lateral slip; however, Plio-Quaternary reactivation was normal left-lateral, as shown by abundant kinematical criteria (slickensides) and geomorphic evidence. It shows an average displacement rate of 0.02 mm/yr for the vertical component of movement in the approximately last 2.5 Ma. It is proposed that the normal displacements on the AF result from tangential longitudinal strain on the upthrown block of the VMF above a convex ramp of this main reverse structure. According to this model of faults interaction, the AF is interpreted to work as a bending-moment fault sited above the VMF thrust ramp. Consequently, it is expected that the displacements on the AF increase towards the topographic surface with the increase in the imposed extension, declining downwards until they vanish above or at the VMF ramp. In order to constrain the proposed scheme, numerical modeling was performed, aiming at the reproduction of the present topography across the faults using different geodynamic models and fault geometries and displacements.     

Crustal attenuation in the Southern Andean retroarc (38°–39°30′ S) determined from tectonic and gravimetric studies: The Lonco-Luán asthenospheric anomaly

The western retroarc of the Southern Andes between 38° and 40° S is formed by a NNW-elongated ridge not associated with stacked thrust sheets. On the contrary, during the last 4–3 Ma this ridge was affected by extensional deformation, regional uplift and related folding on a very broad scale. Receiver function analysis shows that the drainage divide area and adjacent retroarc lie over an attenuated crust. Expected crustal thickness at these latitudes is around 38 km, whereas in this part of the retroarc the thickness is less than 32 km. The causes for such attenuation have been linked to a moderate steepening of the subducted Nazca plate beneath the South American plate, which is suggested by a westward shift and narrowing of the magmatic arc during the last 4 to 5 Ma. Gravimetric studies show that the upper plate did not react homogeneously to slab steepening, but ancient sutures and lithospheric discontinuities deeply buried under Mesozoic to Cenozoic sequences in the retroarc were locally reactivated. These processes resulted in an asthenospheric anomaly that correlates at the surface with the area of Pliocene to Quaternary doming, widespread extension and three radial troughs. Two of the troughs have accommodated substantial amounts of extension, but the third was probably aborted at an early stage. Moreover, the presence of an anomalous concentration of calderas and large volcanic centers over the proposed asthenospheric anomaly, and their age distribution, may indicate minor migration of the asthenospheric anomaly between 4 and 2 Ma through the western South American plate.     

Revised Mesozoic–Cenozoic orogenic architecture and gold metallogeny in the northern Circum-Pacific

The orogenic collages of the northern Circum-Pacific between Japan and Alaska revealed an endowment of about 450 Moz Au in various deposit types and diverse Mesozoic–Cenozoic tectonic settings. The area consists of predominantly late Paleozoic to Cenozoic turbidite to island arc terranes as well as Precambrian cratonic terranes that can be grouped into the Kolyma–Alaska, Kamchatka–Aleutian, and Nipponide collages. The latter can be linked via the Mongol–Okhotsk suture with the late Paleozoic to early Mesozoic terranes in the Mongolides.The early Yanshanian magmatic arc terranes in the fossil Kolyma–Alaska collage host copper–gold porphyry deposits, which have only recently received much attention. Exploration has revealed a large and growing gold endowment of more than 30 Moz Au in some individual deposits, with smaller role of epithermal deposits. This mineralization, formed at 140–125 Ma, is partly coeval with the collisions of magmatic arcs with the passive margin sequences of the Siberian craton and related granitoid magmatism. About 200 Moz of gold is known in the Kolyma–Alaska collage in the Mesozoic orogenic gold deposits and related Quaternary placers. The Central Kolyma, Indigirka, South Verkhoyansk, and North Chukotka subprovinces of the collage revealed an endowment of more than 10 Moz Au each. A similar and coeval event in the Mongolides in relation to the collision between Siberia and North China is largely reflected in still poorly dated intrusion-related gold deposits clustered along the Mongol–Okhotsk suture.The overlapping Yanshanian magmatic arcs in Transbaikalia and northeast China and the Okhotsk–Chukotka magmatic arc in the Russian Far East stitch the Kolyma–Alaska collage with the Paleozoic Central Asian supercollage and adjacent cratons. While the Okhotsk–Chukotka arc reveals a relatively simple and broad oroclinal pattern, the Yanshanian arcs in Mongolia, and NE China form a tightly deformed giant Z-shaped feature that was bent in response to the southward movement of the Siberian craton and northward translation of the Nipponides and North China craton to close the Mongol–Okhotsk suture in late Jurassic to Cretaceous times. The Yanshanian arcs host mostly small to medium-sized 100–70 Ma Au–Ag deposits, with the largest endowment discovered in the Baley district in Transbaikalia and at Kupol in the northern part of the Okhotsk–Chukotka arc. Some intrusion-related gold deposits were formed synchronously with this arc magmatism, with the largest known examples in the Tintina belt in Alaska formed at 104 and 93–91 Ma.The Kamchatka–Aleutian collage is still evolving in front of the westward-subducting Pacific plate. It's late Cretaceous to Paleogene magmatic arc rocks form immature island arc terranes, extending from the Aleutian islands towards the Nipponides via Kamchatka peninsula, Kuril islands and eastern Sakhalin. However, in the Nipponides, the Sikhote–Alin portion of the magmatic arc overlaps the Mesozoic turbidite terranes. The oroclinal pattern of this more than 8000 km-long magmatic arc indicates its westward translation in agreement with the movement of the Pacific plate so that the arc is presently colliding with itself along the island of Sakhalin, a seismically active intraplate lineament and a boundary between the Nipponide and Kamchatka–Aleutian collages. This magmatic arc is usually interpreted to be of intra-oceanic origin, with subsequent docking to Asia from the south; however, presence of the Sea of Okhotsk cratonic terrane between Sakhalin and Kamchatka suggests that it may be rather considered as an external arc system that separated from the rest of Asia due to backarc spreading events, therefore, forming the most external arc system at the active margin with the Pacific plate. The subduction-related events in the collage produced numerous late Mesozoic to Cenozoic 1–3 Moz gold epithermal deposit in Kamchatka and Sikhote–Alin as well as Au–Cu porphyry deposits, with currently largest gold endowment in the pre-Tertiary Pebble Copper deposit in Alaska. The westward translation of the Kamchatka–Aleutian collage might have controlled the emplacement of this porphyry deposit, as well as up to 30 Moz into intrusion-related gold deposits at 70–65 Ma in the Kuskokwim belt, immediately north from the porphyry cluster.     

Expanding the Cenozoic paleoceanographic record in the Central Arctic Ocean: IODP Expedition 302 Synthesis

The Arctic Coring Expedition (ACEX) proved to be one of the most transformational missions in almost 40 year of scientific ocean drilling. ACEX recovered the first Cenozoic sedimentary sequence from the Arctic Ocean and extended earlier piston core records from ≈1.5 Ma back to ≈56 Ma. The results have had a major impact in paleoceanography even though the recovered sediments represents only 29% of Cenozoic time. The missing time intervals were primarily the result of two unexpected hiatuses. This important Cenozoic paleoceanographic record was reconstructed from a total of 339 m sediments. The wide range of analyses conducted on the recovered material, along with studies that integrated regional tectonics and geophysical data, produced surprising results including high Arctic Ocean surface water temperatures and a hydrologically active climate during the Paleocene Eocene Thermal Maximum (PETM), the occurrence of a fresher water Arctic in the Eocene, ice-rafted debris as old as middle Eocene, a middle Eocene environment rife with organic carbon, and ventilation of the Arctic Ocean to the North Atlantic through the Fram Strait near the early-middle Miocene boundary. Taken together, these results have transformed our view of the Cenozoic Arctic Ocean and its role in the Earth climate system.     

Rift propagation at craton margin.: Distribution of faulting and volcanism in the North Tanzanian Divergence (East Africa) during Neogene times

A revised kinematic model is proposed for the Neogene tectono-magmatic development of the North Tanzanian Divergence where the axial valley in S Kenya splits southwards into a wide diverging pattern of block faulting in association with the disappearance of volcanism. Propagation of rifting along the S Kenya proto-rift during the last 8 Ma is first assumed to have operated by linkage of discrete magmatic cells as far S as the Ngorongoro–Kilimanjaro transverse volcanic belt that follows the margin of cratonic blocks in N Tanzania. Strain is believed to have nucleated throughout the thermally-weakened lithosphere in the transverse volcanic belt that might have later linked the S Kenya and N Tanzania rift segments with marked structural changes along-strike. The North Tanzanian Divergence is now regarded as a two-armed rift pattern involving: (1) a wide domain of tilted fault blocks to the W (Mbulu) that encompasses the Eyasi and Manyara fault systems, in direct continuation with the Natron northern trough. The reactivation of basement fabrics in the cold and intact Precambrian lithosphere in the Mbulu domain resulted in an oblique rift pattern that contrasts with the orthogonal extension that prevailed in the Magadi–Natron trough above a more attenuated lithosphere. (2) To the E, the Pangani horst-like range is thought to be a younger (< 1 Ma) structure that formed in response to the relocation of extension S of the Kilimanjaro magmatic center. A significant contrast in the mechanical behaviour of the stretched lithosphere in the North Tanzanian diverging rift is assumed to have occurred on both sides of the Masai cratonic block with a mid-crustal decoupling level to the W where asymmetrical fault-basin patterns are dominant (Magadi–Natron and Mbulu), whereas a component of dynamical uplift is suspected to have caused the topographic elevation of the Pangani range in relation with possible far-travelled mantle melts produced at depth further N.     

Cretaceous–Tertiary geology of the Gangdese Arc in the Linzhou area, southern Tibet

Knowledge of the Cretaceous–Tertiary history of upper crustal shortening and magmatism in Tibet is fundamental to placing constraints on when and how the Tibetan plateau formed. In the Lhasa terrane of southern Tibet, the widely exposed angular unconformity beneath uppermost Cretaceous–lower Tertiary volcanic-bearing strata of the Linzizong Formation provides an excellent geologic and time marker to distinguish between deformation that occurred before vs. during the Indo-Asian collision. In the Linzhou area, located  30 km north of the city of Lhasa, a > 3-km-thick section of the Linzizong Formation lies unconformably on Cretaceous and older rocks that were shortened by both northward- and southward-verging structures during the Late Cretaceous. The Linzizong Formation dips northward in the footwall of a north-dipping thrust system that involves Triassic–Jurassic strata and a granite intrusion in the hanging wall. U–Pb zircon geochronologic studies show that the Linzizong Formation ranges in age from 69 Ma to at least 47 Ma and that the hanging wall granite intrusion crystallized at  52 Ma, coeval with dike emplacement into footwall Cretaceous strata. ⁴⁰Ar/³⁹Ar thermochronologic studies suggest slow cooling of the granite between 49 and 42 Ma, followed by an episode of accelerated cooling to upper crustal levels beginning at  42 Ma. The onset of rapid cooling was coeval with the cessation of voluminous arc magmatism in southern Tibet and is interpreted be a consequence of either (1) Tertiary thrusting in this region or (2) regional rock uplift and erosion following removal of overthickened Gangdese arc lower crust and upper mantle or break-off of the Neo-Tethyan oceanic slab.     

宁武盆地及周缘岩体裂变径迹对山西地块中—新生代构造演化的约束

宁武盆地及周缘岩体的抬升剥蚀对于山西地块中—新生代构造演化具有重要的指示意义。本文对宁武盆地及周缘岩体进行裂变径迹分析,磷灰石裂变径迹年龄97～47 Ma,锆石裂变径迹年龄161～141 Ma。裂变径迹记录了早白垩世早期(145～125 Ma)、晚白垩世(85～70 Ma)、古新世晚期—始新世早期(59～53 Ma)和渐新世晚期(28 Ma)的4次抬升剥蚀事件。综合分析山西地块的裂变径迹数据,表明隆起区晚古生代以来发生了多期抬升剥蚀事件。山西地块中—新生代构造演化具有时空差异。周缘岩体样品的裂变径迹年龄大于盆地内沉积地层样品的年龄,指示了周缘山体先于盆地抬升剥蚀。晋东北抬升剥蚀时限早于晋西南。山西裂谷系西南端裂开较早。裂谷系发育具有由南向北扩展的特征,这与地层保留记录相一致。山西地块现今地貌格局是在中生代发育一系列雁行状排列的复背斜和复向斜构造基础上发展而成的。     

Petrology and geochemistry of post-collisional Middle Eocene volcanic units in North-Central Turkey: Evidence for magma generation by slab breakoff following the closure of the Northern Neotethys Ocean

The Eocene volcano-sedimentary units of Northern Anatolia are confined into a narrow zone trending parallel to the Intra Pontide and İzmir–Ankara–Erzincan sutures, along which the northern branch of the Neotethys Ocean was closed during a period between Late Maastrichtian and Paleocene. The Middle Eocene formations overlie both the imbricated and highly deformed units of the suture zone, which are Paleocene or older in age, as well as the formations of adjacent continental blocks with a regional disconformity. Therefore, they can be regarded to be post-collisional. These units are composed of subaerial to shallow marine sedimentary beds (i.e. the Örencik formation) at the base and a subaerial volcanic unit (i.e. the Hamamözü formation) in the middle and at the top. This sudden facies change from marine to subaerial environment in the Middle Eocene is a common phenomenon across northern Turkey, implying that a regional uplift event occurred possibly across the suture zone before the initiation of the volcanism during Lutetian. The Middle Eocene lavas span the whole compositional range from basalts to rhyolites and display a calc-alkaline character except for alkaline to mildly-alkaline lavas from the top of the sequence. All lavas display a distinct subduction signature. Our geochemical data indicate that calc-alkaline lavas were derived from a subduction-modified source, whereas alkaline to mildly-alkaline lavas of the late stage were possibly sourced by an enriched mantle domain. Magmas evolved in magma chambers emplaced possibly at two different crustal levels. Magmas in deeper (> 13 km) and possibly larger chambers fractionated hydrous mafic minerals (e.g. amphibole and biotite), two pyroxenes and plagioclase and assimilated a significant amount of crustal material. Intermediate to acid calc-alkaline lavas and pyroclastics were derived from these chambers. Magmas in the shallower chambers, on the other hand (~ < 12 km), crystallized anhydrous mineral assemblages, assimilated little or no crustal material and fed basic to intermediate lavas in the region. Both deep and shallow chambers were periodically replenished by mafic magmas. We argue that a slab breakoff model explains better than any alternative model (i) why the volcanism during the Middle Eocene was confined into a rather narrow belt along the suture zone, (ii) why it initiated almost contemporaneous with a regional uplift after the continental collision event, (iii) why it postdated arc volcanism along the Pontides in the north by 15–20 My, (iv) why it assimilated significant amount of crustal material, and (v) why alkalinity of lavas increased in time.     

Morphological characteristics and emplacement mechanism of the seamounts in the Central Indian Ocean Basin

The morphotectonic features of the Central Indian Ocean Basin (CIOB) provide information regarding the development of the basin. Multibeam mapping of the CIOB reveals presence of abundant isolated seamounts and seamount chains sub-parallel to each other and major fracture zones along 73° E, 79° E and 75°45′ E. Morphological analyses were carried out for 200 seamounts that occur either as isolated edifies or along eight sub-parallel chains. The identified eight parallel seamount chains that trend almost north–south and reflecting the absolute motion of the Indian plate, probably originated from the ancient propagative fractures. Inspite of the differences in their height, the seamounts of these eight chains are morphologically correlatable. In the study area the seamounts are clustered north and south of 12° S latitude. Interestingly, in the area north of 12° S (area II: 9°–12° S) the seamounts are distinctly smaller (≤ 400 m height) whereas, the area south of 12° S (area I: 12°–15° S) has a mixed population of seamounts. The normalized abundance of the CIOB seamount is 976 seamounts/10⁶ km² but on a finer scale this value varies from 500 to 1600 seamounts/10⁶ km², which is less than the seamount concentrations of the Pacific and Atlantic oceans (9000 to 16,000 seamounts/10⁶ km²). Three categories of seamounts are present in the CIOB e.g. (1) single-peaked (2) multi-peaked and (3) composite. The study indicate that single-peaked seamounts are dominant (89%) while multi-peaked is less (8%) and composite ones are rare (3%) in the CIOB.The progressive northward movement of the Indian continent caused collision between India and Asia at around 62 Ma ago. A majority of the near-axis originated seamounts in the CIOB seemed to have formed as a consequence of the temporally widespread (Cretaceous  65 Ma to late Eocene < 49 Ma) collision between India and Eurasia. The regional stress patterns in the Indian plate vary N to NE in the continent and N to NW in Indian Ocean areas. The combined effect of the regional stress patterns maintained the orientation of the seamount chains and the local stress regime helped in the upwelling of magma and formation of seamounts. The low heat flow, morphological features and geochemical signature indicate that the morphotectonic structures formed contemporaneously with the oceanic crust.