Rapid exhumation of the Tianshan Mountains since the early Miocene: Evidence from combined apatite fission track and (U-Th)/He thermochronology

Combined apatite fission track(AFT)and(U-Th)/He(AHe)thermochronometries can be of great value for investigating the history of exhumation of orogenic belts.We evaluate the results of such a combined approach through the study on rock samples collected from the Baluntai section in the Tianshan Mountains,northwestern China.Our results show that AFT ages range from~60 to 40 Ma and AHe ages span~40–10 Ma.Based on the strict thermochronological constraints imposed by AHe ages,forward modeling of data derived from AFT analyses provides a well-constrained Cenozoic thermal history.The modeled results reveal a history of relatively slow exhumation during the early Cenozoic times followed by a significantly accelerated exhumation process since the early Miocene with the rate increasing from<30 m/Myr to>100 m/Myr,which is consistent with the inference from the exhumation rates calculated based on both AFT and AHe age data by age-closure temperature and mineral pair methods.Further accelerated exhumation since the late Miocene is recorded by an AHe age(~11 Ma)from the bottom of the Baluntai section.Together with the previous low-temperature thermochronological data from the other parts of the Tianshan Mountains,the rapid exhumation since the early Miocene is regarded as an important exhumation process likely prevailing within the whole range.     

青藏高原东南缘大凉山新生代隆升建造过程——多封闭系统低温热年代学与热模型限制

长波长、低起伏度大凉山构造带新生代隆升剥露与建造过程是解译青藏高原东向扩展过程的关键核心地区之一.本文基于大凉山构造带喜德剖面和沐川剖面9件样品的多封闭系统低温热年代学年龄(即磷灰石(U-Th)/He(AHe)、磷灰石裂变径迹(AFT)和锆石(U-Th)/He(ZHe))定年,揭示出多封闭系统热年代学年龄与古岩性柱深度具有明显的正相关性,即伴随古岩性柱深度增大,多封闭系统热年代学年龄明显减小.喜徳剖面多封闭系统低温热年代学AHe、AFT和ZHe年龄值分别为7—9Ma、14—22Ma和25—38Ma;沐川剖面多封闭系统低温热年代学AHe和AFT年龄值分别为10—26Ma、23—85Ma,ZHe年龄值为未完全退火年龄.多封闭系统热年代学和QTQt热史模拟揭示,大凉山构造带喜徳和沐川剖面岩性柱所有样品都经历大致相似的三阶段热演化过程,尤其是晚新生代快速隆升剥露阶段(30—20 Ma以来),其平均剥露速率分别为~0.15mm·a-1和~0.20mm·a-1,抬升剥露量分别为~3.0km和~1.5km.结合区域低温热年代学特征的大凉山构造带地表隆升动力学模型,揭示出重力均衡作用下地壳缩短与剥露作用(即构造隆升剥露机制)控制形成了现今大凉山造山带长波长、低起伏和高海拔地貌建造过程.     

Spatial distribution of the apatite fission‐track ages in the Toki granite,central Japan: Exhumation rate of a Cretaceous pluton emplaced in the East Asian continental margin

The Cretaceous Toki granitic pluton of the Tono district, central Japan was emplaced in the East Asian continental margin at about 70 Ma. The Toki granite has apatite fission‐track (AFT) ages ranging from 52.1 ±2.8 Ma to 37.1 ±3.6 Ma (number of measurements, n = 33); this indicates the three‐dimensional thermal evolution during the pluton's low‐temperature history (temperature in the AFT partial annealing zone: 60–120 °C). The majority of the Toki granite has a spatial distribution of older ages in the shallower parts and younger ages in the deeper parts, representing that the shallower regions arrived (were exhumed) at the AFT closure depth earlier than the deeper regions. Such a cooling pattern was predominantly constrained by the exhumation of the Toki granitic pluton and was related to the regional denudation of the Tono district. The age–elevation relationships (AERs) of the Toki granite indicate a fast exhumation rate of about 0.16 ±0.04 mm/year between 50 Ma and 40 Ma. The AFT inverse calculation using HeFTy program gives time‐temperature paths (t–T paths), suggesting that the pluton experienced continuous slow cooling without massive reheating since about 40 Ma until the present day. A combination of the AERs and AFT inverse calculations represents the following exhumation history of the Toki granite: (i) the fast exhumation at a rate of 0.16 ±0.04 mm/year between 50 Ma and 40 Ma; (ii) slow exhumation at less than 0.16 ±0.04 mm/year after 40 Ma; and (iii) exposure at the surface prior to 30–20 Ma. The Tono district, which contains the Toki granite, underwent slow denudation at a rate of less than 0.16 ±0.04 mm/year within the East Asian continental margin before the Japan Sea opening at 25–15 Ma and then within the Southwest Japan Arc after the Japan Sea opening, which is in good agreement with representative denudation rates obtained in low‐relief hill and plain fields.     

Apatite-fission-track geochronology and its tectonic correlation in the Dabieshan orogen,central China

The thermochronology based on a combination ofradiometric age and closure temperature (Tc) is widelyaccepted as a sensitive method to provide constraintson tectonic evolution of the orogen, and numerousadvances as a result of thermochronological applica-tions have been made recently in the Dabieshan oro-gen[1—5]. Zircon U/Pb, hornblende Ar/Ar and biotiteAr/Ar ages with Tc= 800—300℃[6] will yield a wealthof information on the cooling and exhumation of rockscrossing crustal levels of 30—1…     

晚白垩世以来川东北地区的剥蚀历史——多类低温热年代学数据综合剖面的制约

低温热年代学数据是一个与热历史过程紧密相关的资料类型,与高温年代学不同,低温热年代学表观年龄本身在很多情况下没有直接的地质意义.当且仅当样品线性持续冷却的情况下,表观年龄才可以被直接解释为样品经过其封闭温度的大致时间.因此,只有结合地质约束通过对低温热年代学数据进行热历史模拟才能更好地揭示其所蕴含的地质信息.对川东北地...     

Constraining the long-term evolution of the slip rate for a major extensional fault system in the central Aegean,Greece, using thermochronology

The brittle/ductile transition is a major rheologic boundary in the crust yet little is known about how or if rates of tectonic processes are influenced by this boundary. In this study we examine the slip history of the large-scale Naxos/Paros extensional fault system (NPEFS), Cyclades, Greece, by comparing published slip rates for the ductile crust with new thermochronological constraints on slip rates in the brittle regime. Based on apatite and zircon fission-track (AFT and ZFT) and (U–Th)/He dating we observe variable slip rates across the brittle/ductile transition on Naxos. ZFT and AFT ages range from 11.8 ± 0.8 to 9.7 ± 0.8 Ma and 11.2 ± 1.6 to 8.2 ± 1.2 Ma and (U–Th)/He zircon and apatite ages are between 10.4 ± 0.4 to 9.2 ± 0.3 Ma and 10.7 ± 1.0 to 8.9 ± 0.6 Ma, respectively. On Paros, ZFT and AFT ages range from 13.1 ± 1.4 Ma to 11.1 ± 1.0 Ma and 12.7 ± 2.8 Ma to 10.5 ± 2.0 Ma while the (U–Th)/He zircon ages are slightly younger between 8.3 ± 0.4 Ma and 9.8 ± 0.3 Ma. All ages consistently decrease northwards in the direction of hanging wall transport. Most of our new thermochronological results and associated thermal modeling more strongly support the scenario of an identical fault dip and a constant or slightly accelerating slip rate of ∼ 6–8 km Myr^− 1 on the NPEFS across the brittle/ductile transition. Even the intrusion of a large granodiorite body into the narrowing fault zone at ∼ 12 Ma on Naxos does not seem to have affected the thermal structure of the area in a way that would significantly disturb the slip rate. The data also show that the NPEFS accomplished a minimum total offset of ∼ 50 km between ∼ 16 and 8 Ma.     

琼东南盆地构造沉降史及其主控因素

以28口钻井、3条主干地震剖面最新解释成果资料为依据的构造沉降史恢复结果表明：琼东南盆地新生代存在三期快速沉降过程和一期缓慢沉降过程：第一期为始新世,第二期为渐新世—早中新世,第三期存在时空差异：在盆地西部乐东凹陷为晚中新世以来,在盆地中东部陵水凹陷和松南-宝岛凹陷为上新世以来;缓慢沉降过程亦存在时空差异,盆地西部中中新世沉降缓慢,盆地中东部中—晚中新世沉降缓慢.第一期快速沉降过程受东亚陆缘扩张和红河断裂左旋走滑共同影响,第二期快速沉降受南海海底扩张和红河断裂左旋走滑联合作用,第三期快速沉降主要受红河断裂右旋走滑控制,缓慢沉降过程与南海海底扩张停止以及红河断裂构造活动处于宁静阶段相耦合.     

Exhumation history of the Jiaodong and its adjacent areas since the Late Cretaceous:Constraints from low temperature thermochronology

The thermal history of the Jiaodong region and adjacent provinces(Shandong and northern Jiangsu) have been extensively studied,particularly by apatite fission track(AFT) dating.However,the AFT ages from surface outcrops range broadly and do not show an apparent relationship between age and elevation.This work provides a multiple low temperature thermochronological dataset including zircon and apatite(U-Th)/He ages(ZHe and AHe),and AFT ages from a 1000-m-deep borehole at the Jiaojia goldneld in the northwest of Jiaodong Peninsula.ZHe,AFT and AHe ages range from-100-70,-85-50and-65-50 Ma,respectively.These data conform to the principles of age vs.closure temperature and age vs.elevation and thus can be employed to estimate the exhumation history.Based on the density histogram of fission track length calculation,thermal history modeling,and previously published AFT ages from the Chinese Continental Science Drill program,this work concludes that compared to the AFT ages from surface outcrops,the low temperature thermochronological ages from the boreholes show a better relationship between age,elevation and closure temperature,and the age becomes younger with increasing depth.In addition,the exhumation history in the Jiaodong and adjacent areas can be divided into two distinct stages:a short,rapid tectonic exhumation(~100-95 Ma) and a long,slow exhumation since 95 Ma.The rate and amount of tectonic exhumation since 95 Ma are inferred as ~30 m Ma~(-1) and ~3 km,respectively.     

REGIONAL GEOTHERMAL EXPLORATION IN EGYPT*

Although Egypt is not characterized by abundant Cenozoic igneous activity, its location in the northeastern corner of the African plate suggests that it may possess geothermal resources, especially along its eastern margin. Regional geothermal exploration has been carried out in Egypt using the thermal gradient/heat flow technique and groundwater temperature/chemistry technique. In the thermal gradient/heat flow study, existing oil-well bottom-hole temperature data as well as subsurface temperature measurements in existing boreholes were utilized before special thermal gradient holes were drilled. Groundwater temperature and chemistry data were used to extend the geographic range of the direct subsurface thermal measurements. On a very modest budget, a regional thermal high has been discovered along the eastern margin of Egypt, and a local thermal anomaly has been discovered in this zone. Published geological information suggests that the sandstones of the Nubian Formation may be a suitable reservoir for geothermal fluids. The new data indicate that temperatures of 150°C or higher may be found in this reservoir in the Gulf of Suez and Red Sea coastal zone where it lies at a depth of 4 km and deeper.     

江南隆起带幕阜山岩体新生代剥蚀冷却的低温热年代学证据

江南隆起位于扬子与华夏地块的碰撞汇聚带,是研究华南大地构造演化的关键地质单元.本文采用磷灰石裂变径迹及(U-Th-Sm)/He年龄分布特征定性分析与径迹长度分布数据定量模拟相结合,主要研究了幕阜山岩体新生代的隆升与剥蚀过程,并在此基础上结合区域构造背景, 对其构造-热演化之间的关系进行了探讨.自晚白垩世持续隆升以来,幕阜山岩体经历的平均剥蚀厚度约4800 m.在不同岩体间,隆升过程及幅度存在差异,空间上具有非均匀性.热史结果显示幕阜山岩体经历了3期剥蚀, 其中两期快速剥蚀分别发生在晚白垩世-古近纪(80~50 Ma)和10 Ma以来,而这之间为一期缓慢剥蚀过程.研究区古近纪的快速剥蚀反映了中-下扬子喜山期大规模伸展断陷作用造成的肩部块体快速剥蚀事件; 约10 Ma以来的快速剥蚀是对太平洋板块向西运动的响应.幕阜山岩体自燕山晚期以来的隆升剥蚀作用具有良好的盆地沉积响应, 三期隆升剥蚀事件与研究区构造演化的动力学背景相吻合.     

New data on the Late Cenozoic basaltic volcanism in Syria,applied to its origin

New data on geology and 21 K–Ar dates of the Late Oligocene–Quaternary basalts in Syria, combined with analysis of the new and previous data are used to reconstruct the volcanic history and relations between it and tectonic events. Volcanism began at the end of Oligocene (26–24 Ma) and was concentrated in the Late Oligocene–Early Miocene along a N-trending band, which stretches from the Jebel Arab (Harrat Ash Shaam) up to Kurd Dagh and southern Turkey. Activity waned in the Middle Miocene (17–12 Ma), but was resumed in the same band in the Tortonian and increased in the Messinian and Early Pliocene (6.3–4 Ma), when volcanism spread to the Shin Plateau and its coastal extension. After a brief hiatus ～ 4–3.5 Ma, volcanism became still more intensive and spread from the N-trending band to the east into the northern margin of the Mesopotamian Foredeep and to the west into the Dead Sea Transform zone. Additional eruptions continued into the Holocene.Volcanism lasted > 25 million years in the Jebel Arab Highland and > 15 million years in the Aleppo Plateau. The long duration of volcanism in the same parts of the moving Arabian plate and absence of records of one-way migration of the activity mean that the magmatic sources moved together with the plate, i.e., they were situated within the lithosphere mantle. Coincidence of the tectonic and volcanic stages of the Arabian plate development proves that volcanic activity depended on the geodynamic situation, caused by the plate motion. Situated within the lithosphere, magmatic sources within this transverse band were possibly caused by thermal and deforming influences of the asthenospheric lateral flow, moved laterally from the Ethiopia–Afar deep superplume.     

Constraining exhumation pathway in an accretionary wedge by (U-Th)/He thermochronology—Case study on Meliatic nappes in the Western Carpathians

This study reconstructs the late stages in the exhumation history of a nappe derived from the Meliatic accretionary wedge in the Western Carpathians by means of zircon and apatite (U-Th)/He dating. The Meliatic accretionary wedge formed due to the closure of the Neotethyan Triassic–Jurassic Meliata–Hallstatt Ocean in the Late Jurassic. The studied fragments of the blueschist-bearing Meliatic Bôrka Nappe were metamorphosed at low-temperature and high- to medium-pressure conditions at ca. 160–150 Ma and included into the accretionary wedge. The time of the accretionary wedge formation constrains the beginning of the Bôrka Nappe northward thrusting over the Gemeric Unit of the evolving Central Western Carpathians (CWC) orogenic wedge. The zircon (U-Th)/He data on four samples recorded three evolutionary stages: (i) cooling through the ∼180 °C isotherm at 130–120 Ma related to starting collapse of the accretionary wedge, following exhumation of the high-pressure slices in the Meliatic accretionary wedge; (ii) postponed exhumation and cooling of some fragments through the ∼180 °C isotherm from 115 to 95 Ma due to ongoing collapse of this wedge; and (iii) cooling from 80 to 65 Ma, postdating the thrusting (∼100–80 Ma) of the Bôrka Nappe slices during the Late Cretaceous compression related to formation of the CWC orogenic wedge. The third stage already documents cooling of the Meliatic Bôrka Nappe slices in the CWC orogenic wedge. The apatite (U-Th)/He data may indicate cooling of a Bôrka Nappe slice to near-surface temperatures at ∼65 Ma. The younger AHe age clusters indicate that at least one, or possibly two, reheating events could have occurred in the longer interval from ∼40 to ∼10 Ma during the Oligocene–Miocene. These were related to sedimentary burial and/or the magmatism as documented in other parts of the CWC.     

BURIAL AND EXHUMATION OF THE XIGAZE FORE-ARC BASIN FROM LOW TEMPERATURE THERMOCHRONOLOGICAL EVIDENCE

The Xigaze fore-arc basin is adjacent to the Indian plate and Eurasia collision zone. Understanding the erosion history of the Xigaze fore-arc basin is significant for realizing the impact of the orogenic belt due to the collision between the Indian plate and the Eurasian plate. The different uplift patterns of the plateau will form different denudation characteristics. If all part of Tibet Plateau uplifted at the same time, the erosion rate of exterior Tibet Plateau will be much larger than the interior plateau due to the active tectonic action, relief, and outflow system at the edge. If the plateau grows from the inside to the outside or from the north to south sides, the strong erosion zone will gradually change along the tectonic active zone that expands to the outward, north, or south sides. Therefore, the different uplift patterns are likely to retain corresponding evidence on the erosion information. The Xigaze fore-arc basin is adjacent to the Yarlung Zangbo suture zone. Its burial, deformation and erosion history during or after the collision between the Indian plate and Eurasia are very important to understand the influence of plateau uplift on erosion. In this study, we use the apatite fission track(AFT)ages and zircon and apatite(U-Th)/He(ZHe and AHe)ages, combined with the published low-temperature thermochronological age to explore the thermal evolution process of the Xigaze fore-arc basin. The samples' elevation is in the range of 3 860~4 070m. All zircon and apatite samples were dated by the external detector method, using low~U mica sheets as external detectors for fission track ages. A Zeiss Axioskop microscope(1 250×, dry)and FT Stage 4.04 system at the Fission Track Laboratory of the University of Waikato in New Zealand were used to carry out fission track counting. We crushed our samples finely, and then used standard heavy liquid and magnetic separation with additional handpicking methods to select zircon and apatite grains. The new results show that the ZHe age of the sample M7-01 is(27.06±2.55)Ma(Table 2), and the corresponding AHe age is(9.25±0.76)Ma. The ZHe and AHe ages are significantly smaller than the stratigraphic age, indicating suffering from annealing reset(Table 3). The fission apatite fission track ages are between(74.1±7.8)Ma and(18.7±2.9)Ma, which are less than the corresponding stratigraphic age. The maximum AFT age is(74.1±7.8)Ma, and the minimum AFT age is(18.7±2.9)Ma. There is a significant north~south difference in the apatite fission track ages of the Xigaze fore-arc basin. The apatite fission track ages of the south part are 74~44Ma, the corresponding exhumation rate is 0.03~0.1km/Ma, and the denudation is less than 2km; the apatite fission track ages of the north part range from 27 to 15Ma and the ablation rate is 0.09~0.29km/Ma, but it lacks the exhumation information of the early Cenozoic. The apatite(U-Th)/He age indicates that the north~south Xigaze fore-arc basin has a consistent exhumation history after 15Ma. The results of low temperature thermochronology show that exhumation histories are different between the northern and southern Xigaze fore-arc basin. From 70 to 60Ma, the southern Xigaze fore-arc basin has been maintained in the depth of 0~6km in the near surface, and has not been eroded or buried beyond this depth. The denudation is less than the north. The low-temperature thermochronological data of the northern part only record the exhumation history after 30Ma because of the young low-temperature thermochronological data. During early Early Miocene, the rapid erosion in the northern part of Xigaze fore-arc basin may be related to the river incision of the paleo-Yarlungzangbo River. The impact of Great Count Thrust on regional erosion is limited. The AHe data shows that the exhumation history of the north-south Xigaze fore-arc basin are consistent after 15Ma. In addition, the low-temperature thermochronological data of the northern Xigaze fore-arc basin constrains geographic range of the Kailas conglomerate during the late Oligocene~Miocene along the Yarlung Zangbo suture zone. The Kailas Basin only develops in the narrow, elongated zone between the fore-arc basin and the Gangdese orogenic belt. The southern part of the Xigaze fore-arc basin has been uplifted from the sea level to the plateau at an altitude of 4.2km, despite the collision of the Indian plate with the Eurasian continent and the late fault activity, but the plateau has been slowly denuded since the early Cenozoic. The rise did not directly contribute to the accelerated erosion in the area, which is inconsistent with the assumption that rapid erosion means that the orogenic belt begins to rise.     

Timescale of magma chamber processes revealed by U–Pb ages,trace element contents and morphology of zircons from the Ishizuchi caldera,Southwest Japan Arc

U–Pb geochronology and trace element chemistry of zircons in a microscale analysis were applied to the Ishizuchi caldera in the Outer Zone of Southwest Japan in order to estimate the timescale of the magma process, in particular, the magma differentiation. This caldera is composed mainly of ring fault complexes, major pyroclastic flow deposits, and felsic intrusion including central plutons. Using SHRIMP‐IIe, our new U–Pb zircon ages obtained from the major pyroclastic flow deposits (Tengudake pyroclastic flow deposits), granitic rocks from central plutons (Soushikei granodiorite and Teppoishigawa quartz monzonite), and rhyolite from the outer ring dike (Tenchuseki rhyolite) and the inner ring dike (Bansyodani rhyolite) are 14.80 ±0.11 Ma, 14.56 ±0.10 Ma, 14.53 ±0.12 Ma, 14.55 ±0.11 Ma and 14.21 ±0.19 Ma, respectively. Based on the U–Pb ages, the Hf contents and the REE patterns of the zircons, three stages are recognized in the evolutionary history of the magma chamber beneath the Ishizuchi caldera: (i) climactic Tengudake pyroclastic flow eruption; (ii) Tenchuseki rhyolite intrusion into the outer ring dike and central pluton intrusion; and (iii) Bansyodani rhyolite intrusion in the inner ring dike. These results indicate a magma evolution history of the Ishizuchi caldera system which took at least ca 600 kyr from the climatic caldera‐forming eruption to the post‐caldera intrusions. Our new geochronological data suggest that the Ishizuchi caldera formed as part of the voluminous and episodic magmatism that occurred in the wide zone along the Miocene forearc basin of Southwest Japan during the inception of the young Philippine Sea Plate subduction.     

青藏高原东缘峨眉山新生代加速抬升剥蚀作用

本文通过峨眉山基底卷入构造带低温热年代学(磷灰石和锆石裂变径迹、锆石(U-Th)/He)研究,结合典型构造-热结构特征诠释峨眉山晚中-新生代冲断扩展变形与热年代学耦合性.峨眉山磷灰石裂变径迹(AFT)和锆石(U-Th)/He(ZHe)年龄值分别为4~30Ma和16~118Ma.ZHe年龄与海拔高程关系揭示出ZHe系统抬升剥蚀残存的部分滞留带(PRZ).低温热年代学年龄与峨眉山构造分带性具有明显相关性特征:万年寺逆断层上盘基底卷入构造带AFT年龄普遍小于10Ma,万年寺逆断层下盘扩展变形带AFT年龄普遍大于10 Ma;且空间上AFT年龄与断裂带具有明显相关性,它揭示出峨眉山扩展变形带中新世晚期以来断层冲断缩短构造活动.低温热年代学热史模拟揭示峨眉山构造带晚白垩世以来的多阶段性加速抬升剥蚀过程,基底卷入构造带岩石隆升幅度大约达到7~8km,渐新世以来抬升剥蚀速率达0.2~0.4mm·a-1,其新生代多阶段性构造隆升动力学与青藏高原多板块间碰撞过程及其始新世大规模物质东向扩展过程密切相关.     

Constraining the stepwise migration of the eastern Tibetan Plateau margin by apatite fission track thermochronology

Granites sampled from Garzê-Litang thrust, Longmen Shan thrust, Garzê and Litang strike-slip faults in the eastern Tibetan Plateau have been analyzed with apatite fission track thermochronological method in this study. The measured fission track apparent ages, combined with the simulated annealing mod- eling of the thermal history, have been used to reconstruct the thermal evolutionary histories of the samples and interpret the active history of the thrusts and faults in these areas. Thermal history mod- eling shows that earlier tectonic cooling occurred in the Garzê-Litang thrust in Miocene (~20―16 Ma) whereas the later cooling occurred mainly in the Longmen Shan thrust since ~5 Ma. Our study sug- gests that the margin of eastern Tibetan Plateau was extended by stages: through strike-slip faults deformations and related thrusts, the upper crust formed the Garzê-Litang margin in the Miocene epoch and then moved to the Longmen Shan margin since ~5 Ma. During this process, the deformations of different phases in the eastern Tibetan Plateau were absorbed by the thrusts within them and conse- quently the tectonic events of long-distance slip and extrusion up to hundreds of kilometers have not been found.     

Heat flow in Eastern Egypt: The thermal signature of a continental breakup

The Red Sea is a modern example of continental fragmentation and incipient ocean formation. Heat flow data have been collected from eastern Egypt to provide information relating to the mode and mechanism of Red Sea opening. Preliminary heat flow data, including new data reported here, are now available from twenty-five sites in eastern Egypt and one site in western Sinai. A pattern of low to normal heat flow (35–55 mW m⁻²) inland with high heat flow (75–100 mW m⁻²) in a zone within 30 to 40 km of the coast is indicated.Moderately high heat flow (around 70 mW m⁻²) is indicated for the Gulf of Suez. The coastal zone thermal anomaly appears continuous with high heat flow previously reported for the Red Sea shelf. Heat production data indicate that the coastal thermal anomaly is not primarily related to crustal radiogenic heat production. The effects of rapid erosion may contribute to the anomaly, but are not thought to be the primary cause of the anomaly. If the anomaly is caused by lateral conduction from hot, extended, offshore lithosphere, the extension must have been active for the last 30 Ma or so, and a minimum of 100% extension is indicated. Alternatively, the anomaly is primarily caused by high mantle heat flow causing lithospheric thinning, centred beneath the Red Sea. The Red Sea is probably underlain by dominantly basic crust, formed either by intrusion into attenuated continental crust or sea-floor spreading, and for most purposes the crust formed in these two modes of extension may be essentially indistinguishable. Fission-track ages from eastern Egypt indicate that uplift started prior to, or at latest at the time of initial Red Sea opening, and this result, together with thermo-mechanical considerations, suggests an active asthenospheric upwelling beneath the Red Sea and high temperature in the lithosphere prior to extension.     

~(40)Ar/~(39)Ar dating of shear deformation of the Xianshuihe fault zone in west Sichuan and its tectonic significance

TheNW-SEstrikingXianshuihefaultzoneslicesthesoutheasternTibetanPlateauandconnectssoutheastwardwiththeAnninghe-Zemuhe-Xiaojiangfaultzone,whichformahuge,activesinistralstrike-slipfaultzone(fig.1).ThisfaultzoneisanimportantseismicfaultineastTibet[1-5].EarthquakegeologystudiesandoffsetpatternsofyounggeologicalfeatureshaveshownthatlateQuaternarysinistralsliprateoftheXianshuihefaultzonereaches13mm/a[1,2].TheXianshuhefaultzoneconsistsoftwomainbranches,theDaofufaultbranchinthewestandtheXianshuih…     

Rapid cooling and geospeedometry of granitic rocks exhumation within a volcanic arc: A case study from the Central Slovakian Neovolcanic Field (Western Carpathians)

U–Pb Sensitive High‐Resolution Ion MicroProbe (SHRIMP) dating of zircon in combination with (U–Th)/He dating of zircon and apatite is applied to constrain the emplacement and exhumation history of the youngest granitic rocks in the Western Carpathians collected in the Central Slovakian Neovolcanic Field. Two samples of diorite from the locality Banky, and granodiorite from Banská Hodru?a yield the U–Pb zircon concordia ages of 15.21 ±0.19 Ma and 12.92 ±0.27 Ma, respectively, recording the time of zircon crystallization and the intrusions’ emplacement. Zircon (U–Th)/He ages of 14.70 ±0.94 (Banky) and 12.65 ±0.61 Ma (Banská Hodru?a), and apatite (U–Th)/He ages of 14.45 ±0.70 Ma (diorite) and 12.26 ±0.77 Ma (granodiorite) are less than 1 Myr younger than the corresponding zircon U–Pb ages. For both diorite and granodiorite rocks their chronological data thus document a simple cooling process from magmatic crystallization/solidification temperatures to near‐surface temperatures in the Middle Miocene, without subsequent reheating. Geospeedometry data suggest for rapid cooling at an average rate of 678 ±158 °C/Myr, and the exhumation rate of 5 mm/year corresponding to active tectonic‐forced exhumation. The quick cooling is interpreted to record the exhumation of the studied granitic rocks complex that closely followed its emplacement, and was likely accompanied by a drop in the paleo‐geothermal gradient due to cessation of volcanic activity in the area.     

Cenozoic geodynamic evolution of the Andaman–Sumatra subduction margin: Current understanding

The Andaman–Sumatra margin displays a unique set‐up of extensional subduction–accretion complexes, which are the Java Trench, a tectonic (outer arc) prism, a sliver plate, a forearc, oceanic rises, inner‐arc volcanoes, and an extensional back‐arc with active spreading. Existing knowledge is reviewed in this paper, and some new data on the surface and subsurface signatures for operative geotectonics of this margin is analyzed. Subduction‐related deformation along the trench has been operating either continuously or intermittently since the Cretaceous. The oblique subduction has initiated strike–slip motion in the northern Sumatra–Andaman sector, and has formed a sliver plate between the subduction zone and a complex, right‐lateral fault system. The sliver fault, initiated in the Eocene, extended through the outer‐arc ridge offshore from Sumatra, and continued through the Andaman Sea connecting the Sagaing Fault in the north. Dominance of regional plate dynamics over simple subduction‐related accretionary processes led to the development and evolution of sedimentary basins of widely varied tectonic character along this margin. A number of north–south‐trending dismembered ophiolite slices of Cretaceous age, occurring at different structural levels with Eocene trench‐slope sediments, were uplifted and emplaced by a series of east‐dipping thrusts to shape the outer‐arc prism. North–south and east–west strike–slip faults controlled the subsidence, resulting in the development of a forearc basins and record Oligocene to Miocene–Pliocene sedimentation within mixed siliciclastic–carbonate systems. The opening of the Andaman Sea back‐arc occurred in two phases: an early (～11 Ma) stretching and rifting, followed by spreading since 4–5 Ma. The history of inner‐arc volcanic activity in the Andaman region extends to the early Miocene, and since the Miocene arc volcanism has been associated with an evolution from felsic to basaltic composition.