Mountain building across a lithospheric boundary during arc construction: The Cretaceous Peninsular Ranges batholith in the Sierra San Pedro Martir of Baja California, Mexico

The Jura-Cretaceous Peninsular Ranges batholith (PRB) of Southern and Baja California contains a remarkable example of variation in crustal composition and structure across a batholith-parallel lithospheric-scale discontinuity. This lithospheric boundary between western oceanic-floored and eastern continental-floored crust influenced contractional deformation, arc magmatism, and differential exhumation of western and eastern zones in the batholith during its evolution.In the Sierra San Pedro Martir of Baja California, Mexico, a ca. 20 km wide, doubly vergent fan structure occurs across the PRB basement transition that consists of inward-dipping mylonite thrust sheets on the sides of the fan that gradually transition to a steeply-dipping tectonized zone in the center. A dramatic inverted metamorphic gradient occurs on the western side of this structure where mid-crustal amphibolite metamorphic grade rocks with peak pressures of 5–6 kbar in the center of the fan were thrust over upper-crustal sub-greenschist grade rocks (peak pressures < 2 kbar) in the western zone footwall. An inverted but smaller gradient occurs on the eastern side of the structure where rocks of the fan interior have been thrust eastwards over amphibolite to upper greenschist grade rocks (peak pressures 4–5 kbar).Gradients in cooling ages determined by ⁴⁰Ar/³⁹Ar biotite and K-feldspar and apatite fission track methods coupled with U–Pb zircon ages and Al in hornblende thermobarometry studies on plutons across this zone indicate that structures focused along the transition zone between contrasting lithosphere in the PRB accommodated nearly 15 km of the differential exhumation of western and eastern basement in the orogen. The western zone of the batholith was a major forearc depo-center for thick clastic sequences derived from the uplifting eastern PRB and remained at low average elevation during the Late Cretaceous and Paleogene. In contrast the eastern zone experienced dramatic uplift subsequent to achieving a crustal thickness in excess of 55 km by mid-Cretaceous time. This region had the isostatic potential for 4–5 km surface elevations, and likely formed a topographically high orogenic plateau. Exhumation of the fan structure initiated after 100 Ma and was largely complete by 85 Ma. Eastward-migrating unroofing of the rest of the eastern PRB continued into the Paleogene.A variety of factors were responsible for exhumation in this region. Structural thickening of the eastern zone of the orogen resulted from more than 30 million years of episodic contractional deformation in the fan structure, much of which followed island arc accretion of the western zone along this segment of the batholith. An episode of voluminous magmatism involving the intrusion of the 99–92 Ma La Posta-type magmatic suite across the eastern zone of the PRB triggered exhumation in the fan structure. Denudation in this region appears to have been solely by erosion; no evidence has been found for extensional tectonics during this time. This arc orogen demonstrates the important influence of inherited tectonic boundaries in controlling the spatial distribution of structural thickening and magmatism. It also displays the complex interrelationships among structural thickening, exhumation, and the role of magmatism in triggering exhumation episodes within orogens.     

GIS在农用地分等中的应用

农用地等别是衡量农用地质量的基本指标,是确定土地使用税、土地征用补偿及租赁费等的重要依据。GIS强大的数据输入、管理、查询、分析和输出功能,为农用地分等提供了全新的方法和技术手段。本文以广东省四会市农用地分等工作为例,探讨了GIS在农用地分等中的应用。     

农用地分等中ARCVIEW的技术

从实际出发,分析了ARCVIEW在地图编绘、图层提取、空间分析等方面的作用,揭示ARCVIEW的特点,希望ARCVIEW在土地分等中发挥更为重要的作用。     

北祁连地区早古生代弧岩浆作用及变质作用事件:以门源-柯柯里地区为例

位于祁连地块北缘的门源-柯柯里杂岩体出露一套早古生代高级变质岩,其中长英质片麻岩的矿物组合以石榴子石+夕线石+黑云母+长石+石英为特征,变基性岩以石榴子石+单斜辉石+角闪石+斜长石+石英为特征,具有典型的高角闪岩相-麻粒岩相组合特征。岩相学观察、相平衡模拟和地质温压计估算结果显示门源地区石榴黑云斜长片麻岩和石榴角闪岩的峰期变质作用的温压条件分别为780℃、9.0kbar和790℃、8.4kbar。相平衡模拟揭示石榴黑云斜长片麻岩经历了近等压降温的逆时针P-T轨迹,可能反映了处于中地壳的岩石经历了与洋壳俯冲相关的弧岩浆岩和弧火山岩所带来大量的热所导致的高温变质及随后的等压冷却过程。锆石U-Pb定年显示门源-柯柯里地区2个高级变质岩的变质年龄和1个闪长岩的弧岩浆作用年龄分别为495±2Ma、493±3Ma和495±3Ma,表明门源-柯柯里一带共同经历了北祁连洋壳俯冲导致的500Ma左右的弧岩浆作用和变质作用。门源-柯柯里高温低压变质带和其北侧百经寺-清水沟一带出露的HP/LT变质岩组成了双变质带,并指示了北祁连洋早古生代时期的向南俯冲。     

徐州西南部地质构造特征及成因

以大地构造理论为基础,通过对徐州西南部的小构造尺度的断层、褶皱及其相互联系的研究,结合区域构造的特点,对前人的观点提出新的认识,并合理补充徐宿弧形构造的成因机制。结合地层的岩性特征,认为该构造的形成前期以板块挤压伴随重力滑覆为主,后期以多向重力滑脱为主,研究区的构造特征是由两期构造共同结合的产物。     

A semi-analytical model for computation of capillary entry pressures and fluid configurations in uniformly-wet pore spaces from 2D rock images

A novel semi-analytical model for computation of capillary entry pressures and associated fluid configurations in arbitrary, potentially non-convex, 2D pore space geometries at uniform wettability is developed. The model computes all possible centre positions of circular arcs, and physically sound criteria are implemented to determine the set of these arcs that correspond to geometrically allowed interfaces. Interfaces and pore boundary segments are connected to form closed boundaries of identified geometrical regions. These regions are classified as either oil regions, located in the wider parts of the pore space, or as water regions located in pore space constrictions. All possible region combinations are identified and evaluated for each radius value in an iterative procedure to determine the favourable entry radius and corresponding configuration based on minimisation of free energy. The model has been validated by comparison with known analytical solutions in idealised pore geometries. In cases where different analytical solutions are geometrically possible, the model generates several oil and water regions, and the valid solution is determined by the region combination that corresponds to the most favourable entry pressure, consistent with the analytical solution. Entry pressure radii and configurations are computed in strongly non-convex pore spaces extracted from an image of Bentheimer sandstone, which demonstrates that the model captures successfully well-known characteristics of capillary behaviour at different wetting conditions. The computations also demonstrate the importance of selecting the fluid configuration of minimum change in free energy. In some cases, a merged region formed by a combination of oil and water regions corresponds to the favourable entry configuration of oil, whereas in other cases, an individual oil region may correspond to the favourable oil entry configuration. It is also demonstrated that oil entry configurations may constitute merged regions for weakly water-wet conditions and individual oil regions for strongly water-wet conditions in the same pore space. The computations show that the ratio of pore area to perimeter multiplied by the cosine of the contact angle under-predicts the entry pressure radii in Bentheimer sandstone pore spaces. An alternative formula is proposed for prediction of entry radii for nonzero contact angles based on the entry radii obtained for zero contact angles.     

Flood risk curves and uncertainty bounds

The Scotia Arc is one of two regions in the Atlantic Ocean with greater potential for tsunami generation from seismic and volcanic sources. A numerical modeling study was undertaken to determine tsunami generation from postulated sources along the Arc and tsunami wave amplification or attenuation along the Patagonian continental shelf. Sea level oscillation represented by a simple sinusoidal wave function applied at the boundary of the numerical grid, which simulated the tsunami entering the computational domain, was implemented as forcing. The validation of this model was carried out by comparing the maximum amplitudes recorded and simulated at Santa Teresita and Mar del Plata (Buenos Aires province) after the occurrence of earthquake and subsequent tsunami in Sumatra (December 2004). From numerical simulations it can be seen that the tsunami propagation is highly affected by bathymetric refraction on the Patagonian continental shelf and the wave amplitude is significantly attenuated on the inner continental shelf. Maximum amplifications were obtained around Malvinas (Falkland) Islands and Burdwood bank because the wave propagates almost without refracting and the shoaling effect is highly significant there.     

Anatomy of a Cambrian suture in Gondwana: Pacific-type orogeny in southern India?

Southern India occupies a central position in the Late Neoproterozoic–Cambrian Gondwana supercontinent assembly. The Proterozoic mosaic of southern India comprises a collage of crustal blocks dissected by Late Neoproterozoic–Cambrian crust-scale shear/suture zones. Among these, the Palghat–Cauvery Suture Zone (PCSZ) has been identified as the trace of the Cambrian suture representing Mozambique Ocean closure during the final phase of amalgamation of the Gondwana supercontinent. Here we propose a model involving Pacific-type orogeny to explain the Neoproterozoic evolution of southern India and its final amalgamation within the Gondwana assembly. Our model envisages an early rifting stage which gave birth to the Mozambique Ocean, followed by the initiation of southward subduction of the oceanic plate beneath a thick tectosphere-bearing Archean Dharwar Craton. Slices of the ocean floor carrying dunite–pyroxenite–gabbro sequence intruded by mafic dykes representing a probable ophiolite suite and invaded by plagiogranite are exposed at Manamedu along the southern part the PCSZ. Evidence for the southward subduction and subsequent northward extrusion are preserved in the PCSZ where the orogenic core carries high-pressure and ultrahigh-temperature metamorphic assemblages with ages corresponding to the Cambrian collisional orogeny. Typical eclogites facies rocks with garnet + omphacite + quartz and diagnostic ultrahigh-temperature assemblages with sapphirine + quartz, spinel + quartz and high alumina orthopyroxene + sillimanite + quartz indicate extreme metamorphism during the subduction–collision process. Eclogites and UHT granulites in the orogenic core define P–T maxima of 1000 °C and up to 20 kbar. The close association of eclogites with ultramafic rocks having abyssal signatures together with linear belts of iron formation and metachert in several localities within the PCSZ probably represents subduction–accretion setting. Fragments of the mantle wedge were brought up through extrusion tectonics within the orogenic core, which now occur as suprasubduction zone/arc assemblages including chromitites, highly depleted dunites, and pyroxene bearing ultramafic assemblages around Salem. Extensive CO₂ metasomatism of the ultramafic units generated magnesite deposits such as those around Salem. High temperature ocean floor hydrothermal alteration is also indicated by the occurrence of diopsidite dykes with calcite veining. Thermal metamorphism from the top resulted in the dehydration of the passive margin sediments trapped beneath the orogenic core, releasing copious hydrous fluids which moved upward and caused widespread hydration, as commonly preserved in the Barrovian amphibolite facies units in the PCSZ. The crustal flower structure mapped from PCSZ supports the extrusion model, and the large scale north verging thrusts towards the north of the orogenic core may represent a fold-thrust belt. Towards the south of the PCSZ is the Madurai Block where evidence for extensive magmatism occurs, represented by a number of granitic plutons and igneous charnockite massifs of possible tonalite–trondhjemite–granodiorite (TTG) setting, with ages ranging from ca. 750–560 Ma suggesting a long-lived Neoproterozoic magmatic arc within a > 200 km wide belt. All these magmatic units were subsequently metamorphosed, when the Pacific-type orogeny switched over to collision-type in the Cambrian during the final phase of assembly of the Gondwana supercontinent. One of the most notable aspects is the occurrence of arc magmatic rocks together with high P/T rocks, representing the deeply eroded zone of subduction. The juxtaposition of these contrasting rock units may suggest the root of an evolved Andean-type margin, as in many arc environments the roots of the arc comprise ultramafic/mafic cumulates and the felsic rocks represent the core of the arc. The final phase of the orogeny witnessed the closure of an extensive ocean — the Mozambique Ocean — and the collisional assembly of continental fragments within the Gondwana supercontinent amalgam. The tectonic history of southern India represents a progressive sequence from Pacific-type to collision-type orogeny which finally gave rise to a Himalayan-type Cambrian orogen with characteristic magmatic, metasomatic and metamorphic factories operating in subduction–collision setting.     

Deformation-related microstructures in magmatic zircon and implications for diffusion

An undeformed glomeroporphyritic andesite from the Sunda Arc of Java, Indonesia, contains zoned plagioclase and amphibole glomerocrysts in a fine-grained groundmass and records a complex history of adcumulate formation and subsequent magmatic disaggregation. A suite of xenocrystic zircon records Proterozoic and Archaean dates whilst a discrete population of zoned, euhedral, igneous zircon yields a SHRIMP U-Pb crystallisation age of 9.3 ± 0.2 Ma. Quantitative microstructural analysis of zircon by electron backscatter diffraction (EBSD) shows no deformation in the inherited xenocrysts, but intragrain orientation variations of up to 30° in 80% of the young zircon population. These variations are typically accommodated by both progressive crystallographic bending and discrete low angle boundaries that overprint compositional growth zoning. Dispersion of crystallographic orientations are dominantly by rotation about an axis parallel to the zircon c-axis [001], which is coincident with the dominant orientation of misorientation axes of adjacent analysis points in EBSD maps. Less common <100> misorientation axes account for minor components of crystallographic dispersion. These observations are consistent with zircon deformation by dislocation creep and the formation of tilt and twist boundaries associated with the operation of <001>{100} and <100>{010} slip systems. The restriction of deformation microstructures to large glomerocrysts and the young magmatic zircon population, and the absence of deformation within the host igneous rock and inherited zircon grains, indicate that zircon deformation took place within a low-melt fraction (<5% melt), mid-lower crustal cumulate prior to fragmentation during magmatic disaggregation and entrainment of xenocrystic zircons during magmatic decompression. Tectonic stresses within the compressional Sunda Arc at the time of magmatism are considered to be the probable driver for low-strain deformation of the cumulate in the late stages of initial crystallisation. These results provide the first evidence of crystal plastic dislocation creep in zircon associated with magmatic crystallisation and indicate that the development of crystal-plastic microstructures in zircon is not restricted to high-strain rocks. Such microstructures have previously been shown to enhance bulk diffusion of trace elements (U, Th and REE) in zircon. The development of deformation microstructures, and therefore multiple diffusion pathways in zircon in the magmatic environment, has significant implications for the interpretation of geochemical data from igneous zircon and the trace element budgets of melts due to the potential enhancement of bulk diffusion and dissolution rates.     

浅析江淮区中强震活动与琉球弧强震活动的相关性

江淮地震区地处我国东部地区,属于5-6级地震为主的中强震活动区,琉球弧地处西太平洋地震带的西支,属于7级以上地震为主的强震活动区。本文运用列表和回归分析法对江淮区中强震活动和琉球弧强震活动的相关性进行了分析。结果表明,两者之间的相关性较好,对应率达80％,其平均滞后时间在18个月。并且讨论了产生相关性的原因。分析结论可能对江淮区中强震的中期预报具有一定的作用。