A GPR study of subsidence-creep-fault processes in Morelia, Michoacán, Mexico

In 1983, inhabitants of the City of Morelia, Michoacán, Mexico, began to observe a series of differential settlements causing damages to constructions along linear trends parallel to a system of regional faults. The same phenomenon occurs in others cities of the Mexican Volcanic Belt (MVB), such as Celaya, Aguascalientes, and Querétaro, and is linked to a structurally controlled subsidence, caused by groundwater withdrawal, and the presence of geological faults. We define this subsidence type as Subsidence-Creep-Fault Processes (SCFP), based on the necessary elements for their generation, and we studied them through geophysical and geotechnical techniques. In Morelia, the geophysical investigations have been carried out using ground-penetrating radar (GPR). GPR profiles, perpendicular to the axis of the surface fault generated by the SCFP were carried out. The common-offset single-fold profiling was used, with a central frequency of 50 MHz. In all cases it has been possible to visualize a fault plane dividing two blocks, the presence of synthetic and antithetic faults, influence zones from 20 m to 40 m, and a maximum “net throw” of 4 m. Exploration trenches followed the same direction of the profiles obtained with GPR (perpendicular to the axis of the surface fault). These trenches exposed a fault plane dividing two blocks with different lithology, generating a maximum “net throw” of 4.40 m; as well they help in the determination of influence zones that varied from 14 m to 40 m.     

马里亚纳弧前区锥形海山岩石学及成因

锥形海山是马里亚纳弧前区非火山成因的海山。它是由俯冲的太平洋板块对仰冲板块的上推作用,地壳下的上地幔物质底辟侵入的结果。海山顶部的岩石是由蛇纹石化超镁铁质方辉橄榄岩组成。其主要矿物成分为斜方辉石、橄榄石、尖晶石、磁铁矿、蛇纹石和碳酸盐。手标本呈显晶质块状构造,薄片中呈粒状、残留状、筛网状及斑状变晶结构。岩石化学特征表现为:低Si和Al,高M/F值,含水,亏损了所有过渡金属。岩石的成因属超基性岩浆直接结晶,后经海水的参入,在热液作用下普遍发生蛇纹石化和碳酸盐化。     

A depocenter of organic matter at 7800 m depth in the SE Pacific Ocean

The Atacama trench, the deepest ecosystem of the southern Pacific Ocean (ca. 8000 m depth) was investigated during the Atacama Trench International Expedition. Sediments, collected at three bathyal stations (1040–1355 m depth) and at a hadal site (7800 m) were analyzed for organic matter quantity and biochemical composition (in terms of phytopigments, proteins, carbohydrates and lipids), bacterial abundance, biomass and carbon production and extracellular enzymatic activities. Functional chlorophyll-a (18.0±0.10 mg m⁻²), phytodetritus (322.2 mg m⁻²) and labile organic carbon (16.9±4.3 g C m⁻²) deposited on surface sediments at hadal depth (7800 m) reached concentrations similar to those encountered in highly productive shallow coastal areas. High values of bacterial C production and aminopeptidase activity were also measured (at in situ temperature and 1 atm). The chemical analyses of the Atacama hadal sediments indicate that this trench behaves as a deep oceanic trap for organic material. We hypothesize that, despite the extreme physical conditions, benthic microbial processes might be accelerated as a result of the organic enrichment.     

High heat flow anomalies on an old oceanic plate observed seaward of the Japan Trench

Thirty-three new measurements on the seaward slope and outer rise of the Japan Trench along a parallel of 38°45′N revealed the existence of high heat flow anomalies on the subducting Pacific plate, where the seafloor age is about 135 m.y.. The most prominent anomaly with the highest value of 114 mW/m² is associated with a small mound on the outer rise, which was reported to be a kind of mud volcano. On the seaward slope of the trench, heat flow is variable: high (70–90 mW/m²) at some locations and normal for the seafloor age (about 50 mW/m²) at others. The spatial variation of heat flow may be related to development of normal faults and horst/graben structures due to bending of the Pacific plate before subduction, with fluid flow along the fault zones enhancing the vertical heat transfer. Possible heat sources of the high heat flow anomalies are intra-plate volcanism in the last several million years like that discovered recently on the Pacific plate east of the Japan Trench.     

全水深多波束测深系统Seabeam3012在西太平洋马里亚纳海山区地形测量中的应用

中国科学院海洋研究所在开展西太平洋马里亚纳海山区多学科综合科学考察的过程中,利用"科学"轮船载的全水深多波束测深系统Seabeam3012对多个海山进行了地形测量工作。针对作业过程中遇到的恶劣海况导致采集数据质量差、多波束系统易检测错误海底信息、测线布设难度大等问题,提出了基于船体姿态对数据质量影响分析的多波束测线方向优化、基于地形变化并参考浅地层剖面资料的作业参数优化和基于实时采集情况的多波束采集测线布设优化等一系列措施,有效地提高了海山区多波束数据采集质量,并提高了作业效率。获得的高品质地形数据,为多学科协同研究奠定了基础,为ROV等设备的现场作业提供了安全保障。     

Coral assemblages and reef growth in the Commonwealth of the Northern Mariana Islands (Western Pacific Ocean)

Reef development varies considerably around the high, raised‐limestone islands of the Commonwealth of the Northern Mariana Islands (CNMI). Here we examine the modern assemblages at 30 sites for coral composition, colony density, colony size, and fidelity. We defined four reef types and hypothesize the presence of environmentally driven ecological stasis, whereby the environment continuously selects for coral species membership, defines colony sizes, and over time creates the noted reef types. Our results show that constructional spur‐and‐groove reefs supported significantly larger coral‐colony sizes and higher coral species richness compared with high‐relief interstitial framework, low‐relief incipient, and non‐constructional coral assemblages. Non‐constructional reefs supported much smaller coral colony sizes, despite similar population densities, and were consistently found in association with high wave exposure. The distinct coral assemblages found on interstitial framework and low‐relief incipient reefs were not affiliated with any wave exposure regime, but were located adjacent to large watersheds and on islands with unique geological history. These assemblages were nested within the spur‐and‐groove species pool. Overall, modern coral cover was well predicted by bathymetric slope and watershed size, while species richness was additively influenced by two proxies of pollution, suggesting the latter is better suited for establishing management targets. In contrast with previous studies that suggested modern assemblages were biologically controlled in the CNMI, we show reef assemblages and reef development are highly influenced by long‐term environmental forcing.     

日本海沟俯冲带热结构与深源地震

本文利用有限差分方法,计算了全地幔对流模式和双层地幔对流模式下日本海沟俯冲带热结构、浮力及P波速度异常分布,基于亚稳态橄榄石相变模型推测亚稳态橄榄石的存在范围,同时分析了热传导系数、热膨胀系数和热源对俯冲带热结构的影响,以及俯冲带所受浮力与俯冲带形态的关系.结果表明,双层地幔对流模式下模拟的P波速度异常分布与层析成像结果更为相符,也与深源地震的分布有较好的相关性.板块内部亚稳态橄榄石的存在范围随热传导系数和热膨胀系数的减小而增大,同时忽略相变潜热和剪切生热的影响也会造成模型所预测的亚稳态橄榄石范围偏大.俯冲带所受负浮力在400 km深度附近达到最大值,亚稳态橄榄石的存在使负浮力逐渐减小,甚至在板块内部产生正浮力,不利于俯冲带穿透660 km相变界面.