Progressive,episodic deformation in the Mexican Fold–Thrust Belt (central Mexico): evidence from isotopic dating of folds and faults

We used illite Ar/Ar dating to obtain absolute ages of folds and shear zones formed within the Mexican Fold–Thrust Belt (MFTB). The methodology takes advantage of illite dating in folded, clay-bearing layers and the ability to obtain accurate ages from small-size fractions of illite using encapsulated Ar analysis. We applied our approach to a cross-section that involves folded Aptian–Cenomanian shale-bentonitic layers interbedded with carbonates of the Zimapán (ZB) and Tampico–Misantla (TMB) Cretaceous basins in central-eastern Mexico. Basinal carbonates were buried by syn-tectonic turbidites and inverted during the formation of the MFTB in the Late Cretaceous. Results from folds and shear zones record different pulses of deformation within this thin-skinned orogenic wedge.

Mineralogical compositions, variations in illite polytypes, illite crystallite size (CS), and Ar/Ar ages were obtained from several size fractions in limbs and hinges of the folds and in the shear zones. 1M_d-illite polytype (with CS of 6–9 nm) dominates in two folds in the TMB while 2M₁-illlite (with CS of 14–30 nm) dominates in the third fold, in the ZB, and in the fold/shear zone. From west (higher grade) to east (lower grade): Ar retention ages indicate shearing occurred at ~84 Ma in the westernmost shear zone, folding at ~82 Ma in the ZB with subsequent localized shearing at ~77 Ma, and Ar total gas ages constrain the time of folding at ~64 Ma on the west side of the TMB and ~44 Ma on the eastern edge. These results are consistent with the age and distribution of syn-tectonic turbidites and indicate episodic progression of deformation from west to east.     

Magnetic Characterization of Amphibolites from the Fomopéa Pluton (West Cameroon): Their Implication in the Pan-African Deformation of the Central African Fold Belt

The Fomopea granitic pluton is emplaced in gnessic and amphibolitic basement.These gneissic and amphibolitic basement rocks are represented in the pluton's body as sub-rounded,elongated or stretched xe...     

甘肃省北山地区罗雅楚山—大红山一带区域构造格局、褶皱样式及断裂系

甘肃北山地区罗雅楚山—大红山一带,赋存青白口纪沉积变质铁矿,由元古宙以来,本区经历了多期次构造变动,形成了现今相互叠置的复杂地质体,其加里东运动及海西运动奠定了本区总体褶皱样式。印支运动发育了逆冲断裂系。最为特色的是燕山运动导致本区大规模由南向北推覆的推覆构造,喜山运动为相对稳定的升降运动。     

海南岛西部青白口系石碌群褶皱变形特征及其控矿分析

青白口系石碌群属于一套海相沉积经变质的以细粒碎屑岩、泥岩及镁质碳酸岩夹火山岩为原岩的变质岩系组合,赋存有沉积变质型铁矿和热液型铜钴矿。岩石经历了多期次变形作用的改造,总体表现为由褶皱变形往韧脆性断裂变形演化的特征,褶皱变形包含了三个变形序列的构造形迹组合。与铁矿主成矿期密切有关的NW向石碌复向斜属于海西．印支期造山运动...     

阿尔金山东段大平沟地区褶皱构造的特征及其成因

大平沟地区位于北东向阿尔金走滑断裂北侧与东西向阿尔金北缘断裂所夹持的区域。区内出露卓阿布拉克组火山-沉积岩系,其中以灰黑色粉砂质泥岩为代表的沉积岩系出露宽度3～5km,是相邻20km的东侧喀腊大湾地区出露厚度（约1km）的3～5倍。通过野外详细的地质调查,特别是运用小褶皱分析方法、褶皱层理与轴面劈（片）理关系判别分析方法,首次对阿尔金山东段大平沟地区的卓阿布拉克组火山-沉积岩系中的沉积岩夹层沿剖面开展追索研究,恢复了大平沟地区褶皱构造的轮廓,并对其成因进行初步探讨。认为南侧喀腊达坂-阿克达坂断裂以南的元古宇碳酸盐岩系较强硬的岩层、北侧阿尔金北缘断裂以北的太古宇结晶基底的刚性岩块、以灰黑色粉砂质泥岩为代表的细碎屑岩的柔性特点、冰沟岩体隆升造成其东侧盖层向东收缩是基础,而北北东向区域挤压构造应力场作用下北东东向卓阿布拉克断裂的左行走滑造成的卓阿布拉克组沉积岩系西端的侧向挤压是褶皱形成的直接原因。     

Geomorphic response to growing fault-related folds: example from the foothills of central Taiwan

Abstract

In this paper we concentrate particularly on the geomorphological indicators left by active tectonics. In the central foothills of Taiwan, we used topography, drainage pattern and structural data to perform quantitative morphometric analysis and to determine relative age of fault-related anticlines. The Tiehchen, Tatu and Pakua ridge belt is a fault-related anticline system located in the hanging wall of the Changhua fault along the western thrust front of the foothills. Geomorphic systems are analysed with intent to detect the various responses of landforms and drainage pattern to late Quaternary deformation. Topography and drainage basin register uplift and are valuable tools to discriminate lateral propagation of an active frontal fold. Geomorphic field evidence and quantitative morphometric parameters are used to define the evolution of the rising anticline ridges and to infer tectonism style along an active front. Geometry of alluvial fans, formed along the frontal side of the anticlines, and weathered terrace deposits provide relevant information on neotectonics. Knowledge concerning these younger anticline ridges, makes this area a good example of an actively forming mountain front. We discuss in detail the origin of N045°, N095 and N120° trending oblique fault scarps which delimite numerous fault blocks. The fault scarps morphology is characterized by imbricate talus facets. Steeper topography accompanied by breaks in the slope along some transverse profiles, seems to correspond to the traces of successive uplifts. © 2001 Éditions scientifiques et médicales Elsevier SAS     

九瑞矿集区燕山期构造-岩浆作用及其与铜金多金属成矿关系研究

九瑞地区是长江中下游成矿带中最重要的铜金多金属矿集区,成矿作用可分为海西期喷流沉积期及燕山期岩浆热液期,其中燕山期的构造-岩浆热事件对成矿至关重要。本文在详细的野外矿田构造-地层-岩浆岩调查与分析及室内成岩成矿机理研究基础上,对九瑞矿集区构造系统、岩浆系统和成矿系统进行了综合研究。梳理厘定出该区主要控岩成矿断裂构造为NEE（近EW）向脆韧性走向叠掩逆冲断裂带和SN（近SN）向张扭性横向破碎断裂带。两组断裂形成“井”字形的构造格架,制约着该区的岩浆岩分布并与成矿密切相关。区内岩浆岩种类繁多,与成矿有关的主要是花岗闪长斑岩和石英闪长玢岩。两类岩体侵位的时期都集中在燕山早中期（138~148Ma）。其产状一般为岩株、岩锥、岩枝和岩墙等。岩枝、岩墙多沿NEE向逆冲断层带,尤其是沿其特有的“Y”字形断裂分布。而与成矿关系最为密切的岩株、岩锥则都侵位于NEE向和SN向断裂的交汇处。本文特别强调前人关注不够的SN（近SN）向张裂带及其控岩成矿作用。在本区确定的4组SN向张扭性带中,其中第2组宋家湾-严家村一线同NEE断裂带的几处交汇处是值得进一步详查的预测区。九瑞矿集区矿床成因类型主要包括矽卡岩型、斑岩型（包括隐爆角砾岩型）、沉积-热液叠加改造型和热液脉型等。在不同矿区,各种不同成因类型的矿床可以叠加复合为不同的矿床式,如①武山式（沉积-热液叠加改造型+矽卡岩型矿化）;②丰山洞式（矽卡岩型+隐爆角砾岩型矿化）;③城门山式（斑岩型+矽卡岩型+沉积-热液叠加改造型矿化）;④洋鸡山式（隐爆角砾岩型+斑岩型矿化）。     

五河县大巩山地区褶皱构造浅析

大巩山西蝈堆组蚌埠期褶皱可利用含白云质大理岩透镜体的蛇纹岩作为标志层来恢复,推测其为轴面倒向北的紧密同斜褶皱,S0近南北向,枢纽西倾,地层由东向西变新。     

煤与瓦斯突出中单个煤壳解体突出的突变理论分析

在煤与瓦斯突出中,煤体自组织地形成最具抗力的球盖壳形式来抵抗被突出,煤壳一旦解体便丧失承载能力,高位势煤岩系统中与单个煤壳突出相应的各种能量得以释放。根据反映煤岩破坏特性的塑性变形局部化理论,导得突出前煤壳形变、位移时相应于偏应力的功、能增量关系,建立了煤壳失稳解体的突变模型,并对由于偏应力作用导致煤壳失稳解体机制进行了研究,给出失稳瞬间释放的偏应力应变能表达式、煤岩系统体积应变能释放量表达式和孔隙瓦斯粉碎解体后的碎煤及与煤层瓦斯渗流形成瓦斯-煤粉两相流描述。所得结果可深化对煤、地应力与瓦斯压力三因素对煤与突出演化规律综合作用的认识。     

海拉尔盆地乌尔逊-贝尔凹陷挤压构造的发现及其地质意义

海拉尔盆地是叠置于内蒙古-大兴安岭古生代碰撞造山带之上的中-新生代盆地,总体上呈北东向展布,由三坳、两隆五个一级构造单元组成,自西向东依次为扎贲诺尔坳陷、嵯岗隆起、贝尔湖坳陷、巴彦山隆起和呼和湖坳陷, 其中乌尔逊凹陷、贝尔凹陷是位于贝尔湖坳陷凹陷南部的两个二级构造单元。研究表明,乌尔逊-贝尔凹陷在早白垩世大磨拐河期-伊敏期早期、伊敏期末期分别经历了北西 南东向、近东西向两次较长时间的挤压作用,使伸展断陷中的部分控陷正断层发生反转作用,在研究区形成了新的断层传播褶皱、断层转折褶皱以及叠瓦构造和双重构造、突发构造、构造三角带等。由于挤压构造的几何学、运动学特征的不同及隆升速率与同构造生长地层沉积速率的比值在空间上的差异,同挤压地层层序在不同构造部位也表现出不同的沉积特点,其构造特征与造山带十分相似。乌尔逊-贝尔凹陷中两次挤压构造的发现,说明中国东北地区的地壳厚度在早白垩世并非一直处于伸展减薄状态,而是在上述两次挤压时期地壳处于叠置增厚的特点,在空间上形成挤压坳陷型盆地和陆内(板内)造山带相间分布的构造格局。