Tectonic Evolution of the Southern Part of Aravalli Mountain Belt and its Environs: Possible Causes and Time Constraints

Structural studies on Proterozoic rocks belonging to the Lunavada Group, Southern Aravalli Mountain Belt (SAMB), India, have shown that they underwent three episodes of deformation which have led to the formation of various regional scale interference patterns. Whilst the northern parts of the SAMB underwent brittle-ductile deformation, the southern portion underwent more ductile deformation. On the basis of structural as well as metamorphic studies it has been established earlier that the region was subjected to uplift orogenesis during its evolutionary history. In the present paper an attempt is made to visualize the possible causes that led to deformation of the SAMB, the structural geology of which has been established by the authors, and to constraint the timing of these events on the basis of already available geochronological data. A “working-hypothesis” is proposed according to which it is suggested that deformation of the SAMB is a result of the accretion of the three protocontinents viz. Aravalli, Dharwar and Singhbhum during the Mesoproterozoic. It is envisaged that the accretion of Aravalli and Singhbhum Protocontinents occurred between 1600 and 1400 Ma along the NE-SW trending Son Suture and this event led to development of NE-SW trending structures in the SAMB. Suturing of Aravalli and Dharwar Protocontinents between 1400 and 935 Ma along the E-W Narmada Suture was responsible for the E-W to NW-SE trending D₃ structures of the SAMB. It is postulated that the Satpura orogeny which resulted in deformation of rocks in Satpura mountain range lying to the south of Narmada Suture was coeval with the accretion of Aravalli and Dharwar Protocontinents.     

Comparing Phoebe’s 2005 opposition surge in four visible light filters

We observed Phoebe for 13 nights over a period of 55 days before, during, and after the 2005 Saturn opposition with the New Mexico State University (NMSU) 1-m telescope at Apache Point Observatory (APO) in Sunspot, NM and characterized the width and magnitude of Phoebe’s opposition surge in BVRI filters. Our observations cover a phase angle range of 4.87° to 0.0509°. We use a Hapke reflectance model incorporating shadow hiding and coherent backscatter to investigate the wavelength dependence of Phoebe’s opposition surge. We find a significant opposition surge magnitude of 55-58% between phase angles of 5° and 0°. We find the strongest opposition surge for phase angles less than 2° in the I-band. The coherent backscatter angular width is on the order of 0.50°. We find Phoebe’s albedo to be spectrally flat within our error limits, with a B-band albedo of 0.0855 ± 0.0031, a V-band albedo of 0.0856 ± 0.0023, an R-band albedo of 0.0843 ± 0.0020, and an I-band albedo of 0.0839 ± 0.0023. We compare Phoebe’s albedo, color, and opposition surge magnitudes and slopes with those of other outer solar system bodies and find similarities to Centaurs, Nereid, Puck, and Comets 19P/Borrelly, 9P/Tempel 1, and 81P/Wild 2. We find that this comparison supports the idea that Phoebe originated in the Kuiper Belt. We also discuss the caveats of using results from a Hapke reflectance model to derive specific surface particle properties.     

Optical and infrared colors of transneptunian objects observed with HST

We present optical colors of 72 transneptunian objects (TNOs), and infrared colors of 80 TNOs obtained with the WFPC2 and NICMOS instruments, respectively, on the Hubble Space Telescope (HST). Both optical and infrared colors are available for 32 objects that overlap between the datasets. This dataset adds an especially uniform, consistent and large contribution to the overall sample of colors, particularly in the infrared. The range of our measured colors is consistent with other colors reported in the literature at both optical and infrared wavelengths. We find generally good agreement for objects measured by both us and others; 88.1% have better than 2 sigma agreement. The median absolute magnitude, H_V, magnitude of our optical sample is 7.2, modestly smaller (∼0.5 mag) than for previous samples. The median H_V in our infrared sample is 6.7. We find no new correlations between color and dynamical properties (semi-major axis, eccentricity, inclination and perihelion). We do find that colors of Classical objects with i < 6° come from a different distribution than either the Resonant or excited populations in the visible at the >99.99% level with a K-S test. The same conclusion is found in the infrared at a slightly lower significance level, 99.72%. Two Haumea collision fragments with strong near infrared ice bands are easily identified with broad HST infrared filters and point to an efficient search strategy for identifying more such objects. We find evidence for variability in (19255) 1999 VK₈, 1999 OE₄, 2000 CE₁₀₅, 1998 KG₆₂ and 1998 WX₃₁.     

藏南古堆地区的新生代构造样式

藏南古堆地区具南北分带特征。北部地区靠达拉岩体,受也拉香波穹窿影响,断层以倾向南的正断层为主,地层呈多期次挤压褶皱形态,且出现呈楔形构造夹片出露的红柱石板岩、石榴石片岩等变质核杂岩地层;中部地区断层、褶皱较发育,褶皱呈紧闭的层间同斜褶皱,断层以倾向北的叠瓦状脆-韧性逆冲断层为主;南部地区为相对稳定区,该区地层相对较完整,褶皱以宽缓向斜形式出现,且越往南越宽缓。这一构造样式是印度板块与欧亚板块碰撞之后,在喜马拉雅造山运动影响及后期伸展作用的背景之下,由北向南的挤压推覆的结果,总体上它是一套挤压褶皱～推覆逆冲断层的组合,呈叠瓦状展布的隆子断裂是主推覆断层。     

The persistence and role of basin structures on the 3D architecture of the Marañón Fold-Thrust Belt,Peru

The 3D architecture of fold-thrust belts commonly involves thin-skinned and thick-skinned deformation. Both thick- and thin-skinned deformation styles have been suggested to occur in the Marañón Fold-Thrust Belt (MFTB) in Peru, but the relative timing and strain partitioning associated with them are not well understood. We demonstrate that inherited basement structures along the Peruvian convergent margin reactivated during the evolution of the MFTB. We present results from field mapping, interpretation of remote sensing imagery, and cross section construction and restoration. The results show that the Chonta Fault, a median pre-folding basin normal fault, was inverted and acted as a mechanical buttress during initial east-vergent contraction of the fold-thrust belt. This fault separates the belt into two domains of distinctly different structural styles. During the Eocene, units to the west of the Chonta Fault deformed by folding, using the fault as a buttress, and subsequently propagated eastward by thin-skinned thrusting. This was followed in the Miocene by west-vergent, basement-involved deformation, which overprinted the earlier east-vergent, thin-skinned structures. The proposed tectonic model of the MFTB highlights the role of basement-fault reactivation during orogenesis and the involvement of deep structures in partitioning deformation styles.     

安徽铜陵姚家岭锌金多金属矿床成岩成矿年代学研究

姚家岭锌金多金属矿床位于铜陵断隆区与繁昌断凹区的过渡部位,是长江中下游成矿带新近发现的大型锌金多金属矿床,也是铜陵矿集区中迄今发现的首例大型锌金多金属矿床。在详细野外地质工作及室内研究的基础上,对姚家岭矿床花岗闪长斑岩体开展了LA-ICPMS锆石U-Pb定年工作,获得206Pb/238U加权平均年龄为141.4±1.7Ma(MSWD=0.78,n=19),代表成岩年龄;选取了该矿床金属矿物辉钼矿进行Re-Os同位素测年,获得模式年龄范围为141.2±2.1Ma~141.6±2.7Ma,平均年龄为141.4±2.4Ma,可代表热液成矿作用的时代,与成岩年龄一致。姚家岭锌金多金属矿床为早白垩世早期岩浆热液作用形成的,是铜陵断隆区燕山期大规模成矿作用的产物,铜陵断隆区具有进一步寻找锌金矿床的有利前景。     

内蒙古霍各乞Cu-Pb-Zn矿床的剪切带与岩性控矿特征及意义

内蒙古霍各乞大型Cu-Pb-Zn矿床地处内蒙西部狼山地区,为狼山-渣尔泰山成矿带的典型矿床。前人依据该矿裂谷沉积赋矿、岩性控矿、具有层状矿体等宏观特征判断其为裂谷环境下形成的热水沉积矿床,但也有部分学者提出其为受构造控制的后生热液矿床。本次研究以岩矿相学观察为基础,结合热力学模拟,提出该矿为受剪切带控制的变质热液矿床。矿区内赋矿围岩普遍经历角闪岩相区域变质作用及韧性剪切变形。宏观上,矿体与矿区内剪切带产状协调。硫化物总体沿围岩糜棱岩片理产出,但局部切割围岩糜棱岩组构,含矿微裂隙具有脆韧性剪切变形特征,可见硫化物交代围岩峰期变质矿物。热液脉石矿物组合及矿物温压计均表明成矿期具有低绿片岩相的温压条件。上述特征表明Cu-Pb-Zn矿化受剪切带控制,发生于剪切变形晚期,围岩退变质抬升阶段。热力学模拟显示当含矿流体流经由碳质千枚岩、石英二云母片岩和富铁夹层构成的岩性柱子,碳质千枚岩层位发生Pb-Zn矿化,而石英二云母片岩层位发生Cu矿化,富铁层位形成富矿体,这与霍各乞的地质事实相一致,矿相学观察也表明围岩岩性对成矿具有控制作用,因此后生热液流体与围岩的水岩反应导致了霍各乞矿床的层控、岩矿特征。通过上述研究建立起的剪切带控矿的成矿模式既符合该矿的显微观察结果,又可以解释该矿层状矿体、岩性控矿等宏观特征。     

浙江璜山造山型金矿的确认及其意义

造山型金矿是一种重要的金矿类型,在我国主要造山带都有分布,但在华南地区的江南造山带却少有人研究.通过与典型造山型金矿进行对比,认为浙江璜山金矿在成矿构造环境、赋矿地体变质特征、成矿时间与构造变形、峰期变质时间的关系、成矿元素富集特点以及成矿流体特征上,均与造山型金矿类似,所以确认璜山金矿为典型造山型金矿.璜山造山型金矿的确认对江南造山带区域找矿、璜山金矿深部找矿均具有十分重要的启发和借鉴意义,而且对江南造山带构造演化提供了约束.     

Re–Os Geochronology on Molybdenites from the Donggebi Mo Deposit in the Eastern Tianshan of the Central Asia Orogenic Belt and its Geological Significance

The Donggebi Mo deposit is located in the eastern section of the Eastern Tianshan Orogenic Belt, Central Asia Orogenic Belt. Rhenium and osmium isotopes of molybdenites from the Donggebi porphyry Mo deposit have been used to determine the timing of mineralization and the source of osmium and, by inference, ore metals. Molybdenite was collected mainly from a granite porphyry stock, which is characterized by moderate to strong silicification. Rhenium concentrations in molybdenite samples are between 10 and 63 μg g^?1. Analysis of eight molybdenite samples yields an isochron age of 234.3 ± 1.6 Ma (2σ) with an initial ¹⁸⁷Os of 0.04 ± 0.45 (MSWD = 0.25). The data support the hypothesis that a significant part of the metals and magmas have a mixed (crust + mantle) origin. Based on the geological history and spatial‐temporal distribution of the granitoids, it is proposed that the Mo deposits in the eastern part of the East Tianshan Orogenic Belt were related to a post‐collision extensional setting in the Early Mesozoic.     

http://www.sciencedirect.com/science/article/pii/S1674987113000364

The South Tianshan Orogen and adjacent regions of Central Asia are located in the southwestern part of the Central Asian Orogenic Belt.The formation of South Tianshan Orogen was a diachronous,scissors-like process,which took place during the Palaeozoic,and its western segment was accepted as a site of the fnal collision between the Tarim Craton and the North Asian continent,which occurred in the late Palaeozoic.However,the post-collisional tectonic evolution of the South Tianshan Orogen and adjacent regions remains debatable.Based on previous studies and recent geochronogical data,we suggest that the fnal collision between the Tarim Craton and the North Asian continent occurred during the late Carboniferous.Therefore,the Permian was a period of intracontinental environment in the southern Tianshan and adjacent regions.We propose that an earlier,small-scale intraplate orogenic stage occurred in late Permian to Triassic time,which was the frst intraplate process in the South Tianshan Orogen and adjacent regions.The later largescale and well-known Neogene to Quaternary intraplate orogeny was induced by the collision between the India subcontinent and the Eurasian plate.The paper presents a new evolutionary model for the South Tianshan Orogen and adjacent regions,which includes seven stages:(I)late Ordovicianeearly Silurian opening of the South Tianshan Ocean;(II)middle Silurianemiddle Devonian subduction of the South Tianshan Ocean beneath an active margin of the North Asian continent;(III)late Devonianelate Carboniferous closure of the South Tianshan Ocean and collision between the Kazakhstan-Yili and Tarim continental blocks;(IV)early Permian post-collisional magmatism and rifting;(V)late PermianeTriassic the frst intraplate orogeny;(VI)JurassicePalaeogene tectonic stagnation and(VII)NeoceneeQuaternary intraplate orogeny.