Evidence for Proterozoic Collision from Airborne Magnetic and Gravity Studies in Southern Granulite Terrain, India and Signatures of Recent Tectonic Activity in the Palghat Gap

The composite airborne total intensity map of the Southern Granulite Terrain (SGT) at an average elevation of 7000' (≈ 2100 m) shows bands of bipolar regional magnetic anomalies parallel to the structural trends suggesting the distribution of mafic/ultramafic rocks that are controlled by regional structures/shear zones and thrusts in this region. The spectrum and the apparent susceptibility map computed from the observed airborne magnetic anomalies provide bands of high susceptibility zones in the upper crust associated with known shear zones/thrusts such as Transition Zone, Moyar-Bhavani and Palghat-Cauvery Shear Zones (MBSZ and PCSZ). The quantitative modelling of magnetic anomalies across Transition Zone, MBSZ and PCSZ suggest the presence of mafic rocks of susceptibility (1.5-4.0 × 10⁻³ CGS units) in upper crust from 8-10 km extending up to about 21-22 km, which may represent the level of Curie point geotherm as indicated by high upper mantle heat flow in this section.Two sets of paired gravity anomalies in SGT and their modelling with seismic constraints suggest gravity highs and lows to be caused by high density mafic rocks along Transition Zone and Cauvery Shear Zone (CSZ) in the upper crust at depth of 6-8 km and crustal thickening of 45-46 km south of them, respectively. High susceptibility and high density rocks (2.8 g/cm³) along these shear zones supported by high velocity, high conductivity and tectonic settings suggest lower crustal mafic/ultramafic granulite rocks thrusted along them. These signatures with lower crustal rocks of metamorphic ages of 2.6-2.5 Ga north of PCSZ and Neoproterozoic period (0.6-0.5 Ga) south of it suggest that the SGT represents mosaic of accreted crust due to compression and thrusting. These observations along with N-verging thrusts and dipping reflectors from Dharwar Craton to SGT suggest two stages of N-S directed compression: (i) between Dharwar Craton and northern block of SGT during 2.6-2.5 Ga with Transition Zone and Moyar Shear towards the west as thrust, and (ii) between northern and southern blocks of SGT with CSZ as collision zone and PCSZ as thrust during Neoproterozoic period (0.6-0.5 Ga). The latter event may even represent just a compressive phase without any collision related to Pan-African event. The proposed sutures in both these cases separate gravity highs and lows of paired gravity anomalies towards north and south, respectively. The magnetic anomalies and causative sources related to Moyar Shear, MBSZ and PCSZ join with those due to Transition Zone, Mettur and Gangavalli Shears in their eastern parts, respectively to form an arcuate-shaped diffused collision zone during 2.6-2.5 Ga.Most of the Proterozoic collision zones are highlands/plateaus but the CSZ also known as the Palghat Gap represents a low lying strip of 80-100 km width, which however, appears to be related to recent tectonic activities as indicated by high upper mantle heat flow and thin crust in this section. It is supported by low density, low velocity and high conductive layer under CSZ and seismic activity in this region as observed in case of passive rift valleys. They may be caused by asthenospheric upwarping along pre-existing faults/thrusts (MBSZ and PCSZ) due to plate tectonic forces after the collision of Indian and Eurasian plates since Miocene time.     

Heterogeneity in crustal structure across the Southern Granulite Terrain (SGT): Inferences from an analysis of gravity and magnetic fields in the Periyar plateau and adjoining areas

The study region forms the western part of the Madurai block (southern block) and shares several lithological characteristics of the Proterozoic exhumed South Indian Granulite Terrain (SGT). The crustal structure of the area has been derived from gravity data, constrained partly by aeromagnetic data. The Bouguer anomaly map of the region prepared based on detailed gravity observations shows a number of features (i) the Periyar lineament separates two distinctly different gravity fields, one, a high gravity gradient tending to be positive towards the coast in south west and significant gravity lows ranging from − 85 to as low as − 150 mGal in the NE covering a large part of the Periyar plateau (ii) within the broad gravity low, three localised circular anomalies of considerable amplitude occur in the region of Munnar granite. A magnetic low region in the central part coincides with the area of retrogressed charnockites and the major lineaments suggestive of a genetic link and considerable downward extent. The crustal models indicate that the upper layer containing exhumed lower crustal rocks (2.76 gm/cc) is almost homogeneous, most part of the gravity field resulting from variations in intracrustal layers of decharnockitised hornblendic gneisses and granite bodies. Below it, a denser layer (2.85 gm/cc) of unknown composition exists with Moho depth ranging from 36 to 41 km. The structure below the region is compared with that of two other segments of the SGT from which it differs markedly. The Wynad plateau forming the western part of the Northern Block of the SGT is characterised by a heterogeneity due to the presence of contrasting crustal blocks on either side of the Bavali shear zone, possibly a westward extension of the Moyar shear zone and presence of high density material in the mid-to-lower crustal portions. The crust below the Kuppam–Palani transect has a distinctive four-layer structure with a mid-crustal low density layer. The differences in crustal structure are consistent with the different tectonic settings of the three regions discussed in the paper. It is suggested that the crustal structure below the Kuppam–Palani transect corridor is not representative of the SGT as a whole, an aspect of great relevance to intra-continental comparisons and trans-continental reconstructions of continent configurations of the Gondwanaland.     

重磁对应分析方法在东北地区实际资料处理中的应用

本文从泊松定理出发,设计三度体理论模型,运用重磁对应分析方法,研究理论模型的重磁异常相关系数R、斜率B和截距A。由其等值线图得出点、线距的疏密不影响分析窗口半径的选择,确定了分析窗口半径的选择范围在模型半径的1.5倍左右以及截距的变化是剩磁反映的结论。进而从东北地区实际资料的重磁对应分析方法处理中推断出:该区强磁性岩石的剩磁较强,且由重磁异常的斜率B和截距A确定了强磁与剩磁的分布范围。     

重磁三维可视化反演系统的设计与实现

重磁三维可视化反演解释系统一直是国内外重磁勘探领域的研究重点之一。介绍了基于Visual C++与OpenGL环境研发的重磁三维可视化反演系统,详细阐述了系统的设计思想与实现的关键技术,包括可视化技术、面向对象技术、图形拾取技术、碰撞检测技术及反演约束技术等。系统具有人机交互几何反演与最优化物性自动反演两种功能,可满足目标异常、区域模拟和盆地建模。     

青藏高原区域重磁异常的东西向分区及其构造地质特征

自从大陆整合以来作为一个整体的青藏高原继续受着印度板块向北俯冲的影响,也必定不断地改造着原各地体的结构构造,形成了高原整体意义上东西向的差异。这种差异与原本各地体的组成、结构和东西向延伸不一致。这不仅表现在南北向断裂构造跨各单个地体范围的出现,而且,逐步形成了东西的分区。这种分区突出地表现在区域重力与磁场的特征上,这不仅是局部的岩石磁性与密度变化的结果,而且是由于印度板块向北俯冲过程中,在其前缘的不同部位上经受的压力不同,以及地块的隆升与扩张作用的差异造成了高原东西各区段的地壳组分与厚度的变化。青藏高原的南北向断裂构造并非地壳上层的局部断裂,它具有深层的原因。由于印度板块向北推进的过程中不是均匀地齐头并进,而是在帕米尔高原以东的青藏高原范围内存在着推进速度和俯冲深度的差异,随着高原隆升的加剧高原本身出现断裂,自中新生代以来就存在着一定差异,所以南北向的断裂构造比目前地表见到的多些,而且具有较大的深度,Moho面的深度和地壳厚度都受南北向断裂的控制,并形成了区域重磁场的变化。同时,高原的东西向拉张作用也使南北断裂带发育加剧。     

赣南新林屋-均村地区低磁性隐伏岩体的识别:来自重磁场的证据

通过在赣南新林屋-均村地区开展1∶5万高精度磁法测量工作,结合1∶20万重力测量数据,以岩石物性为基础,分析该区重磁异常并圈定隐伏花岗岩体.新林屋-均村地区低磁性、低密度的花岗岩侵入至具有磁性的前寒武纪变质岩中,形成磁低重低的异常特征;推断新林屋-均村地区存在1个呈NW向展布的隐伏花岗岩体,该隐伏花岗岩体浅部受SN-NE向弧状断裂控制,深部受NW向断裂控制,浅部分布范围较深部更广.这些信息为该区进一步找矿提供了基础地球物理资料,也为同类地区寻找隐伏花岗岩体提供参考.     

鲁西金刚石原生矿床区域重磁异常特征及深部地质构造背景

鲁西金刚石原生矿床成矿背景复杂,由于缺乏地壳结构、岩浆侵入体位置、断裂规模等信息作为参考依据,深部构造特征及其在成矿过程中的作用尚不明确。本文基于重磁数据和地震剖面资料,采用Parker-Oldenburg界面法、功率谱法和2.5D重磁震联合反演等方法,获得了鲁西地区地壳结构、断裂规模、岩浆侵入体位置等信息,在此基础上,探讨了金刚石原生矿床的深部构造背景。结果表明:鲁西金刚石原生矿床分布于区域布格重力异常低值区、磁异常中-低值区,布格重力异常平均值低于-100×10^-5 m/s²,磁异常变化范围较大,介于-160～60 nT之间;矿床位于莫霍面、居里面等值线密集的梯度带,即稳定区域与活化区域的过渡带,莫霍面深度约为31.2～32.2 km,居里面深度约27.5～30 km;矿床分布区NW向断裂构造切割深度均超过20 km,其中蒙山断裂切割深度为35 km,深达上地幔,新泰-垛庄断裂切割深度为28.5 km,深达居里面,泰山-铜冶店断裂切割深度为20.5 km;金伯利岩于古生代形成后,受中生代伸展构造影响,被NW向断裂逐级抬升、剥蚀,直至出露地...