25 million years to break a continent : early to middle Miocene rifting and syn-extensional magmatism in the southern Gulf of California

Cenozoic extension in western Mexico has been divided into two episodes separated by the change from convergence to oblique divergence at the plate boundary. The Gulf Extensional Province is thought to have started once subduction ended at ~12.5 Ma whereas early extension is classified as Basin and Range. Mid-Miocene volcanism of the Comondú group has been considered as a subduction-related arc, whereas post ~12.5 Ma volcanism would be extension-related. Our new integration of the continental onshore and offshore geology of the south-east Gulf region, backed by tens of Ar-Ar and U-Pb ages and geochemical studies, document an early-mid Miocene rifting and extension-related bimodal to andesitic magmatism prior to subduction termination. Between ~21 and 11 Ma a system of NNW-SSE high-angle extensional faults rifted the western side of the Sierra Madre Occidental (SMO) ignimbrite plateau. In Nayarit, rhyolitic domes and some basalts were emplaced along this extensional belt at 18-17 Ma. These rocks show strong antecrystic inheritance but an absence of Mesozoic and older xenocrysts, suggesting a genesis in the mid-upper crust triggered by extension-induced basaltic influx. In Sinaloa, large grabens were floored by huge dome complexes at ~21-17 Ma and filled by continental sediments with interlayered basalts dated at 15 Ma. Mid-Miocene volcanism, including the largely volcaniclastic Comondú strata in Baja California, was thus emplaced in rift basins and appears associated to decompression melting rather than subduction. Along the coast, flat-lying basaltic lava flows dated at 11-10 Ma are exposed just above the present sea level. Here crustal thickness is 25-20 Km, almost half that in the core of the SMO, implying significant lithosphere stretching before ~11 Ma. This mafic pulse, with relatively high Ti but still clear Nb-Ta negative spikes, may be related to the detachment of the lower part of the subducted slab, allowing asthenosphere to flow into parts of the mantle previously fluxed by subduction fluids. Very uniform OIB-like lavas appear in late Pliocene and Pleistocene, only 18 m.y. after the onset of rifting and ~9 m.y. after the end of subduction. Our study shows that rifting began much earlier than Late Miocene and progressively overwhelmed subduction in generating magmatism.

The structure and petrology of the Johnny Lyon Hills area, Cochise County, Arizona

The Johnny Lyon Hills area is located in Cochise County in southeastern Arizona. The rocks of the area include a central core of Lower pre-Cambrian igneous and metamorphic rocks surrounded by a complexly faulted and tilted section of Upper pre-Cambrian and Paleozoic strata. Limited exposures of Mesozoic and Tertiary sedimentary and volcanic rocks are present at the north end of the map area. Late Tertiary and Quaternary alluvium almost completely surrounds and overlaps upon the older rocks.

The older pre-Cambrian rocks include a section of more than 9000 feet of generally moderately metamorphosed graywackes, slates and conglomerates of the Pinal schist injected in zones by somewhat younger rnyolite sheets. The original sediments were deposited in a geosyncline whose extent probably included large parts of Arizona, New Mexico and west Texas. During the Mazatzal Revolution the Pinal schist was deformed into northeast-trending, steeply dipping and plunging structures and the entire local section was overturned steeply toward the northwest. The pre-Cambrian Johnny Lyon granodiorite was emplaced as a large epi-tectonic pluton which modified the metamorphic character of part of the Pinal schist. Larsen method determinations indicate an age of about 715 million years for this rock, which is about the minimum age compatible with the geologic relations.

The Laramide orogeny produced numerous major thrust faults in the area involving all rocks older than and including the Lower Cretaceous Bisbee group. Major compression from the southwest and subsequent superimposed thrusting from the southeast and east are indicated. Minimum thrust displacements of more than a mile are clear and the probable displacements are of much greater magnitude. The crystalline core behaved as a single structural unit and probably caused important local divergences from the regional pattern of northeast-trending compressive forces. The massif was rotated as a unit 40 degrees or more about a northwest-trending axis overturning the pre-Cambrian fold axes in the Pinal schist.

Swarms of Late Cretaceous(?) or Early Tertiary(?) lamprophyric dikes cross the Laramide structures and are probably related to the large Texas Canyon stock several miles southeast of the map area. Intermittent high angle faulting, both older and younger than the dikes, has continued since the Laramide orogeny and has been superimposed on the older structures. This steep faulting combined with the fundamental northwesterly Laramide structural grain to produce the northwesterly trends characteristic of the mountain ridges and valleys of the area.

渤海湾地区前第三系构造特征及其演化研究

Based on fine structural interpretation on seismic profiles of buried-hills in Huanghua depression, structural interpretation and balanced cross-section restoration of regional seismic profiles, drawing structural maps of main seismic interfaces, residual strata distribution of different ages in the Bohai Bay region and structural survey in the western Shandong uplifted area and the intracontinental orogeny of Yanshan mountain, the paper has studied pre-tertiary structural styles and tectonic evolution of the Bohai Bay region. There mainly develop 5 types of pre-tertiary structural style that are extension structure, compression structure, strike-slip structure, negative inversion structure and sliding structure in the Bohai Bay region. Among these 5 types of structural style, extension structure develops detachment fault and its controlling fault terrain structure and fault break slop; compression structure develops reverted fold, fault propagation fold, fault bent fold, imbricate thrust structure and triangle zone; strike-slip structure develops positive flower structure, negative flower structure, en-echelon structure and brush structure; negative reversion structure develops Indosinian compression and Yanshanian extension negative reversion structure, late Yanshanian compression and Cenozoic extension negative reversion structure; sliding structure develops interlayer sliding structure and detachment structure. According to Cangdong fault of SN direction, Zhangjiakou – Penglai fault and Qihe – Guangrao fault of NWW direction, the Bohai Bay region can be divided into 6 sub-regions in which structural direction and style is different from each other. Structural maps of bottom boundary of Cenozoic and upper Paleozoic manifest that main NNE structural direction is formed from late Yanshanian to Himalayan movement and minor NWW structural direction and a string of area more than 8000m are mainly suggest that Indosinian tectonic pattern strongly influence on Yanshanian and Himalayan movement. Residual strata distribution characteristics of middle to upper Neoproterozoic in the Bohai Bay region manifest that middle- to neo- aulacogen position may be corresponding to late Mesozoic uplifted zone. Residual Paleozoic distribution characteristics of main ENN suggest that structural alteration should be resulted from late Yanshanian to Himalayan movement while which of minor NWW structures suggest that deeper structure should restrict shallower structure. Structural patterns of main EW fold direction in the Bohai Bay region and thrust structure in eastern part are formed late Triassic in studied area. Granite magma intrusion of early to middle Jurassic mainly develops Yanshan mountain zone. Late Mesozoic rifting basins of NEE direction are widely distributed in the Bohai Bay region and granite magma intrusions are mainly distributed in Tancheng – Rongcheng zone. Mesozoic structural evolution in the Bohai Bay region is related to scissor convergent from east to west between North China plate and Yangtze plate and gradually reinforcing of the west circum-pacific tectonic tract while basin and range province of late Jurassic and early Cretaceous may be mainly related to lithospheric thinning of North China craton in late Mesozoic.

东北亚构造-盆地演化及油气资源控制作用

The oil and gas potential of Northeast Asia is enormous, but the degree of exploration is very low in Northeast Asia (the degree is below 3%-10%).The reasons are as follows: First, it is relatively difficult to study the oil and gas bearing basins(OGB), which are of multiple types, in different tectonic settings, with complex geologic frameworks and with long-term geologic evolution. Secondly, because of the non-equilibrium in development of economy and regional market, application of theories and techniques and the research levels in different countries, the conclusions are not conformable, and even contradictory. Thirdly, most of the former researches were limited to one territory or one basin, and lack of systematical and in-depth study on geotectonic evolution, classification of basins, and the evaluation of hydrocarbon resources. In this thesis, integrated study of the regional tectonic feature and basin features of Northeast Asia was done, to understand the basin evolution history and the controlling action on oil and gas. Then, new conclusions are and exploration proposals are as following: 1. Geotectonic evolution in Northeast Asia: The main structural motion system in Paleozoic Era was longitudinal, and in Meso-cenozoic was latitudinal with the Pacific Ocean. The whole evolution history was just the one of pulling-apart, cutting-out, underthrusting and collision of the Central Asia- Mongolia Ocean and the Pacific Ocean. 2. The evolution characteristics of basins in Northeast Asia: mainly developed from longitudinal paste-up, collision and relaxation rifting motion in Paleozoic-Early Mesozoic Era and from underthrust, accretion, and receding of subducted zone of the Pacific Ocean in Late Mesozoic Era-Cenozoic Era. 3. The research in basin classification of Northeast Asia: According to geotectonic system, the basins can be classified into three types: intracratonic, pericratonic and active zone basin. And they can be further classified into 18 different types according to genetic mechanism and dynamic features. 4. The master control factors of oil and gas accumulation in Northeast Asia: high quality cap-rock for craton and pericrationic basin, the effective source rock and high quality cap-rock for Mesozoic rifted basins, intra-arc, fore-arc and back-arc basins. Graded exploration potential of oil and gas for basin in Northeast Asia according to 7 factor, hereby, divided the oil and gas potential of basins into 5 levels. 5. Evaluation of hydrocarbon resources: The difference of resource potential among these basins is huge in Northeast Asia. The evaluation of Mesozoic rifted basin and Pacific Ocean basin showed that the large scale rifted basin and retroarc basin(including backarc marginal sea basin) have great resource potential. 6. The writer believes that the next step should pay more attention to the evaluation of petroleum resource in Far East part of Russia and trace them. On the other hand, according to integrated analysis of oil/gas resource potential and the operation difficulty in this area, suggests that East-Siberia basin, East-Gobi-Tamchag basin, Sakhalin basin, North-Okhotck basin, West-Kamchatka basin could be as cooperation priority basins in future.

黄海海域前新生代残留盆地综合地质地球物理研究

China’s annual oil import volume has been increasing in recent years, but the oil price in the international market fluctuates and poses a severe threat to China’s economic development and national security. Therefore, it is of great importance to study the gas and oil exploration of Pre-Cenozoic Residual Basins in Yellow Sea. Yellow Sea has widespread and thick Mesozoic and Paleozoic strata that contain multilayer source rock. Hence, Yellow Sea Mesozoic and Paleozoic strata have good conditions of forming Pre-Cenozoic hydrocarbon reservoirs. Pre-Cenozoic Residual Basins are usually buried deep and then transformed many times in its long evolutional history. These characteristics make it difficult to apply a single method in exploring Pre-Cenozoic Residual Basins. On the other hand, it is highly effective to solve key problems of gas and oil exploration of Pre-Cenozoic Residual Basins in Yellow Sea by using integrated geological and geophysical methods which make full use of the advantages of various exploring techniques. Based on the principle of “the region controls the local; the deep restricts the shallow,” this study focuses on Pre-Cenozoic Residual Basins in Yellow Sea to describe the structure frame of its distribution, with gravity, magnetic, seismic, drill-hole and geological data and previous research findings. In addition, the distribution characteristics of Pre-Cenozoic Residual Basins in Yellow Sea are also analyzed. This paper explores the characteristics of error between gravity forward with constant density and gravity forward with variable density through the study on 2-D and 3-D gravity forward in frequency domain. The result shows that there is a linear relationship between error and depth of 2-D geological model but there is a nonlinear relationship between error and depth of 3-D geological model. The error can be removed according to its linear characteristics or statistical nature of nonlinear characteristics. There is also error between gravity inversion with constant density and gravity inversion with variable density due to variable density and edge-effect. Since there are not noticeable rules between the error and the two causes as variable density and edge-effect, this study adopts gravity inversion with variable density and methods to eliminate the edge-effect in basement inversion to improve inversion accuracy. Based on the study on the rock physical properties and strata distribution of Yellow Sea and adjacent regions, this study finds that there is a big density contrast between Cretaceous-Jurassic strata and their substratum. The magnetic basement of south Yellow Sea is regarded as top of Archeozoic-Proterozoic early strata, and there are double magnetic basements in north Yellow Sea. Gravity and magnetic data are used to inverse the gravity basement and magnetic basement of Yellow Sea, with seismic and drill-hole data as constrains. According to data of gravity and magnetic basement distribution, the depth of Cenozoic strata and previous research findings, this paper calculates the thickness of the Mesozoic and Pre-Mesozoic Residual Basins, draws the distribution outline of Pre-Cenozoic Residual Basins in Yellow Sea, and analyzes its macro-distribution characteristics. Gravity inversion is applied on a typical geological profile in Yellow Sea to analyze the characteristics of its fractures and magnetic basements. The characteristics of Pre-Cenozoic Residual Basins distribution outline in Yellow Sea and the fractures and magnetic basements of its typical profile shown by profile inversion provides new geophysical evidence for these structure views such as “the South Yellow Sea and the North Yellow Sea belong to different structural units” and “Sino-Korea and Yangtze blocks combine along Yellow Sea East Fractured Zone in Yellow Sea”.

华北克拉通新生代玄武岩携带的地幔橄榄岩Fe同位素地球化学及含金云母地幔岩岩石学研究

In recent years, thanks to the improvement of analytical methods and the use of MC-ICP-MS, Fe isotope can be measured precisely. Fe isotope shows considerable variation both in biological and inorganic processes (from low T to high T) in nature, Therefore, Fe isotope has become one of the exciting frontier sciences and has favorable prospects of the application to the geosciences and life sciences. Based on a comprehensive review of available references in the related field, this study focuses on the development of techniques for high-precision measurement of iron isotope using MC-ICP-MS, and application of the techniques developed to study the Fe isotopes as well as major and trace element compositions of minerals (Ol, Opx, Cpx and Sp) from spinel peridotitic xenoliths from Cenozoic alkaline basalts to investigate Fe isotopic features of the lithospheric mantle beneath the North China Craton. The minerals from these xenoliths are similar to those off-cratonic peridotites world-wide, but are remarkably different from those on-cratonic peridotites and clinopyroxenes from these spinel lherzolites exhibit two types of chondrite-normalized REE patterns i.e. LREE-depleted and flat or spoon-shaped. It is noted that total abundances of REE in clinopyroxenes from these peridotites show a broad negative correlation with Cr# numbers of Cpx and Sp. The Fe isotope results show that the spinel peridotitic xenoliths have small but distinguishable Fe isotopic variations in minerals (generally Ol < Opx < Cpx) and samples, and the isotopic range in spinel is relatively large. Positive linear relationship with the ε57Fecpx/ε57Feopx ratio close to one unit has been observed between Fe isotopes of coexistent Opx and Cpx, indicating that the Cpx and Opx have generally reached Fe isotopic equilibrium. However, Fe isotopes between the Ol and Sp show apparent disequilibrium. The broadly negative correlation between mineral Fe isotopes and oxygen fugacity (fo2), metasomatic indexes such as spinel Cr#, (La/Yb) N and (La/Sm) N ratios of clinopyroxenes suggest that Fe isotopic variations in different minerals and peridotites were probably produced by melt-peridotite interaction. This study further confirms the previous observation that the lithospheric mantle has distinguishable and heterogeneous Fe isotopic variations at a scale of xenoliths. Mantle metasomatism that induces the interaction of the lithospheric mantle peridotite with metasomatic agent is a most potential mechanism for the Fe isotope fractionation in mantle peridotites. Therefore, Fe isotope could be a new and powerful tool to probe the evolution of the lithospheric mantle. We also report mineral compositions, clinopyroxene trace element concentrations and Sr-Nd isotopes for newly-discovered phlogopite-bearing spinel lherzolite and olivine clinopyroxenite xenoliths from three different localities （Hannuoba, Hebei Province; Jining Sangyitang, Inner Mongolia; Hebi, Henan Province）of the North China Craton. Systematic comparisons with phlogopite-free spinel lherzolite xenolith from the same locality reveals that the phlogopite-bearing peridotitic xenoliths have relatively higher Al2O3, CaO, Na2O, K2O, TiO2 contents and lower MgO contents than those phogopite-free counterparts. The former also has higher LREE concentrations, but relatively less radiogenic Sr-Nd isotopic ratios. This demonstrates that mantle metasomatism can not only enrich the basaltic components and trace element concentrations, but also make a decrease in Mg# of the peridotites and olivines and a relative depletion in Sr-Nd isotopes. 87Rb/86Sr-87Sr/86Sr isochrons of the phlogopite-bearing xenoliths indicate that mantle metasomatism happened in the Mesozoic and/or Cenozoic time. The metasomatic agent was derived from the asthenosphere. The result also manifests that the widespread similarity of the geochemical features such as major and trace elements and isotopic compositions in the Cenozoic lithospheric mantle beneath the North China Craton to those “oceanic” lithospheric mantle could be as a result of the ubiquitous presence of the interaction between the old refractory peridotites and the infiltrated asthenospheric melt, rather than the actually newly-accreted lithospheric mantle.

塔里木盆地塔北、塔中地区早古生代白云岩形成机理

In Tarim Basin, extensive carbonates of Lower Paleozoic occur, in which thick Cambrian and Lower Ordovician dolostones are widespread and show a potential perspective in hydrocarbon exploration. So they are viewed as an important target for exploration. Tarim Basin is a poly phase composite basin, which underwent multiphase tectonic modification and volcanic activities; these exerted significant influences on the basin-fills and basin fluid evolution, thereby the diagenetic history, particularly on the deep-buried Lower Paleozoic dolostones. Referring to the classification of dolomite texture proposed by Gregg & Sibley (1984) and Sibley & Gregg (1987). In view of crystal size, crystal shape, crystal surface and contact relation, eight genetic textures of dolomite crystals are identified, based on careful petrographic examinatoins. These textures include: 1) micritic dolomite; 2) relict mimetic dolomite; 3)finely crystalline, planar-e(s), floating dolomite; 4)finely crystalline, planar-e(s) dolomite; 5) finely-coarse crystalline, nonplanar-a dolomite; 6)coarse crystalline, nonplanar saddle dolomite; 7) finely-medium crystalline, planar-e(s) dolomite cement; 8) coarse crystalline, nonplanar saddle dolomite cement, in which the former six textures occurs as in matrix, the latter two in the cements. Detailed geochemistry analysis is carried out on the basis of genetic textures of dolomite and related minerals such as quartz and calcite. The result showed that the calcite has the highest average content in Sr, which can be sorted into two groups; micritic dolomite has the highest average content in Sr among all kinds of dolomites; the REE patterns of all kinds of dolomites is similar to those of marine limestone samples. Saddle dolomite cement has δ13C values from -2.44‰ to 1.27‰ PDB, and δ18O values from -13.01‰ to -5.12‰ PDB, which partially overlap with those of matrix dolomite (δ13C values from -2.83‰ to 2.01‰ PDB, δ18O values from -10.63‰ to -0.85‰ PDB). Saddle dolomite cement has 87Sr/86Sr ratios from 0.7086 to 0.7104, which totally overlap with those of matrix dolomite (0.7084 ~ 0.7116). Compared with saddle dolomite derived from other basins all over the world, the saddle dolomites of Tarim Basin have similar δ13C, δ18O and 87Sr/86Sr ratios values with those of matrix dolomite. This scenario reflects the unusual geological setting and special dolomitizing liquid of Tarim Basin. The values of δ18O, δ13C and 87Sr/86Sr ratios of calcite also can be sorted out two groups, which may been resulted from the one stage of extensive uplift of Tarim Basin from Mesozoic to Cenozoic. Fluid inclusion microthermometry data of the diagenetic mineral indicates that matrix dolomite has relatively low homogenization temperatures (Th) of 80～105oC and salinities of 12.3％ (wt% NaCl equivalent); saddle dolomite has highest Th values, which concentrate in 120～160oC and salinities of 13.5～23.7％ (wt% NaCl equivalent); quartz has relatively low Th of 135～155oC and salinities of 17.8～22.5％ (wt% NaCl equivalent); calcite has relatively low Th of 121～159.5oC and salinities of 1.4～17.5％ (wt% NaCl equivalent). These data suggest that the saddle dolomites could have formed in thermal brine fluids. Based on comprehensive petrographical study, detailed geochemistry and fluid inclusion microthermometry analysis on Lower Paleozoic dolomite of Tarim Basin, three types of dolomitisation mechanism are proposed: Penecontemporaneous dolomitisation (Sabkha dolomitisation & Reflux dolomitisation); Burial dolomitisation (shallow-intermediate burial dolomitisation & Deep burial dolomitisation ); Hydrothermal cannibalized dolomitisation. In view of host-specified occurrences of hydrothermal dolomite, the low abundance of saddle dolomite and high geochemical similarities between saddle dolomite and host dolomite, as well as highest Th and high salinities , the hydrothermal dolomite in Tarim Basin is thus unique, which could have been precipitated in modified fluid in the host dolomite through intraformational thermal fluid cannibalization of Mg ions from the host. This scenario is different from the cases that large scale dolomitizing fluid migration took place along the fluid pathways where abundant saddle dolomite precipitated. Detailed observations on 180 petrographic and 60 casting thin sections show original pores in Lower Paleozoic dolomite were almost died out by complicated diagenetic process after a long time geologic evolution. On the other hand, deep-buried dolomite reservoirs is formed by tectonic and hydrothermal reforming on initial dolomites. Therefore, the distribution of structure-controlled hydrothermal dolomite reservoirs is predicted in Tabei and Tazhong Area of Tarim Basin based on the geophysical data.

Tectônica rúptil e sismicidade na área de inundação do açude do Castamhão (CE): implicações para o risco sísmico

This study describes brittle deformation and seismicity in the Castanhão Dam region, Ceará State, Brazil. This reservoir will include a hidroeletric power plant and will store about 6,7 billions m3 of water. Five main litostratigraphic unit were identified in the region: gneissic-migmatitic basement, metavolcanosedimentary sequence, granitoid plutons of Brasiliano age, Mesozoic basaltic dike swarm, and Cenozoic fluvial terraces of the Jaguaribe river. The region has experienced several faulting events that occurred at different crustal levels. Faults formed at depths less than about 12 km present left-lateral movement and are associated with epidote and quartz infillings. Faults formed at depths less than 7 km are mainly strike-slip present cataclastic rocks, fault breccia and gouge. Both fault groups form mainly NE-trendind lineaments and represent reactivation of ductile shear zones or new formed faults that cut across existing structures. Seismically-induced liquefaction fractures take place in Cenozoic terraces and indicate paleoearthquakes that may have reached at leat 6,8 MS. In short, this work indicate that the level of paleoseismicity is much greater than one observed in the instrumental record. Several faults are favourably oriented for reactivation and induced seismicity should be expected after the Castanhão Dam impoudment

North-western Africa-North-eastern Brazil. Major tectonic links and correlation problems

The establishment of a geological correlation between northwest Africa and northeast Brazil faces a series of problems of both a virtual and a real nature. Several aspects are summarised in this work that include pre-Mesozoic and Mesozoic features on both continental sides. © 2002 Elsevier Science Ltd. All rights reserved.

Evolução tectono-sedimentar mesozóico-cenozóica da região de Franca

Pós-graduação em Geologia Regional - IGCE