Tectonic geomorphology of the easternmost extension of the Gulf of Corinth (Beotia, Central Greece)

The easternmost sector of the Gulf of Corinth, the Beotia area in Central Greece, is an area with active normal faults located between the two major rift structures of Central Greece, the Gulf of Corinth and the North Gulf of Evia. These active normal faults include WNW to E–W and NE to ENE-trending faults affect the landscape and generate basin and range topography within the Beotia. We study four normal fault zones and drainage basin geometry in the easternmost sector of the Gulf of Corinth to document the impact of active tectonics on the landscape evolution. Fault and drainage geometry are investigated based on detailed field mapping and high-resolution digital elevation models. Tectonic geomorphic analysis using several parameters of active tectonics provides information concerning the relative tectonic activity and fault growth. In order to detect areas of lateral stream migration that could indicate recent tectonic activity, the Transverse Topographic Symmetry Factor and the Asymmetry Factor are used to analyse drainage basin geometry in six large drainage basins and a drainage domain covering the study area. Our results show that vertical motions and tilting associated with normal faulting influence the drainage geometry and its development. Values of stream-gradient indices (S_L) are relatively high close to the fault traces of the studied fault zones suggesting high activity. Mountain-front sinuosity (S_mf) mean values along the fault zones ranges from 1.08 to 1.26. Valley floor width to valley height ratios (V_f) mean values along the studied fault zones range between 0.5 and 1.6. Drainage basin shape (B_S) mean values along the fault zones range from 1.08 to 3.54. All these geomorphic parameters and geomorphological data suggest that the analyzed normal faults are highly active. Lateral fault growth was likely produced by primarily eastward propagation, with the WNW to E–W trending faults being the relatively more active structures.     

Spatial Variation in Diameter Structures of Forests in Lassen Volcanic National Park,California*

Variation in seedling/sapling densities and stand diameter forms for six coniferous tree species is related to stand structural development and to elevation and topography in Lassen Volcanic National Park, California. Understory density patterns reflect differences in species tolerance; densities decrease with stand development for shade intolerant pines, but increase for shade tolerant firs and mountain hemlock. Pine species exhibit reverse-J diamter structures on recently disturbed sites, and decreaser and random forms elsewhere. More tolerant fir species show topographically mediated patterns of diameter structure, with reverse-J form common on northerly exposures and upland sites, but with decreaser and random forms on more xeric slopes. Interaction among species tolerance, environmental setting, and disturbance history yields a complex mosaic of stand diameter structures in the Lassen landscape.     

What do fault patterns reveal about the latest phase of extension within the Northern Snake Range metamorphic core complex,Nevada, USA?

The Northern Snake Range is a classic example of a metamorphic core complex, Basin-and-Range province, United States. It is composed of a plastically deformed footwall and a brittlely deformed hanging wall, separated by the Northern Snake Range low-angle detachment (NSRD). Brittle deformation, however, is not confined to the hanging wall.This paper focuses on exposures in Cove Canyon, located on the SE flank of the Northern Snake Range, where penetrative, homogeneous faults are well exposed throughout the hanging wall, footwall and NSRD, and overprint early plastic deformation. These late-stage fault sets assisted Eocene-Miocene extension. Detailed analysis of the faults reveals the following: (1) The shortening direction defined by faults is similar to the shortening direction defined by the stretching lineation in the footwall mylonites, indicating that the extensional kinematic history remained unchanged as the rocks were uplifted into the elastico-frictional regime. (2) After ∼17 Ma, extension may have continued entirely within elastic-frictional regime via cataclastic flow. (3) This latest deformation phase may have been accommodated by a single, continuous event. (3) Faults within NSRD boudins indicate that deformation within the detachment zone was non-coaxial during the latest phase of extension.     

小兴安岭“晚古生代”地层的时代与物源：地质与碎屑锆石 U-Pb 年代学证据

本文对出露于小兴安岭的"晚古生代"红山组和黑龙宫组进行了碎屑锆石LA-ICP-MS U-Pb定年,旨在准确限定红山组和黑龙宫组的沉积时限,并揭示其物源组成。样品中大多数锆石呈自形—半自形,显示典型的岩浆生长环带或条痕状吸收,暗示其岩浆成因。研究结果显示,采自红山组标准剖面泥板岩中的碎屑锆石42个分析点产生以下年龄峰值:747、807、849、903、956、1 167和1 811 Ma,表明红山组沉积于747 Ma之后;采自伊春地区黑龙宫组泥板岩中的碎屑锆石97个分析点产生以下年龄峰值:700(发生Pb丢失)、805、902、1 764、2 446和2 467Ma,确定黑龙宫组沉积于805Ma之后。近年来在该地区"晚古生代"地层中碎屑锆石的定年结果显示普遍存在561 Ma年龄,鉴于红山组和黑龙宫组中缺乏上述锆石年龄组合,认为研究区的红山组和黑龙宫组的形成时代分别为747~561 Ma和805~561 Ma,时代置于新元古代。基于两组碎屑锆石的年龄频数和区域地质年代学资料的对比分析,两个地层单元中出现大量新元古代岩浆锆石,证明研究区可能存在新元古代岩浆事件,岩浆产物为两组地层提供物源;而中—古元古代碎屑锆石的存在,同时暗示该区沉积时地表或地表浅部应存在更为古老的前寒武纪残片。     

内蒙古大兴安岭地区火山岩型铅锌矿矿产预测

内蒙古自治区已发现的火山岩型铅锌矿(陆相)规模较大,集中分布于大兴安岭地区,与中晚侏罗世火山活动联系密切,已知矿床的深部及外围找矿潜力巨大。依据前人的地质、矿产、物探、化探等资料,采用"火山岩型"预测方法类型,分别以比利亚谷铅锌矿、甲乌拉铅锌矿和扎木钦铅锌矿作为典型矿床,在全区选取比利亚谷、甲乌拉和扎木钦3个预测工作区进行矿产预测。圈定铅锌矿最小预测区109个,其中A级最小预测区18个,B级最小预测区37个,C级最小预测区54个。预测334-1级别Pb+Zn总资源量1732548t,334-2级别Pb+Zn总资源量588930t,334-3级别Pb+Zn总资源量4608682t。可利用的Pb+Zn总资源量为3179267t。反映内蒙古大兴安岭地区中生代火山岩型铅锌矿(陆相)找矿潜力较大。     

Dynamic interactions between crustal shortening,extension, and magmatism in the North American Cordillera

This paper examines the first-order dynamic interactions between crustal shortening, extension, and volcanism in tectonic evolution in the North American Cordillera. The protracted crustal compression in the Mesozoic and early Cenozoic (110–55 Ma) contributed to the subsequent Tertiary extension by thermally weakening the lithosphere and producing an overthickened (>50 km) and gravitationally unstable crust. In addition to post-kinematic burial heating, synkinematic thermal processes including conduction are shown significantly because of the long period of crustal contraction and the slow shortening rates (<4 mm/yr). The effects of shear heating were probably limited for the same reasons. Localized delamination of the lithospheric mantle may have contributed to the abundant plutonism and high crustal temperature in the southeastern Canadian Cordillera at the end of the orogeny. Most early-stage extension in the Cordillera, characterized by formation of metamorphic core complexes, resulted from gravitational collapse of the overthickened crust. Plutionism may have facilitated strain localization, causing widespread crustal extension at relatively low stress levels. Crustal collapse, however, was unlikely the direct cause of the Basin-Range extension, because the gravitational stresses induced by crustal thickening are limited to the crust; only a small fraction of the gravitational stresses may be transmitted to the lithospheric mantle. Nor could core complex formation induce the voluminous mid-Tertiary volcanism, which requires major upwelling of the asthenosphere. While the causes of the asthenospheric upwelling are not clear, such processes could provide the necessary conditions for the Basin-Range extension: the driving force from thermally induced gravitational potential and a thermally weakened lithosphere. The complicated spatial and temporal patterns of volcanism and extension in the Basin and Range province may be partially due to the time-dependent competing effects of thermal weakening and rheological hardening associated with intrusion and underplating of mantle-derived magmas.     

Pan-African Magmatism, and Sedimentation in the NW Himalaya

Correlation of early Palaeozoic, Pan-African (500 ± 50 Ma) granites that intruded the Chail, Salkhala, Haimanta Formations in the Lesser Himalaya, Zanskar crystallines, and Lower Taglang La of Tso-Morari crystallines in the northwestern Himalaya, is based on the field relationship, tectonic setting, mineralogical, and geochemical characteristics, and isotope dating of the granites. These granite plutons exhibit identical petrographical, and geochemical character. The mineralogical composition of the granites is quite similar, consisting of quartz, K-feldspar, plagioclase feldspar, biotite, muscovite, garnet, tourmaline, ± cordierite, andalusite, and sillimanite fibrolite. The granite which are massive, and inequigranular in the core of the plutons, show strongly foliated character indicating development of ductile shear zone at the margins. These are peraluminous S-type granites having high A/CNK value (> 1). Presence of normative corundum, rounded shape of zircon, and high initial Sr ratio suggest crustal source of the granites. Mantle normalized spider-diagram exhibits similar characters for all these granitoids. The intrusion of the Pan-African granites mark an abrupt end of the sedimentation that continued virtually uninterrupted from Palaeoproterozoic. The sudden break in sedimentation towards the terminal phases of the Lower Cambrian has been observed in almost all parts in Lesser as well as the Tethys Himalaya. Occurrences of large number of plutons along different tectonic belts of northwestern Himalaya are indicative of widespread tectono-thermal event during early Palaeozoic (500 ± 50 Ma). The bracketing of the two features like, the break in sedimentation during post-Late Cambrian, and the intrusion of granites around 500 ± 50 Ma, is considered to be the result of a strong diastrophic orogenic event correlatable to the late phases of the Pan-African Orogeny in Africa.     

Recognition of Early Paleozoic Magmatisms in the Supposed Proterozoic Basements of Zhalantun, Great Xing’an Range, NE China

The Zhalantun terrane from the Xing'an massif, northeast China, was used to be considered as Proterozoic basements. However, amounts of detrital zircon ages from the meta-sedimentary rocks deny the existence of Precambrian basements recently. Notably, magmatic rocks were barely reported to limit the exact ages of the Zhalantun basements. In this study, we collected rhyolite, gabbro and quartz diorite for zircon in-situ U-Pb isotopic dating, which yield crystallization ages of ～505 Ma, ～447 Ma and ～125 Ma, respectively. Muscovite schist and siltstone define maximum depositional ages of ～499 Ma and ～489 Ma, respectively. Additionally, these dated supracrustal rocks and plutons also yield ancient detrital/xenocryst zircon ages of ～600–1000 Ma, ～1600–2220 Ma, ～2400 Ma, ～2600–2860 Ma. Based on the whole-rock major and trace element compositions, the ～505 Ma rhyolites display high SiO_2 and alkaline contents, low Fe_2O_3T, TiO_2 and Al_2O_3, and relatively high Mg O and Mg#, which exhibit calc-alkaline characteristics. These rhyolites yield fractionated REE patterns and negative Nb, Ta, Ti, Sr, P and Eu anomalies and positive Zr anomalies. The geochemistry, petrology and Lu-Hf isotopes imply that rhyolites were derived from the partial melting of continental basalt induced by upwelling of sub-arc mantle magmas, and then experienced fractional crystallization of plagioclase, which points to a continental arc regime. The ～447 Ma gabbros exhibit low Si O2 and alkaline contents, high Fe2 O3 T, Ti O2, Mg O and Mg#. They show minor depletions of La and Ce, flat MREE and HREE patterns, and negative Nb, Ta, Zr and Hf anomalies. Both sub-arc mantle and N-MORB-like mantle were involved in the formation of the gabbros, indicative of a probable back-arc basin tectonic setting. Given that, the previously believed Proterozoic supracrustal rocks and several plutons from the Zhalantun Precambrian basements were proved to be Paleozoic to Mesozoic rocks, among which these Paleozoic magmatic rocks were generally related to subduction regime. So far, none Proterozoic rocks have been identified from the Zhalantun Precambrian basement, though some ～600–3210 Ma ancient detrital/xenocryst zircons were reported. Combined with ancient zircon ages and newly reported ～2.5 Ga and ～1.8 Ga granites from the south of the Zhalantun, therefore, the Precambrian rocks probably once exposed in the Zhalantun while they were re-worked and consumed during later long tectonic evolutionary history, resulting in absence of Precambrian rocks in the Zhalantun.     

大兴安岭北部岔路口斑岩钼矿床岩浆岩锆石U Pb年龄及其地质意义

黑龙江省岔路口超大型斑岩钼矿床位于大兴安岭北部,是目前我国东北地区最大的钼矿床,矿体赋存于中酸性杂岩体及侏罗系火山-沉积岩内,其中花岗斑岩、石英斑岩、细粒花岗岩与钼矿化关系密切.本文采用LA-ICP-MS锆石U-Pb定年方法,获得了矿区内二长花岗岩、花岗斑岩、石英斑岩、细粒花岗岩、流纹斑岩、闪长玢岩及安山斑岩的结晶年龄分别为162±1.6 Ma、149±4.6 Ma、148±1.6 Ma、148±1.2 Ma、137±3.3 Ma、133±1.7Ma和132±1.6 Ma.岔路口矿区内至少存在3期岩浆活动,其顺序为侏罗纪火山-沉积岩、二长花岗岩→晚侏罗世花岗斑岩、石英斑岩、细粒花岗岩→早白垩世流纹斑岩、闪长玢岩、安山斑岩.岔路口矿床成矿时代为晚侏罗世,是东北亚大陆内部构造-岩浆活化的产物,形成于古太平洋板块俯冲作用引起的挤压向伸展构造体制转折背景,与我国东部大规模钼矿化爆发期相对应.     

Petrogenesis and significance of early Yanshanian syenite-granite complex in eastern Nanling Range

According to the statistics for granitoid distribution map of 1/2000000 Nanling region[1], the granitoids in the Nanling region sum up an area of more than 170000 km2, occupying about one fifth of the entire Nanling region. Granitoid rocks in the Nanlingregion are mainly composed of monzogranites (occu- pying more than 84%), granodiorites (about 11%) and syenogranites (about 3%)[2]. There also exists a small amount of syenites (0.12%) with a sum area of about 94 km2[2]. However, nearly half …