3-D crustal-scale gravity model of the San Rafael Block and Payenia volcanic province in Mendoza,Argentina

The San Rafael Block(SRB)is part of one of the main retroarc volcanic provinces in southern Central Andes in Mendoza,Argentina.This block is located in the Andean foothills between the orogenic front and foreland basement uplifts of late Miocene age.In order to analyze the geochronological evolution of the Quaternary volcanism in the region,several geologic and geophysical studies have been conducted.Nevertheless,the crust,where the SRB is located,has not been well characterized yet.Based on gravimetric and magnetic data,together with isostatic and elastic thickness analyses,we modeled the crustal structure of the area.Information obtained has allowed us to understand the crust where the SRB and the Payenia volcanic province are located.Bouguer anomalies indicate that the SRB presents higher densities to the North of Cerro Nevado and Moho calculations suggest depths for this block between 40 and 50 km.Determinations of elastic thickness would indicate that the crust supporting the San Rafael Block presents values of approximately 10 km,being enough to support the block loading.However,in the Payenia region,elastic thickness values are close to zero due to the regional temperature increase.     

Evolution of Irruputuncu volcano,Central Andes,northern Chile

The Irruputuncu is an active volcano located in northern Chile within the Central Andean Volcanic Zone (CAVZ) and that has produced andesitic to trachy-andesitic magmas over the last ∼258 ± 49 ka. We report petrographical and geochemical data, new geochronological ages and for the first time a detailed geological map representing the eruptive products generated by the Irruputuncu volcano. The detailed study on the volcanic products allows us to establish a temporal evolution of the edifice. We propose that the Irruputuncu volcanic history can be divided in two stages, both dominated by effusive activity: Irruputuncu I and II. The oldest identified products that mark the beginning of Irruputuncu I are small-volume pyroclastic flow deposits generated during an explosive phase that may have been triggered by magma injection as suggested by mingling features in the clasts. This event was followed by generation of large lava flows and the edifice grew until destabilization of its SW flank through the generation of a debris avalanche, which ended Irruputuncu I. New effusive activity generated lavas flows to the NW at the beginning of Irruputuncu II. In the meantime, lava domes that grew in the summit were destabilized, as shown by two well-preserved block-and-ash flow deposits. The first phase of dome collapse, in particular, generated highly mobile pyroclastic flows that propagated up to ∼8 km from their source on gentle slopes as low as 11° in distal areas. The actual activity is characterized by deposition of sulfur and permanent gas emissions, producing a gas plume that reaches 200 m above the crater. The maximum volume of this volcanic system is of ∼4 km³, being one of the smallest active volcano of Central Andes.     

扬子地块奥陶系碳酸盐岩重磁化机制探讨

碳酸盐岩是记录古地磁场信息的重要载体,然而,广泛存在的重磁化现象制约了碳酸盐岩在古地磁研究中的应用,其重磁化机制亟待解决.本文对采自贵州羊蹬地区的319块奥陶系碳酸盐岩定向样品作了详细的古地磁学和岩石磁学研究,其结果表明,94%样品(A类)记录了单一剩磁分量A,其解阻温度低于450℃;在地理坐标系下的平均方向为Dg/Ig=3.1°/48.1°(α₉₅=2.9°),对应的古地磁极(87.0°N,2.8°E,A₉₅=3.0°)与扬子地块古近纪-第四纪的古地磁极重合.6%样品(B类)记录了两个磁化分量,其高温分量(450℃~585℃)与A分量显著不同,但明显远离扬子块体早古生代古地磁极;低温分量(< 450℃)与A分量类似.说明羊蹬剖面奥陶系碳酸盐岩记录了两期重磁化.A分量和B低温分量的主要载磁矿物为磁黄铁矿(胶黄铁矿),B高温分量的主要载磁矿物为磁铁矿.这些磁性矿物都是成岩后的次生矿物.其中,解阻温度高于450℃的磁铁矿可能受晚燕山期造山运动影响生成;磁黄铁矿(胶黄铁矿)等矿物可能与印度板块与欧亚大陆碰撞引起的喜马拉雅造山运动所产生的流体作用有关,以后一期重磁化为主.新生代早期青藏高原隆升产生的流体在流经东南缘的碳酸盐岩等沉积岩层时,与原岩发生相互作用,使磁黄铁矿、胶黄铁矿、磁铁矿等磁性矿物生长并获得化学剩磁,造成了广泛重磁化.     

柴达木-祁连山地块内部震间上地壳块体运动特征与变形模式研究

以青藏高原北缘及东北缘的柴达木-祁连山地块内的活动断裂、由断裂所围限的微小块体为研究对象,系统收集整理区内活动断裂定量参数和GPS速度场等资料,使用球面应变率计算方法分析研究区内GPS 速度场得到现今构造应变率场,讨论区内最大剪应变率、面膨胀率与旋转率等参数与区域构造变形之间的关系;同时,依据区内详实的活动断裂资料建立精细的微小活动块体模型,利用Backslip模型反演断裂所围限的各个块体边界断裂的滑动速率、块体内部统一应变率及块体欧拉运动学参数等,并与活动构造方法获得的滑动速率做对比;最后,讨论研究区内由GPS速度场所揭示的地壳运动变形模式.结果表明:(1)柴达木-祁连山地区地壳运动,在沿着山脉走向上具有带状区域分块运动特征,大范围内具有弥散变形特征;(2)青藏高原北部变形场应是通过不同断裂差异性相对运动、区域内部逆冲挤压和块体旋转共同作用的结果.从鄂拉山到古浪民勤一带具有强烈的逆冲活动,其两侧地壳块体分别具有逆向旋转的运动性质;(3)在研究区东部GPS速度场所呈现顺时针旋转的形态,应是处于不同地块边界处的中下地壳与地幔介质差异驱动机制对上地壳块体所产生的作用,并以近地表断层应变率积累形式表现的结果,是祁连山地块、阿拉善块体、鄂尔多斯地块等大型块体推挤旋转影响下的复杂运动学形态.     

Permian Fusuline Fauna from the Lower Part of the Lugu Formation in the Central Qiangtang Block and its Geological Implications

A Kubergandian （Kungurian） fusuline fauna from the lower part of the Lugu Formation in the Cuozheqiangma area,central Qiangtang Block is described.This fusuline fauna belongs to the Southern Transitional Zone in palaeobiogeography,and is characterised by the presence of the distinctive bi-temperate genus Monodiexodina and many genera common in lower latitude Tethyan areas such as Parafusulina and Pseudodoliolina.The occurrence of Monodiexodina in the fauna confirms that the seamount-type carbonates of the Lugu Formation did not originate from the Palaeotethys Ocean,but rather from a branch of the Neotethys Ocean after the rifting of the Qiangtang Block from the Tethys Himalaya area in the Artinskian.     

羌塘地块南部晚白垩世火山岩离子探针测年及其对红层时代的约束

羌塘地块南部广泛出露陆相红层,1:25万区域地质调查将大部分红层划归为中新统康托组,但缺乏可靠的年代学依据。野外观测发现火山岩与红层之间存在喷发不整合接触关系,室内从火山岩选出很多岩浆锆石;应用离子探针U-Pb同位素测年方法精确测定岩浆锆石年龄,能够为研究火山喷发期次和红层形成时代提供重要依据。对羌塘地块南部红层内部粗面安山岩夹层—比洛错火山岩和扎加藏布北侧红层上覆安山岩,挑选岩浆锆石进行高精度的离子探针U-Pb同位素测年,发现比洛错粗面安山岩锆石206Pb/238U同位素年龄为(83.3±1.3)Ma,扎加藏布北侧安山岩锆石206Pb/238U同位素年龄为(75.65±0.82)Ma。这些年龄良好地揭示了晚白垩世不同期次的火山喷发时代,同时为红层形成时代和红层盆地演化提供了重要的年代学约束。根据比洛错和扎加藏布北侧火山岩的锆石U-Pb同位素测年资料,将羌塘盆地南部红层时代归属上白垩统阿布山组,这对分析羌塘地块南部油气地质构造保存条件和构造地貌演化具有重要意义。     

勐腊县茅草山铜矿区河边矿段矿床成因

铜矿体赋存于白垩系曼宽河组(K2m)及F2断层层间构造破碎带内,构造与层位共同控矿。成因属中低温热液型矿床,具有一定的找矿前景。     

晚古生代冈瓦纳的演化长逾13500 km的基墨利超级地体的裂离及其向亚洲的漂移

The Cimmerian terrane forms an almost unbroken chain stretching >13,500 km, from central southern Europe to western Indonesia, via SE Europe, the Middle East, Afghanistan, Tibet, SW China and Myanmar. Ar-guably, it is Earth’s most spectacular example of a “sliver” terrane, dwarfing in size more recently devel-oped examples, for instance the Palawan Block in the western Philippines, and the Lord Howe Rise in the Tasman Sea. The presentation will first outline the in-triguing geological features associated with this unique tectonic entity. Following that, recently obtained results following paleomagnetic investigations of two lower Permian rift-related basalt suites will be summarized (Abor Volcanics in northeastern India and Woniusi Ba-salts in Yunnan, China). The two studies are part of a larger programme of ongoing research aimed at deducing (I) the geodynamic configuration that generated the un-usual rifting system, and (II) exactly how Cimmeria fit-ted against Gondwana prior to its dispersal in the Early Permian. The critical unit is Baoshan, which we fit against Gondwana within a narrow longitudinal belt close to where northern India and northwestern Australia were once in close proximity (Fig. 1). Furthermore, we suggest that Sibumasu lay to directly the east, offshore of Australia; Qiangtang and Lhasa almost certainly sat to the west (off northern Greater India-SE Arabia), but we are uncertain as to their exact configuration. Our findings will be compared with several rather different models that have been published in recent years. The new pa-leomagnetic constraint highlights the flexibility authors currently have in reconstructing the region, principally because of the overall lack of similar high-quality data from the various blocks. We explain how new data could resolve these ambiguities, thereby offering more robust explanations for eastern Gondwana’s late Paleozoic de-velopment.     

黑濑川地体源于亚洲大陆东缘何处？

The point at issue: The Kurosegawa Terrane is composed of continental fragments transecting Mesozoic terranes of accretionary complex in Southwest Japan (Fig. 1). It is an attenuated tectonic sliver and considered to be allochthonous with respect to the main part of Southwest Japan. The problem of which continental block in the East Asian continental margin is the source of the Kurosegawa Terrane has puzzled Japanese geologists for many years. Firstly, we try to approach this issue based on the analysis of fusulinacean assemblage in accreted terranes composed of subduction complex in the Pacific Rim. Secondly, by applying the result of this analysis we try to locate the source of the continental fragments of the Kurosegawa Terrane. Thirdly, we try to prove its validity with a new paleomagnetic study.     

An alternative Pangea reconstruction for Middle America with the Chortis Block in the Gulf of Mexico: tectonic implications

Understanding Pangea breakup requires a robust reconstruction, and this article focuses on the Middle America sector of the supercontinent. Although most Pangean reconstructions locate the Yucatan Block along the southern USA, the Chortis Block is generally placed off southern Mexico (Pacific model), undergoing sinistral relative motion during the Mesozoic and Cenozoic. However, the Pacific model is inconsistent with the absence of a Cenozoic fault linking the Cayman transforms and the Middle America Trench. We present an alternative Pangean reconstruction, where both the Yucatan and Chortis Blocks are placed in the future Gulf of Mexico, moving Mexico westwards along the Mojave–Sonora megashear to accommodate overlap with South America. Subsequent Mesozoic and Cenozoic evolution is inferred to have occurred in two stages: (i) Jurassic clockwise rotation along the Mojave–Sonora and West Florida megashears, followed by (ii) Cenozoic anticlockwise rotation along the Sierra Madre Oriental and East Yucatan megashears. The first stage is linked to the breakup of Pangea where the Gulf of Mexico formed as a pull-apart basin. The second stage is related to the evolution of the Caribbean where the Chortis and Yucatan Blocks rotated into the trailing side of the Caribbean Plate (pirate model). The new reconstruction is consistent with major parameters, such as (i) gravity, magnetic, and palaeomagnetic data; (ii) the westward continuation of the Cayman transform faults through the Chiapas foldbelt and along the N–S front of the Sierra Madre Oriental foldbelt; (iii) the 27–19 Ma removal of the southern Mexican forearc; (iv) offset of the Cretaceous volcanic arc (Guerrero-Suina); (v) the deflection of the Laramide orogen (Sierra Madre Oriental–Zongolica–Colon); and (vi) the continuity of Cretaceous platformal carbonates containing Caribbean fauna across Middle America. In this latter context, the Motagua high-pressure belt is interpreted as a Cretaceous extrusion zone into the upper plate above a subduction zone rather than as an oceanic suture.