Mesozoic Tectonics Research Articles

Bulk arc strain, crustal thickening, magma emplacement, and mass balances in the Mesozoic Sierra Nevada arc

Quantifying crustal deformation is important for evaluating mass balance, material transfer, and the interplay between tectonism and magmatism in continental arcs. We present a dataset of >650 finite strain analyses compiled from published works and our own studies with associated structural, geochronologic, and geobarometric information in central and southern Sierra Nevada, California, to quantify the arc crust deformation. Our results show that Mesozoic tectonism results in 65% arc-perpendicular bulk crust shortening under a more or less plane strain condition. Mesozoic arc magmatism replaced ∼80% of this actively deforming arc crust with plutons requiring significantly greater crustal thickening. We suggest that by ∼85 Ma, the arc crust thickness was ∼80 km with a 30-km-thick arc root, resulting in a ∼5 km elevation. Most tectonic shortening and magma emplacement must be accommodated by downward displacements of crustal materials into growing crustal roots at the estimated downward transfer rate of 2–13 km/Myr. The downward transfer of crustal materials must occur in active magma channels, or in “escape channels” in between solidified plutons that decrease in size with time and depth resulting in an increase in the intensity of constrictional strain with depth. We argue that both tectonism and magmatism control the thickness of the crust and surface elevation with slight modification by surface erosion. The downward transported crustal materials initially fertilize the MASH zone thus enhancing to the generation of additional magmas. As the crustal root grows it may potentially pinch out and cool the mantle wedge and thus cause reduction of arc magmatism.

Late Mesozoic molybdenum mineralization on Hainan Island, South China: Geochemistry, geochronology and geodynamic setting

Hainan Island has become an important molybdenum (Mo) producer in East China, with the proven and measured metal Mo reserves of >0.29Mt and >0.69Mt, respectively. The known Mo-related deposits and occurrence which genetically might be attributed to porphyry- and hydrothermal vein types are hosted predominantly within the Cretaceous granitoids. These granitoids belong to high-K calc-alkaline, metaluminous to weakly peraluminous I-type series, and have geochemical affinities to arc-related basalts as defined by SiO2 (57.40–77.34wt.%) and K2O (2.45–6.28wt.%) contents, A/CNK ratios (mostly 0.86–1.10), depleted K, Ba, Nb, Sr, P and Ti, and enriched Rb, Th, Pb, Zr and La, and LREE-enriched REE patterns (La/YbN=7.1–66.3) with pronounced negative Eu anomalies to no Eu anomalies (Eu/Eu*=0.35–1.08). The highly variable geochemical compositions might be linked to magmatic processes. The LA-ICP-MS U–Pb dating on zircon have revealed three stages of magma emplacement for the Cretaceous Mo-hosting granitoids during ca. 113–108Ma, ca. 100–94Ma and ca. 90–70Ma, respectively. The initial 87Sr/86Sr ratios (from 0.70567 to 0.71208) and ɛNd(t) values (from −3.9 to −6.9) suggest that the host granitoids were sourced from an enriched subcontinental lithospheric mantle that had been metasomatized by fluids and/or melts released from ancient, subducted oceanic crust under an extension-induced setting. This setting is most favorable to variable degree of magma mixing between mantle-derived and crust-derived melts.With the second event as most important and the third event likely recording the youngest Mo mineralization in East China, the Re–Os dating of molybdenite from two types of Mo deposits on Hainan Island has identified three Mo mineralizing events occurring at ca. 112Ma, ca. 106–95Ma and ca. 89–72Ma. They are consistent with the episodes of the Cretaceous magmatism. The Re concentrations of molybdenites and δ34S values of sulfides (1.36–5.73‰ and average 4.3‰) commonly indicate that the metal Mo is of hybrid origin between mantle and crust but with variable degree of hybridization. Given that the subduction of the Paleo-Pacific plate beneath the eastern Asian margin dominated the Late Mesozoic tectonism of South China, and that Hainan Island has been a part of the Cathaysia Block since Late Permian, the widespread Cretaceous extension and associated Mo mineralizing event(s) on Hainan Island might be a response to episodic rollback of the subducted slab, which led to the underplating of mantle-derived basaltic melts and associated deep crustal melting. Moreover, this tectono-magmatic scenario likely resulted in the migratory pluses of Mo metallogenesis and the change of metal Mo sources from northeast to southwest Hainan Island.

Newly Discovered Jurassic and Cretaceous Strata in the Yidun Area, SW China and their Constraints on the Mesozoic Tectonic Evolution of the Eastern Tethys

The Yushu-Yidun arc magmatic belt has attracted much attention among geologists due to its complete record of the evolutionary process of the Sanjiang tectonic belt in eastern Tethys and its favorable metallogenic conditions.This belt is composed of the Late Triassic volcanicsedimentary rock series,and the Indosinian and Yanshanian granite intrusions.It has long been believed that there are no Jurassic or Cretaceous strata in this area.更多还原

Detrital zircon U–Pb geochronology, Hf isotopes and geochemistry constraints on crustal growth and Mesozoic tectonics of southeastern China

In situ U–Pb geochronological, Hf isotopic and REE geochemical analyses of detrital zircons from Mesozoic sandstones in central Jiangxi and east Hunan of the South China Block (SCB) are used to provide not only information about crustal evolution process, but more importantly new constraints on sedimentary provenances as well as Mesozoic basin evolutions in central SCB. A total of 417 concordant zircon analyses define five U–Pb age populations at ca. 2.6–2.3Ga, ca. 2.0–1.7Ga, ca. 1.0–0.7Ga, ca. 500–400Ma and ca. 300–200Ma. Integrated analyses of zircon U–Pb ages and Hf isotopes of detrital zircons reveal five episodes of juvenile continental crust growth: ca. 2.5Ga, ca. 1.7Ga, ca. 850Ma, ca. 440Ma and ca. 230Ma, with all but the ca. 2.5Ga episode likely represent that of the SCB.None of the three samples from T3–J1 strata showed strong ca. 1850Ma and ca. 230Ma peaks suggesting that the T3–J1 sediments probably sourced from the Yangtze Block. In contrast, stronger peaks of ca. 1850Ma and ca. 250–230Ma in post-J1 strata relative to that of T3–J1 strata suggest a dominantly local Cathaysian provenance. In addition, the distinct low εHf(T) values for zircons of ca. 430Ma from T3–J1 strata in comparison with higher εHf(T) values for that from post-J1 strata also support aforementioned viewpoint. Such sharp changes between the pre-J1 and post-J1 strata coincide with the remarkable change in regional palaeogeography from a broad shallow marine basin in the Late Triassic–Early Jurassic time to a basin-and-range-style province in the Middle Jurassic. The characteristics and time evolution of detrital zircons from the studied area are consistent with the flat-slab subduction model which involves the development of a broad sag basin during the T3–J1 time, and a rapid regional uplift in the Cathaysia Block caused by the reinitiating of normal subduction along the coastal region at ca. 190Ma.

Spatial and temporal distribution of Mesozoic adakitic rocks along the Tan-Lu fault, Eastern China: Constraints on the initiation of lithospheric thinning

The Mesozoic tectonics in East China is characterized by significant lithospheric thinning of the North China Craton, large-scale strike-slip movement along the Tan-Lu fault, and regional magmatism with associated metallogeny. Here we address the possible connections between these three events through a systematic investigation of the geochemistry, zircon geochronology and whole rock oxygen isotopes of the Mesozoic magmatic rocks distributed along the Tan-Lu fault in the Shandong province.The characteristic spatial and temporal distributions of high-Mg adakitic rocks along the Tan-Lu fault with emplacement ages of 134-128Ma suggest a strong structural control for the emplacement of these intrusions, with magma generation possibly associated with the subduction of the Pacific plate in the early Cretaceous. The low-Mg adakitic rocks (127–120Ma) in the Su-Lu orogenic belt were formed later than the high-Mg adakitic rocks, whereas in the Dabie orogenic belt, most of the low-Mg adakitic rocks (143-129Ma) were generated earlier than the high-Mg adakitic rocks. Based on available data, we suggest that the large scale strike-slip tectonics of the Tan-Lu fault in the Mesozoic initiated cratonic destruction at the south-eastern margin of the North China Craton, significantly affecting the lower continental crust within areas near the fault. This process resulted in crustal fragments sinking into the asthenosphere and reacting with peridotites, which increased the Mg# of the adakitic melts, generating the high-Mg adakitic rocks. The gravitationally unstable lower continental crust below the Tan-Lu fault in the Su-Lu orogenic belt triggered larger volume delamination of the lower continental crust or foundering of the root.

First Triassic palaeomagnetic constraints from Junggar (NW China) and their implications for the Mesozoic tectonics in Central Asia

Northwestern China belts result from the Palaeozoic collage of Central Asia and the subsequent reactivations due to far-field effects of the Mesozoic Tibetan and the Cenozoic Himalayan collisions. Triassic is a crucial period to understand and decipher the tectonics related to these two episodes. About 250 oriented palaeomagnetic cores from 43 sites were collected from six sections of Upper Permian to Late Triassic sandstone, in South and West Junggar, Northwestern China. Thermomagnetic, IRM and hysteresis measurements reveal magnetite as the main carrier of the magnetic remanence with minor hematite and maghemite. Stepwise thermal demagnetisation has generally isolated two components. The low temperature component, up to 300–350°C, displays a direction consistent with the present-day geomagnetic field. The locality-mean directions related to the high temperature component (above 350°C) were also calculated. Two out of six sections display intense viscous magnetisation and the occurrence of maghemite reveals a possible Cenozoic chemical remagnetisation for these two localities. For the other four localities, we assume that the magnetisation is primary because: (1) AMS measurements reveal a primary fabric, (2) there are local occurrences of antipodal polarities, and (3) palaeolatitudes of tilt-corrected poles are compatible with previous studies. The consistency between the Early Triassic poles of West and South Junggar indicates that Junggar evolved as a rigid block only since Early Mesozoic. The comparison of the Late Palaeozoic and the Early Mesozoic poles of Junggar and those of Siberia and Tarim shows major rotations between the Late Permian and the Late Jurassic–Early Cretaceous. These periods of discrete rotations are characterized by strike-slip faulting in Tianshan and Altai and they may correlate with the major episodes of coarse-grained detrital sedimentation and uplift of the range. Especially, the counter-clockwise rotations of Junggar relative to Tarim and Siberia, which occurred between the Early and the Late Triassic and between the Late Triassic and the Late Jurassic, are accommodated by transpressive tectonics in the Tianshan and the Altai belts. This reactivation is a far-field effect of Tibetan blocks diachronous collisions. Therefore, these first Triassic palaeomagnetic results from Junggar infer that post-Carboniferous rotations were due to the combined effect of the post-orogenic transcurrent movement and the Mesozoic oblique reactivation.

Early Mesozoic tectonics of the South China block: Insights from the Xuefengshan intracontinental orogen

Intracontinental orogens remain less understood than accretionary or collisional orogens that are related to plate margin interactions. In the center of the South China block, the Xuefengshan Belt provides a well-exposed example of such an intracontinental orogen of Early Mesozoic age. Detailed field tectonic observations indicate that the Xuefengshan Belt can be divided into a Western Outer Zone characterized by km-scale box-fold structures, and an Eastern Zone, separated from the Western Outer Zone by the SE-dipping Main Xuefengshan Thrust. In the Eastern Zone, NW verging folds coeval with a pervasive slaty cleavage and a NW–SE trending lineation are the dominant structures. From west to east, the dip of the cleavage surface exhibits a fan-like pattern. The bulk architecture of the Xuefengshan Belt results from polyphase deformation: D1 is characterized by a top-to-the-NW ductile shearing; D2 corresponds to SE-directed back thrusting and folding; D3 consists of upright folds with vertical cleavage and lineation. At depth, a high strain zone characterized by greenschist facies metamorphic rocks and a top-to-the-NW ductile shearing corresponds to a ductile décollement zone that accommodated the deformation of the Neoproterozoic to Early Triassic sedimentary series. Kinematic compatibility suggests that the synmetamorphic ductile shearing was coeval with the D1 event in the sedimentary cover. The Xuefengshan Belt is interpreted as an Early Mesozoic intracontinental orogen, which possibly originated from the SE-directed continental subduction of a piece of the South China block in response to northwestwards subduction of the Pacific plate.

The Mesozoic Tectonic Dynamics and Chronology in the Eastern North China Block

Mesozoic tectonic events in different areas of the eastern North China Block (NCB) show consistency in tectonic time and genesis. The Triassic collision between NCB and Yangtze results in the nearly S-N strong compression in the Dabie, Jiaodong, and west Shandong areas in Middle Triassic-Middle Jurassic. Compression in the Yanshan area in the north part of NCB was mainly affected by the collision between Mongolia Block and NCB, as well as Siberia Block and North China-Mongolia Block in Late Triassic-Late Jurassic. However, in the eastern NCB, compressive tectonic system in Early Mesozoic was inversed into extensional tectonic system in Late Mesozoic. The extension in Late Mesozoic at upper crust mainly exhibits as extensional detachment faults and metamorphic core complex (MCC). The deformation age of extensional detachment faults is peaking at 120–110 Ma in Yanshan area and at 130–110 Ma in the Dabie area. In the Jiaodong area eastern to the Tan-Lu faults, the compression thrust had been continuing to Late Mesozoic at least in upper crust related to the sinistral strike slipping of the Tan-Lu fault zone.The extensional detachments in the eastern NCB would be caused by strong crust-mantle action with upwelling mantle in Late Mesozoic.

Mapping of salt structures and related fault lineaments based on remote-sensing and gravimetric data: The case of the Monte Real salt wall (onshore west-central Portugal)

The Monte Real (MR) salt wall (MR structure) is a salt structure located in the Monte Real–Pombal Subbasin (onshore west-central Portugal). The MR structure was emplaced during post–Triassic–Mesozoic extensional and Tertiary compressional tectonics and is now partially buried under Miocene to Pliocene–Pleistocene sediments. The region is dominated by NS 20, N45 10E, N30 10W, and N70 10W trending main tectonic systems. Remote-sensing data constrained by gravimetric data allowed the recognition of the two-dimensional surface geometry of this salt structure and its associated structural features. The use of remote-sensing images (optical and microwave data) allow mapping the MR structure based on the heat radiated from rocks and soils, changes in vegetation cover, and texture analysis of synthetic aperture radar imagery. In addition, remote-sensing data, combined with modeled topographic data, also allowed recognition of a lineament pattern consistent with the fracture pattern of the region, probably related to the implantation and deformation of this salt body. The crosscut behavior of the identified structural lineaments can justify the north-northwestern elongated arcuate shape of the MR structure and the corresponding gravimetric anomaly. This work shows that remote-sensing techniques are a powerful tool to study buried salt domes even in the absence of detailed geophysical data.

Detrital zircons from the Tananao metamorphic complex of Taiwan: Implications for sediment provenance and Mesozoic tectonics

Taiwan formed during the Plio-Pleistocene collision of Eurasia with the outboard Luzon arc. Its pre-Tertiary basement, the Tananao metamorphic complex, consists of the western Tailuko belt and the eastern Yuli belt. These circum-Pacific belts have been correlated with the high-temperature/low-pressure (HT/LP) Ryoke belt and the high-pressure/low-temperature (HP/LT) Sanbagawa belt of Japan, respectively. To test this correlation and to reveal the architecture and plate-tectonic history of the Tananao metamorphic basement, detrital zircons were separated from 7 metasedimentary rock samples for U–Pb dating by LA-ICPMS techniques.Results of the present study, coupled with previous data, show that (1) the Tailuko belt consists of a Late Jurassic to earliest Cretaceous accretionary complex sutured against a Permian–Early Jurassic marble±metabasaltic terrane, invaded in the north by scattered Late Cretaceous granitic plutons; the latter as well as minor Upper Cretaceous cover strata probably formed in a circum-Pacific forearc; (2) the Yuli belt is a mid- to Late Cretaceous accretionary complex containing HP thrust sheets that were emplaced attending the Late Cenozoic Eurasian plate–Luzon arc collision; (3) these two Late Mesozoic belts are not coeval, and in part were overprinted by low-grade metamorphism during the Plio-Pleistocene collision; (4) accreted clastic sediments of the Tailuko belt contain mainly Phanerozoic detrital zircons, indicating that terrigenous sediments were mainly sourced from western Cathaysia, whereas in contrast, clastic rocks of the Yuli accretionary complex contain a significant amount of Paleoproterozoic and distinctive Neoproterozoic zircons, probably derived from the North China craton and the Yangtze block±eastern Cathaysia, as a result of continent uplift/exhumation after the Permo-Triassic South China–North China collision; and (5) the Late Jurassic–Late Cretaceous formation of the Tananao basement complex precludes the possibility that the early Yanshanian (Early Jurassic) granitoids in southern China represent a landward arc contemporaneous with the later, outboard Tananao accretionary event.

Late Mesozoic compressional to extensional tectonics in the Yiwulüshan massif, NE China and its bearing on the evolution of the Yinshan–Yanshan orogenic belt: Part I: Structural analyses and geochronological constraints

With a cratonic nucleus, the North China Craton (NCC) experienced a complex tectonic evolution with multiphase compressional and extensional events during Mesozoic times. Along the northern part of the NCC, the Yinshan–Yanshan fold and thrust belt was a typical intraplate orogen. Jurassic and Cretaceous continental sedimentation, magmatism, widespread intraplate characterize the Yinshan–Yanshan orogenic belt. The geodynamic significance of these tectonic events is still in dispute. In the western part of the Liaoning province, the Yiwulüshan massif crops out at the eastern end of the Yinshan–Yanshan orogenic belt. The Yiwulüshan massif presents an elliptical domal shape with a NE–SW striking long axis. The structural evolution of this massif brings new insights for the understanding of the Mesozoic plutonic–tectonic history of the NCC. A multidisciplinary study involving structural geology, geochronology, Anisotropy of Magnetic Susceptibility (AMS) and gravity modeling have been carried out. The presentation of the new results splits into two parts. Part I (this paper) deals with field and laboratory structural observations, and presents the main geochronological results. The AMS, gravity modeling data will be provided in a companion paper (Part II). The early compressional deformation (D1) corresponds to a Late Jurassic to Early Cretaceous southward thrusting. The subsequent deformation is related to the Early Cretaceous exhumation of the Yiwulüshan massif. A detailed structural analysis allows us to distinguish several deformation events (D2, D3, and D4). The Cretaceous extensional structures, such as syntectonic plutons bounded by ductile normal faults, metamorphic core complexes, and half-graben basins are recognized in many places in East Asia. These new data from the Yiwulüshan massif constitute a link between Transbaikalia, Mongolia, North China and South China, indicating that NW–SE extensional Mesozoic tectonics occurred throughout the entire region.

Late Mesozoic compressional to extensional tectonics in the Yiwulüshan massif, NE China and their bearing on the Yinshan–Yanshan orogenic belt: Part II: Anisotropy of magnetic susceptibility and gravity modeling

Granitoids play an important role in deciphering both crustal growth and tectonic evolution of Earth. In the eastern end of the Yinshan–Yanshan belt of North China Craton, the Yiwulüshan massif is a typical region that presents the tectonic evolution features of this belt. Our field work on the host rocks has demonstrated two phases of opposite tectonics: compressional and extensional, however, the deformation is almost invisible in the intrusive rocks. To improve the understanding of the tectonic evolution of the Yiwulüshan massif and the Late Mesozoic tectonics of East Asia, a multidisciplinary study has been carried out. In this study, anisotropy of magnetic susceptibility (AMS) and gravity modeling have been applied on these Jurassic plutons (Lüshan, Jishilazi and Guanyindong), which intrude into the Yiwulüshan massif. According to laboratory measurements and microscopic observations on thin sections, the AMS of the Yiwulüshan massif is characterized by secondary fabrics, indicating that the intensive post solidus deformation has reset the (primary) magmatic magnetic fabrics. A relatively gentle NW dipping magnetic foliation has been identified with two distinct groups of magnetic lineations of N34°E and N335°E orientations, namely LM1 and LM2, relatively. Gravity modeling reveals a southward thinning of the massif with a possible feeding zone rooted in the northern part of the massif. Integrating all results from structural observation, geochronological investigation, AMS measurement and gravity modeling, two tectonic phases have been identified in the Yiwulüshan massif, posterior to the Jurassic (180–160Ma) magmatism in the Yinshan–Yanshan area. The early one concerns a Late Jurassic–Early Cretaceous (~141Ma) compressional event with a top-to-the-south to southwest sense of shear. The second one shows an Early Cretaceous (~126Ma) NW–SE ductile extensional shearing. At that time, sedimentary basins widened and Jurassic plutons started to be deformed under post solidus conditions. In fact, the NW–SE trend of the maximum stretching direction is a general feature of East Asian continent during Late Mesozoic.

Geochronological and geochemical results from Mesozoic basalts in southern South China Block support the flat-slab subduction model

The debate on the Mesozoic tectonics of the South China Block (SCB), featuring a broad orogenic belt and a large continental magmatic province, has been rejuvenated in the past years. There are a number of competing tectonic models, each predicting different time, space, and compositional evolutional trends for the Mesozoic igneous rocks, including sporadic basalts. In this paper, we report high precision 40Ar/ 39Ar ages and geochemical and Sr–Nd isotopic data for basaltic rocks in central–eastern SCB. These results, together with a data compilation, indicate three evolutionary stages for the basalts. Stage 1 (195–160 Ma) basalts occur only in inland SCB. They exhibit low initial 87Sr/ 86Sr ratios (0.7038 to 0.7078), high ε Nd(t) values (− 1.5 to 6.0) and ocean island basalt (OIB)-like geochemical characteristics such as low La/Nb ratios (0.6 to 1.4), implying negligible to minor lithosphere contamination. Stage 2 (160–110 Ma) basalts, mostly from the same inland region with minor occurrence closer to the coast, are characterized by variable initial 87Sr/ 86Sr ratios (0.7053 to 0.7102), εNd(t) values (− 6.0 to 5.0) and low La/Nb ratios (0.9 to 1.8), indicating an enhanced asthenosphere–lithosphere interaction. Stage 3 (110 to ≤ 80 Ma) basalts can be further divided into two sub-groups based on their geographic and geochemical variations. Basalts in the coastal regions are characterized by low εNd(t) values (− 8.1 to 3.7) and variably high La/Nb ratios (up to 4.8) indicating a strong arc signature. On the other hand, basalts in the inland regions have high ε Nd(t) values (− 1.9 to 6.8) and low La/Nb ratios (0.6 to 1.2) that are similar to the stage 1 basalts. We interpret these features as results of geodynamics processes related to the break-up, foundering and retreating of an early Mesozoic flat-subducted oceanic slab.

The junction of the eastern Central Asian Fold Belt and the Siberian Platform: deep structure and Mesozoic tectonics and geodynamics

The tectonic structure of the junction of the eastern Central Asian Fold Belt and the Siberian Platform, along with the deep structure of the Earth's crust and lithosphere in this region, has been described on the basis of new geological and geophysical data (seismic, geoelectric, and space-structural studies as well as new-generation geological maps), combined with new interpretation techniques (processing of the previous data by special software). The data suggest the existence of oblique collision during the convergence of the tectonic plates and, correspondingly, tectonic units composing these plates, when the Mongol–Okhotsk paleobasin closed. Such a scenario within the Aldan–Stanovoi Shield is evidenced by areas of syn- and postcollisional magmatism, with their deep-level and geochemical characteristics, and by the presence of a Late Mesozoic fold–thrust zone. Deep “traces” of these tectonomagmatic events, detected in the course of geological and geophysical modeling, are manifested in inclined deep boundaries between the crustal and lithospheric blocks. On the Earth's surface, they correspond to large fault systems: the Dzheltulak, North and South Tukuringra, Gulyui, and Stanovoi. It has been found that the influence of collision decreases northward with distance from the junction of the eastern Central Asian Fold Belt and the Siberian Platform (Dzheltulak and North Tukuringra transcrustal faults).

Mesozoic paleostress from healed microcracks and fluid inclusions in some granites of South Korea

Mesozoic maximum horizontal stress (σH max) directions are determined from healed microcracks and the secondary fluid inclusions within healed microcracks in granites from the Inje, Jecheon, Wolaksan and Sokrisan areas. The strike of healed microcracks in granite from the Inje area shows a strong peak at N50°W, while N10°W and N80°E are well developed as secondary and the tertiary peaks, respectively. N30°W is the most dominant strike direction of healed microcracks in granites from the Jecheon area, while E-W is the second most dominant strike direction. Orientations of healed microcracks in granite from the Wolaksan area show a strong peak trend of N55°W, while the secondary peak is at N5°E. The most dominant strike direction of healed microcracks in granites from the Sokrisan area is N-S. The age for the formation of healed microcracks is determined from homogenization temperatures of fluid inclusion and the cooling history of granites. The age of healed microcrack formations in the Jecheon granite might be 207∼149 Ma, 176∼160 Ma in the Inje granite, 116∼88 Ma in the Wolaksan granite and 92∼84 Ma in the Sokrisan granite. In addition, the age of the NW-SE healed microcrack in the Wolaksan granite is 95 Ma, and that of the N-S healed microcracks in the Wolaksan and Sokrisan granites is 100∼84 Ma and 92∼84 Ma, respectively. This means that the rotation of principal stress axes occurred at 100∼95 Ma. The σHmax direction in the Korean peninsula was NW-SE in the Late Triassic to Early Cretaceous times, but changed to N-S in the Late Cretaceous. The directions and ages for the formation of healed microcracks in the study area are well correlated with Mesozoic tectonism in South Korea, that is, the Daebo orogeny in the Jurassic∼Early Cretaceous with the σH max direction of NW-SE mainly having an effect on the Jecheon, the Inje and the Wolaksan granite, and the Middle∼Late Cretaceous Bulguksa tectonic event with the σHmax direction of N-S mainly having an effect on the Sokrisan granite.

Mesozoic tectonics and volcanism of Tethyan rifted continental margins in western Sicily

The paleotectonic and volcanic features of the Jurassic–Cretaceous carbonate successions, outcropping in central-western Sicily, allow us to restore the tectono-sedimentary evolution of a sector of the African continental margin. These successions consist of shallow-to-deep-water Mesozoic deposits that have formed in the carbonate platform-to- pelagic plateau depositional setting of the so-called Trapanese paleogeographic domain. Fieldwork, including structural analyses, has indicated the occurrence of lateral facies changes, resedimented materials, volcanic products (pillow lavas and tuffitic deposits), unconformity surfaces and paleofaults of different trends and age. These data, combined with facies and a physical–stratigraphic analysis, allow one to distinguish between the different tectono-stratigraphic settings. A structural low, filled with thick pillow lavas and reworked deposits, appears bordered by two main structural highs: an articulate carbonate-pelagic platform with a stepped fault margin, and the flank of a structural high, where the volcanoclastic deposits are interlayered with reworked reef materials, suggest the occurrence of a submarine volcano, that evolved into an atoll-type carbonate shelf setting. Tectonic and magmatic events punctuated the sedimentary evolution during the Early Jurassic, Middle–Late Jurassic, Early and Late Cretaceous. The horst and graben structures connected by steeped margins and the occurrence of distinctive types of volcanic products suggest that the study areas could represent a sector of the Jurassic–Cretaceous southern Tethyan passive continental margin, situated at the edge of the continental platform bordering those basinal areas where rifting and possible spreading processes were active during the Mesozoic.

Late Mesozoic tectonics of Central Asia based on paleomagnetic evidence

This paper presents paleomagnetic data for Late Mesozoic (Middle Jurassic to end-Cretaceous) rocks of the Siberian platform (Verkhoyansk Trough) and its southwestern margin (Transbaikalian basins and Minusa Trough). We determine a series of key paleomagnetic poles for 165, 155, 135, 120, and 75 Ma, which define the Mesozoic apparent polar wander path (APWP) for Siberia. This quantitative approach provides the opportunity for a general revision of Mesozoic tectonics of Central Asia. Many researchers have considered the Eurasian continent to have been completely stable during the Mesozoic era. However, we demonstrate systematic deviations of corresponding Mesozoic poles from Siberia and Europe, and interpret the discrepancies as evidence for large-scale sinistral strike-slip motion due to clockwise rotation of the Siberian plate relative to the European plate. We conclude that, following its Late Paleozoic assembly, the Eurasian plate was not internally stable, i.e. not rigid. The Mesozoic geological evolution of Siberia was dominated by strike-slip tectonics. Rift-related grabens formed within the basement of the West Siberia sedimentary basin and orogenic events occurred along the southwestern margin of the Siberian craton, within the Central Asia tectonic province. Our paleomagnetic reconstruction indicates also that the Mongol-Okhotsk Ocean was still not closed completely before the end of the Jurassic. We propose that final collision occurred in the Early Cretaceous, and during the Middle to Late Jurassic interval, northward subduction of oceanic lithosphere resulted in oblique, west-to-east ocean closure (a “scissors-like” model). The closure was controlled by significant sinistral strike-slip motion of the Siberian craton. This process is reflected in Transbaikalia by extensive bimodal volcanic activity and development of rift-related structures, including pull-apart basins.

Mesozoic Tectonics and Sedimentation of the Giant Polyphase Nonmarine Intraplate Ordos Basin, Western North China Block

The Mesozoic Ordos Basin is a large intracontinental basin that is characterized by a thick, undeformed stratigraphic succession over a wide region that is bounded by deformed polyphase orogenic belts on all margins. Structural and stratigraphic studies document a diverse history of Late Palaeozoic through Cretaceous shortening, extension and strike‐slip on the northern and western basin margins, and a longer‐lived contractile setting in the southern part of the basin. In response to these structural episodes, associated basins formed within, or adjacent to, the marginal mountain belts and were filled by a variety of nonmarine depositional processes. Synchronously, subsidence across the whole of the Ordos block resulted in a large, integrated basin, also filled by nonmarine systems, that spanned across distinct structural domains. These stratigraphic and structural characteristics are interpreted to suggest that mechanical contrasts between the basin interior and the marginal mountain belts fundamentally control the timing, distribution and styles of deformation and basin formation on the margins of the Ordos block. This mechanical contrast between the Ordos Basin and its margins encouraged the repeated, polyphase deformation on its margins, whereas the vast area of the Ordos block continued to subside uniformly. These characteristics are typical of many large basin systems found on relatively recently assembled continents, like southern Eurasia. Examples that may be similar in many respects to the Ordos Basin include other collisional successor basins of China, such as the Tarim, Junggar and Sichuan basins.

The Ereendavaa Range (north-eastern Mongolia): an additional argument for Mesozoic extension throughout eastern Asia

Mesozoic rift basins locally bounding metamorphic core complexes have been recognized in Transbaikalia and northern China. Numerous basement outcrops located between these two regions, in eastern Mongolia, are considered as pre-Palaeozoic in age. One of these, the Ereendavaa Range, appears as a gneissic core marked by amphibolite-facies metamorphic conditions. The range is overlain to the NW by the unmetamorphosed Mesozoic Onon Basin. Below the basin, the upper part of the range consists of a gently NW-dipping shear zone associated with top-to-the-NW motion. The structural pattern is consistent with syn-extensional exhumation of the range. Preliminary geochronological data indicate that the shear zone is late Jurassic to early Cretaceous in age, coeval with the Onon Basin. These new data from eastern Mongolia constitute a link between Transbaikalia and northern China, indicating that NW–SE extensional Mesozoic tectonics occurred throughout the entire region.

DINOSAUR-BEARING ONCOIDS FROM EPHEMERAL LAKES OF THE LOWER CRETACEOUS CEDAR MOUNTAIN FORMATION, UTAH

Here we document the occurrence of locally common oncoids in the Cedar Mountain Formation of Utah in the Woodside Anticline area of the San Rafael Swell and use them to understand changes in the Early Cretaceous landscape and their effects on the dinosaur fauna. Detailed facies analysis is required to understand the context of these changes within the broader patterns of Mesozoic tectonics and the fossil record. Oncolite crops out in the Cedar Mountain Formation, directly overlying the Buckhorn Conglomerate. Oncolite is not widely distributed outside of the Woodside Anticline area. The oncoids are found in a bimodal population with the majority in the 2–5-cm-diameter range and a smaller population >25 cm in diameter. Nuclei are mostly rounded chert clasts and also include litharenite, polymict conglomerate, limestone, and both abraded and nonabraded dinosaur bone and wood fragments. Cortices are 3–5 mm thick with distinct, penecint (laminae that completely enclose a body; Hofmann, 1969), low-relief laminae. Some laminae are crenulated and comprise ministromatolites. The petrography of the oncoids suggests formation along lake margins where large fragments of reworked sedimentary clasts and dinosaur bones came to rest and were coated by bacterial mats. Caliche cements and coats some of the oncolite; these define a lake shoreline affected by fluctuating lake level. The isotope geochemistry indicates a combination of primary and diagenetic signals consistent with oncoid formation in open, ephemeral freshwater lakes.