Progression of deformation and sedimentation in the southernmost Andes

The chronology of thrust motion in the Fuegian thin-skinned fold-thrust belt was established using data from the Atlantic coast of Tierra del Fuego. A set of original structural–geological maps showing the distribution of structures, unconformities and synorogenic sequences in the last tip of the Andes reveals the cratonward propagation of thrusts and sedimentary depocenters. A succession of syntectonic angular and progressive unconformities occur in the studied zone: (1) an angular unconformity between Danian and Late Paleocene sequences, (2) a series of progressive and syntectonic angular unconformities developed from the Late Early Eocene to the Late Eocene, and (3) a Lower Miocene syntectonic unconformity. Additional evidence for the time–space location of the thrust-front is provided by the presence of seismically triggered sand intrusions in Late Cretaceous, Late Paleocene and Middle Miocene sequences.The integration of data shows that faulting occurred in three main episodes: San Vicente thrusting, ca. 61–55 Ma, Río Bueno thrusting, ca. 49–34, and Punta Gruesa strike-slip event, ca. 24–16 Ma. San Vicente thrusting represents the onset of thrust propagation onto the foreland craton. The thrust-front endured a major cratonward migration through the Río Bueno thrusting, and remained steady afterward. Punta Gruesa constitutes a strike-slip event, associated with the phase of wrench deformation that influences the southernmost Andes since the Oligocene. Although the overall pattern of faulting was progressively younger cratonward, several episodes of out-of-sequence thrusting and folding occurred.Other features in the southernmost Andes can be linked to these three deformation events to broadly characterize the behavior of the Fuegian orogenic wedge in terms of critical taper models. The Fuegian Andes underwent at least three cycles between subcritical, critical and supercritical stages of behavior in terms of deformation, erosion, and sedimentation.     

苗儿山地区中生代酸性脉岩地球化学特征及其成因

苗儿山地区中生代酸性脉岩主要为细粒白云母花岗岩和花岗斑岩,具有较高的Al2O3、K2O、Na2O含量和较低的TiO2、MgO含量,微量元素以富集Rb、Th、U、Ta而亏损Ba、Sr为特征,稀土元素总量偏低,轻稀土富集,Eu亏损强烈。在CaO/Na2O-Al2O3/TiO2和Rb/Sr-Rb/Ba图解上,酸性脉岩具有上地壳富黏土质泥岩的特征。这些特征与豆乍山、香草坪等岩体的地球化学特征相似,表明酸性脉岩与岩体的物质来源相同,即早期酸性花岗岩体的母岩浆滞留于地下深部,经一定程度分离结晶和固结后,晚期熔体沿花岗岩体内外接触带及断裂构造上升形成脉岩。强烈的分离结晶作用可以导致铀等成矿元素在残余熔体中的高度富集,为铀矿床的形成提供必要的物质基础。     

确定大江大河防洪堤高度应考虑地面沉降因素——对1998中国大洪水的反思

世界上抽汲地下水或石油引起地面沉降正越来越普遍。大江大河防洪能力降低是地面沉降灾害现象之一。上海因地面沉降产生严重的洪涝灾害,上海外滩防汛墙高度的主要决定因素是地面沉降。沙市、武汉、哈尔滨和大庆等城市和地区是我国1998年夏特大洪水的重灾地区,这些城市和地区实际上已存在地面沉降问题。几十年来产生的地面沉降很可能已降低了这些地区江河大堤的防洪能力,从而加重了这次洪涝灾害。建议在确定大江大河防洪堤高度应考虑地面沉降因素,并尽快对重点城市和地区进行地面沉降调查与预测研究。     

Petrology and Sr–Nd characteristics of the Nova Lacerda dike swarm,SW Amazonian Craton: new insights regarding its subcontinental mantle source and Mesoproterozoic geodynamics

The NNW-trending Nova Lacerda tholeiitic dike swarm in Mato Grosso State, Central Brazil, intrudes the Nova Lacerda granite (1.46 Ga) and the Jauru granite-greenstone terrain (ca. 1.79–1.77 Ga). The swarm comprises diabases I and II and amphibolites emplaced at ca. 1.38 Ga. Geochemical data indicate that these are evolved tholeiites characterized by high LILE/HSFE and LREE/HSFE ratios. Isotopic modelling yields positive ?_Nd(T) values (+0.86 to?+2.65), whereas values for ?_Sr(T) range from positive to negative (+1.96 to -5.56). Crustal contamination did not play a significant petrogenetic role, as indicated by a comparison of isotopic data (Sr–Nd) from both dikes and country rocks, and by the relationship between isotopic and geochemical parameters (SiO₂, K₂O, Rb/Sr, and La/Yb) of the dikes. We attribute the origin of these tholeiites to fractional crystallization of evolved melts derived from a heterogeneous mantle source. Comparison of the geochemical and isotopic data of the studied swarm and other tholeiitic Mesoproterozoic mafic intrusions of the SW Amazonian Craton – the Serra da Providência, Colorado, and Nova Brasilândia bimodal suites – indicates that parental melts of the Nova Lacerda swarm were derived from the most enriched mantle source. This enrichment was probably caused by the stronger influence of the EMI component on the DMM end-member. These data, coupled with trace element bulk-rock geochemistry of the country rocks, and comparisons with the Colorado Complex of similar age, suggest a continental-margin arc setting for the emplacement of the Nova Lacerda dikes.     

Geochemical,Sr–Nd–Pb isotope,and zircon U–Pb geochronological constraints on the origin of Early Permian mafic dikes,northern North China Craton

Dolerite dike swarms are widespread across the North China Craton (NCC) of Hebei Province (China) and Inner Mongolia. Here, we report new geochemical, Sr–Nd–Pb isotope, and U–Pb zircon ages for representative samples of these dikes. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U–Pb analysis yielded consistent Permian ages of 274.8 ± 2.9 and 275.0 ± 4.5 Ma for zircons extracted from two dikes. The dolerites have highly variable compositions (SiO₂ = 46.99–56.18 wt.%, TiO₂ = 1.27–2.39 wt.%, Al₂O₃ = 14.42–16.20 wt.%, MgO = 5.18–7.75 wt.%, Fe₂O₃ = 8.03–13.52 wt.%, CaO = 5.18–9.75 wt.%, Na₂O = 2.46–3.79 wt.%, K₂O = 0.26–2.35 wt.%, and P₂O₅ = 0.18–0.37 wt.%) and are light rare earth element (LREE) and large ion lithophile element (LILE, e.g. Rb, Ba, and K, and Pb in sample SXG1-9) enriched, and Th and high field strength element (HFSE, e.g. Nb and Ta in sample SXG1-9, and Ti) depleted. The mafic dikes have relatively uniform (⁸⁷Sr/⁸⁶Sr)_i values from 0.7031 to 0.7048, (²⁰⁶Pb/²⁰⁴Pb)_i from 17.77 to 17.976, (²⁰⁷Pb/²⁰⁴Pb)_i from 15.50 to 15.52, (²⁰⁸Pb/²⁰⁴Pb)_i from 37.95 to 38.03, and positive ?_Nd(t) (3.6–7.3), and variable neodymium model ages (T_DM1 = 0.75–0.99 Ga, T_DM2 = 0.34–0.74 Ga). These data suggest that the dike magmas were derived from partial melting of a depleted region of the asthenospheric mantle, and that they fractionated olivine, pyroxene, plagioclase, K-feldspar, and Ti-bearing phases without undergoing significant crustal contamination. These mafic dikes within the NCC formed during a period of crustal thinning in response to extension after Permian collision between the NCC and the Siberian Block.     

赣杭构造带金衢盆地燕山期基性脉岩地球化学特征及成因探讨

赣杭构造带金衢盆地内发育有燕山期基性脉岩,岩石类型主要为辉绿(玢)岩.K-Ar年代学研究显示基性脉岩产出时代为69.5～131.7Ma,为白垩纪岩浆活动的产物.SiO2含量范围为46.70%～50.23%,K2O+Na2O为4.01%～7.82%,可分为碱性和亚碱性岩两类.微量元素相对富集大离子亲石元素(LILE)(K...     

Hydraulic behavior of tsunami backflows: insights from their modern and ancient deposits

Tsunamis are unpredictable, catastrophic events, and so present enormous difficulties for direct studies in the field or laboratory. However, their sedimentary deposits yield evidence of a wide variety of hydrodynamic conditions caused by flow transformations on a spatial and temporal scale. Tsunami deposits ranging from the Miocene to modern times identified at different localities along the Chilean coast are described to provide a database of their characteristics. Among the typical features associated with tsunami deposits are well-rounded megaclasts eroded from coastal alluvial fans or beaches by very dense, competent flows. Sand injections from the base of these flows into the substrate indicate very high dynamic pressures, whereas basal shear carpets suggest hyperconcentrated, highly sheared flows. Turbulence develops in front of advancing debris flows, as indicated by megaflutes at the base of scoured channels.     

Origin of 1.8 Ga zircons in Post Eocene mafic dikes in the Roshtkhar area,NE Iran

The Roshtkhar area is located in the Khaf-Kashmar-Bardaskan volcano-plutonic belt to the northeastern Iran along the regional E–W trending Dorouneh Fault, northeastern of the Lut Block. There are several outcrops of subvolcanic rocks occurring mainly as dikes in the area, which intruded into Cenozoic intrusive rocks. We present U–Pb dating of zircons from a diabase dike and syenite rock using LA-ICP-MS that yielded an age of 1778 ± 10 Ma for the dike, indicating this Cenozoic dike has zircon xenocrysts inherited from deeper sources; and 38.0 ± 0.5 Ma, indicating an Late Eocene crystallization age for the syenite. Geochemically, the dikes typical of high-K calc-alkaline to shoshonitic magmas. Petrographic observations and major and trace element variations suggest that diabase melts underwent variable fractionation of clinopyroxene, olivine, and Fe-Ti oxides and minor crustal contamination during the differentiation process. Primitive mantle-normalized multi-element diagrams display enrichment in LILE, such as Rb, Ba, Th, U, and Sr compared to HFSE, as well as negative anomalies of Nb, Ta, P, and Ti, suggesting derivation from subduction-modified mantle. Chondrite-normalized REE plots show moderately LREE enriched patterns (<3.83 La_N/Yb_N <8.27), and no significant Eu anomalies. Geochemical modelling using Sm/Yb versus La/Yb and La/Sm ratios suggests a low-degree of batch melting (~1–3%) of a phlogopite-spinel peridotite source to generate the mafic dikes. The geochemical signatures suggest that the Roshtkhar mafic dikes cannot be related directly to subduction and likely resulted from melting of upper mantle in an extensional setting where the heat flow was provided from deeper levels. These dikes presumably derived the zircon xenocrysts from the assimilation of upper crust of Gondwanian basement. Processes responsible for partial melting of metasomatized lithospheric mantle and post-collision magmatism in NE Iran was triggered by heating due to asthenospheric upwelling in an extensional setting.     

新疆哈尔里克山石城子地区钠铁闪石花岗斑岩岩墙的发现及构造背景示踪

在新疆哈尔里克山石城子地区新发现了一套侵位于晚古生代后碰撞二长花岗岩中的钠铁闪石花岗斑岩岩墙群。该类岩墙具斑状,斑晶以条纹长石和反条纹长石为主,基质矿物组成为正长石、钠长石、石英、钠铁闪石和霓石,为钠铁闪石花岗斑岩。主量元素特征显示为高硅、高碱,低铝、低镁和低钙;稀土元素总量较高,轻重稀土分馏明显,显著富集轻稀土元素;稀土元素配分模式曲线呈明显右倾的"V"型,Eu亏损强烈;微量元素特征显示为高10~4 Ga/Al、Zr和Zr+Nb+Ce+Y,强烈亏损Ba、Sr、P、Ti,富集Th、U、Zr和Hf;岩石学、地球化学特征显示其为碱性A型花岗岩。正ε_(Nd)(t)(4.13~4.75)和年轻的Nd模式年龄(t_(DM2)),指示其源区为亏损地幔来源的新生陆壳物质。LA-ICP-MS锆石U-Pb年龄结果显示岩墙的结晶年龄为晚二叠世(259.7±4.5Ma),围岩中捕获的老锆石年龄集中在2168~2635Ma和1677~1973Ma两个峰期,说明哈尔里克山地区可能存在前寒武纪古老基底,但已被新生陆壳物质改造。结合区域地质背景认为,哈尔里克山地区晚石炭世-早二叠世,为后碰撞伸展阶段;晚二叠世,进入陆内伸展阶段。     

Structural geology of the Mesozoic Miers Bluff Formation and crosscutting Paleogene dikes (Livingston Island, South Shetland Islands, Antarctica) – Insights into the geodynamic history of the northern Antarctic Peninsula

The Antarctic Peninsula has been part of a magmatic arc since at least Jurassic times. The South Shetland Islands archipelago forms part of this arc, but it was separated from the Peninsula following the Pliocene opening of the Bransfield Strait. Dikes are widespread throughout the archipelago and are particularly accessible on the Hurd Peninsula of Livingston Island. The host rocks for the dikes are represented by the Miers Bluff Formation, which forms the overturned limb of a large-scale fold oriented 63/23 NW. The orientation of minor structures indicates a fold axis oriented NNE–SSW (24/0). Structural analysis of the dikes and their host rocks shows that the tectonic regime was similar to other parts of the archipelago and that only minor changes of the stress field occurred during dike emplacement.Based on crosscutting field relationships and geochemical data, six early Paleocene to late Eocene intrusive events can be distinguished on Hurd Peninsula. In contrast to calc-alkaline dikes from other parts of the South Shetland Islands, the majority of the Hurd Peninsula dikes are of tholeiitic affinity. Nd and Pb isotope data indicate a significant crustal component, particularly during initial magmatic activity.Plagioclase ⁴⁰Ar/³⁹Ar and whole rock K–Ar ages show that dike emplacement peaked during the Lutetian (48.3 ± 1.5, 47.4 ± 2.1, 44.5 ± 1.8 and 43.3 ± 1.7 Ma) on Hurd Peninsula and also further northeast on King George Island. Dike intrusion continued on Livingston Island at least until the Priabonian (37.2 ± 0.9 Ma). The type of magma sources (mantle, slab, crust and sediment) did not change, though their relative magmatic contributions varied with time.During Cretaceous and Early Paleogene times, the Antarctic Peninsula including the South Shetland Islands was situated southwest of Patagonia; final separation from South America occurred not before the Eocene. Thus, the geological evolution of Livingston Island is related as much to the development of Patagonia as of Antarctica, and needs to be considered within the history of southernmost South America.