Rift zone reorganization through flank instability in ocean island volcanoes: an example from Tenerife, Canary Islands

The relationship between rift zones and flank instability in ocean island volcanoes is often inferred but rarely documented. Our field data, aerial image analysis, and ⁴⁰Ar/³⁹Ar chronology from Anaga basaltic shield volcano on Tenerife, Canary Islands, support a rift zone—flank instability relationship. A single rift zone dominated the early stage of the Anaga edifice (~6–4.5 Ma). Destabilization of the northern sector led to partial seaward collapse at about ~4.5 Ma, resulting in a giant landslide. The remnant highly fractured northern flank is part of the destabilized sector. A curved rift zone developed within and around this unstable sector between 4.5 and 3.5 Ma. Induced by the dilatation of the curved rift, a further rift-arm developed to the south, generating a three-armed rift system. This evolutionary sequence is supported by elastic dislocation models that illustrate how a curved rift zone accelerates flank instability on one side of a rift, and facilitates dike intrusions on the opposite side. Our study demonstrates a feedback relationship between flank instability and intrusive development, a scenario probably common in ocean island volcanoes. We therefore propose that ocean island rift zones represent geologically unsteady structures that migrate and reorganize in response to volcano flank instability.Editorial responsibility: T. DruittThis revised version was published online in February 2005 with typographical corrections and a changed wording.     

The Cenozoic Lower Rhine Basin – rifting, sedimentation, and cyclic stratigraphy

In the Cenozoic, the Lower Rhine Basin formed as a rift at the southeastern terminus of the Dutch German Central Graben, while the Rhenish Massif was uplifted. The study focusses on the marginal marine and fluvial fill of the Lower Rhine Basin. A basin model is developed. Support for this study was given by extensive industry outcrop and well data, by new stratigraphical and sedimentological observations. The ingression and subsequent regression of the Cenozoic North Sea is analysed using the concept of base level cyclicity. As the geohistory of the basin was complex, a subsidence curve is constructed. Furthermore, an attempt is made to trace the simultaneous uplift of the Rhenish Massif.     

前陆背景下的两种正断裂现象及其相互作用模式

许多前陆盆地同时存在着下部的裂陷正断裂和上部的造山期的挠曲正断裂。作用在均质不分层和非均质分层板块中的挠曲正断裂具有不同的外观形态和形变的空间协调机制，前者断面陡直、贯穿深远；后者则呈坡坪式形态。裂陷正断裂和挠曲正断裂具有3种相互作用方式、差异甚殊的断裂总体形态和中性面分布位置不同的应力体制。Ⅰ类，挠曲正断裂局限在陆缘期或坳陷期层序中；Ⅱ类，裂陷期正断裂断至断陷期层序，且以裂陷正断裂为基础而存在，二者构成完整而独特的断裂组合；Ⅲ类，挠曲正断裂亦断穿至断陷期层序，但和裂陷正断裂并不存在直接关系。这三类关系对孔隙—裂缝网络体具有不同的影响，并和应力体制一道，对流体尤其是烃类的作用过程有差异很大的影响。地层在跨越同生正断裂和挠曲正断裂时所发生的厚度的变化，起因于二者在物源方向上的完全不同。     

Subsurface structure of the volcanic centre of the České Středohoří Mountains, North Bohemia, determined by geophysical surveys

Former geophysical surveys performed in the region of the volcanic centre of the České Stř edohoří Mts. in North Bohemia (the Ohře Rift zone) showed that anomalous volcanic bodies and features can be effectively identified within sedimentary environment. For this reason we carried out new geophysical measurements in the area of the main mafic intrusion of essexitic character. The target was the exact location and geometry of the intrusion and its relation to other components of the volcanic centre. We used gravity, magnetic, shallow seismic and electromagnetic techniques. The new gravity and magnetic data were tied to the old databases so that we could investigate the area as a whole complex. Electromagnetic measurements were applied in the area of the expected extent of the intrusion, and the seismic measurements in the central part of the intrusion. Based on all the data, mainly on gravity modelling, we delineated not only the surface and subsurface extent of the intrusion, but we also defined the hidden relief of the intrusion. It was found that the intrusion is formed by a single body that has a few protrusions, and not by a set of separate individual intrusions, as indicated by surface outcrops. However, the body of the intrusion is affected by a major fault that caused lithological differences on both sides (essexite/monzodiorite). In detail we show the depth of the debris cover and the thickness of the weathered zone in the central part of the essexite body. We also derived indications of tectonic elements in the area of the intrusion in the main structural/tectonic direction in the region. The results will be utilized to establish a 3D geological model of the whole volcanic centre. This investigation may serve as an example of non-seismic geophysical exploration applied to the study of volcanic centres surrounded by sedimentary rocks.     

攀西裂谷构造─岩浆活动及其成因探讨

本文主要讨论弯窿一火山型攀西裂谷的成因，岩浆深成作用与火山作用过程;弩窿梅 造的发生、发展的演化历史，岩浆分异趋势及双峰式岩浆演化系列及其成因。 攀西裂谷曾经历了岩石圈弯窿一除壳弯窿一次火山弯窿三个发展演化阶段。碱性岩浆作用与 地壳隆开、地慢去气、热流汇聚作用有着密切的成因联系，成弯作用最盛，岩浆碱度最高。随着 陆壳破裂、开放、挥发分散逸，岩浆性质从强碱质一弱碱质一碱酸性转化。 弯窿构造的发展演化阶段有机地控制了岩浆源和二次岩浆房的深度和岩浆演化特点，随着弯 窿构造的发展演化，岩浆活动由深成慢源→中浅成慢源加嵌轻微混染→超浅成壳慢混合源逐渐 演化，因而可以认为:弯窿一火山型裂谷发育的各个阶段，存在有低位→中位→高位的二次岩浆 房. 攀西裂谷属不发育的夭折裂谷，以演化时间长为特点，有利于岩浆深源(二次岩浆房内)结 晶分异、液体不混容性分离作用和陆壳的同化混染作用等得以彻底进行，最终形成“双峰式”岩 浆组合。     

Neotectonics and extension direction of the Southern Kenya Rift, Lake Magadi area

The Magadi area, located in the southern part of the Kenya Rift, is a seismically active region where rifting is still in progress. The recent tectonic activity has been investigated through a seismological survey and the study of neotectonic joints found in Lake Magadi sediments, which were deposited some 5000 years ago. The structural analysis of these open fractures was combined with a quantitative analysis of the orientation and size characteristics of imagery faults. The gathered data demonstrate (1) that the majority of the systematic joints have straight and parallel trajectories with a common en echelon mode of propagation displayed through a rich variety of patterns, and (2) that there is a self-similarity in fault and joint principal directions recognised at the different telescopic scales. SPOT image (1:125,000), aerial photos (1:76,000), and outcrop fieldwork reveal two important structural orientations which are N015°E and N015°W. The N015°E regional direction is consistent with the orientation of the southern segment of the Kenya Rift. Structural analysis is supported by results of a joint microseismic investigation in the Lake Magadi area. Obtained focal mechanism solutions indicate an E–W to ESE–WNW normal faulting extension direction.     

Dynamic causes for the opening of the Baikal Rift Zone: a numerical modelling approach

Previous dynamic models of the Baikal Rift Zone (BRZ) are mostly two-dimensional on vertical plane. In this study, a numerical model of neotectonics in the region on map view was constructed using the adapted PLATES program. The present work is an attempt to test different mechanisms for opening Baikal Rift by comparing the modelled and observed stress and strain rate fields. The following rifting scenarios were tested: (1) pure northwest–southeast extension, (2) pure northeast–southwest compression, (3) oblique rift opening and (4) combined northwest–southeast extension and northeast–southwest compression. The models are calibrated using geologically and GPS-derived strain rates and stress-tensor determinations from fault-slip data and earthquake focal mechanisms. The most successful model requires a combination of NE–SW compression and orthogonal extension. The model results indicate that the present extensional regime in BRZ can be explained by combining the India plate indentation northward into Eurasia, east–west convergence between the North America and Eurasia plates and southeastward extrusion of the Amur plate in northeastern Asia. Predicted fault-slip rates for the best-fit model are consistent with the observed Holocene fault-slip rates in the Lake Baikal region. The generally accepted rotation of the Amur and Mongolia microplates are used as independent constraints for the choice of the best-fit model. These data correlate well with the predicted direction of rotation in our best model.     

Analytic models for orogenic collapse

The temporal and spatial distribution of the aftershock sequences of the Ruwenzori (February 5, 1994, Mb (5.8)), Masisi (April 29, 1995, Mb (5.1)) and Kalehe (October 24, 2002, Mb (5.9)) earthquakes have been studied. It has been found that most of the aftershocks of the Ruwenzori earthquake are located on the eastern flank of the main escarpment and those of the Masisi earthquake are confined to the northwest of Lake Kivu margin where earthquake occurrence of swarm-type was normally observed. The Kalehe earthquake occurred in the central part of Lake Kivu and it was the largest earthquake observed in the Lake Kivu basin since 1900. The rate of decrease in aftershock activity with the time has shown that the p-value for Ruwenzori and Masisi earthquake equals 0.6, somehow smaller than that found in other geotectonic zones where p is close to 1. The p-value of the Kalehe earthquake is a normal value equal to 1.From an area delimited by spatial distribution of aftershocks, the linear dimension of the fault was estimated. The fault area determined in this study correlates well with those of previous studies which occurred in the Western Rift Valley of Africa including the Tanganyika and Upemba Rift.     

华北克拉通中元古代裂解事件:以渣尔泰-白云鄂博-化德裂谷带岩浆与沉积作用研究为例

华北克拉通存在三个主要的中元古代裂谷带,从南到北分别为熊耳裂谷带、燕辽裂谷带以及渣尔泰-白云鄂博-化德裂谷带。其中熊耳群中火山岩的峰期年龄为1780~1750Ma,其上还有形成于被动大陆边缘的五佛山群、汝阳群以及官道口群。中北部的燕辽裂谷带包括长城系、蓟县系和青白口系,其中长城系团山子组和大红峪组火山岩的年龄分别为~1640Ma和1626~1622Ma,蓟县系高于庄组、雾迷山组和铁岭组凝灰岩的年龄分别为1560Ma、1485Ma和1437Ma,而下马岭组凝灰岩年龄为1366~1380Ma。北缘渣尔泰-白云鄂博-化德裂谷带中渣尔泰群书记沟组玄武岩年龄为1743Ma,阿古鲁沟组酸性火山岩年龄为~810Ma,白云鄂博群尖山组中基性火山岩年龄为1728Ma,化德群比鲁特组火山碎屑岩年龄为1515Ma。中元古代岩浆事件除了裂谷带中的火山作用外,还包括三期基性岩墙群(~1780Ma太行-吕梁岩墙群、~1730Ma密云岩墙群和~1620Ma泰山岩墙群)以及1.76Ga到1.65Ga非造山岩浆组合(斜长岩-环斑花岗岩体-A型花岗岩)。中元古代中期,华北克拉通北缘发育了基性岩席(墙)、A型花岗岩以及碳酸岩脉,双峰式岩浆作用说明华北北缘在中元古代中期经历了裂谷作用,与哥伦比亚超大陆的最终裂解有关,并且与白云鄂博巨型REE-Nb-Fe矿床的形成具有成因上的联系。华北克拉通北部两个裂谷带中的地层具有可以对比的层序以及时代,而中元古代中期辉绿岩墙、A型花岗岩以及碳酸岩脉可以与其它克拉通同时期的非造山岩浆作用对比,证明华北克拉通经历了哥伦比亚超大陆的最终裂解。古地磁数据已经证明在哥伦比亚超大陆时期Siberia、Laurentia、Baltica、Amazion以及华北克拉通是连接在一起的,而北缘中元古代中期大陆裂谷相关岩浆岩的发现也说明它是与另一个古大陆相连的。华北克拉通南缘熊耳火山岩的构造背景到底是大陆裂谷还是大陆边缘弧则关系着其是与另一个克拉通相连还是面向大海,这需要我们进一步深入研究。     

东川稀矿山式铁铜矿控矿条件新认识

落因背斜两翼地层构造、地层岩相、含矿的差异性,证明背斜轴部存在生长断层。凡有长断层出现,因民组、落雪组岩相剧变,伴随碱性火山喷发-喷溢活动形成细碧岩、基性熔岩、角砾岩之处成为成矿必要条件。