Thermochronology and exhumation rates of granitic intrusions at Mesa Central,Mexico

ABSTRACT

The Mesa Central of Mexico (MC) is an elevated plateau located 2000 m above sea level in central Mexico, where intrusions outcrop that register the history of exhumation-erosion occurring during the Late Cretaceous-Paleogene. The tectonic history of the region records formation of the Late Cretaceous-Paleogene ‘Mexican orogen’; this was followed by extension of the entire region and several plutons were then exhumed. The age and magnitude of the crustal uplift and erosion occurring during exhumation has not been addressed to date. Therefore, this study reports the crystallization and cooling ages of two plutons, the Tesorera Granodiorite and the Comanja Granite, and estimates their emplacement depths. Based on these data, the exhumation age of the Tesorera Granodiorite is estimated to be between ~73 Ma and ~63 Ma at an exhumation rate of ~528 m/m. y. and that of the Comanja Granite is 52 Ma and 48 Ma at an exhumation rate of ~2500 m/m. y. Exhumation-erosion event of the Tesorera Granodiorite was located on the trace of the San Luis-Tepehuanes Fault System and that of the Comanja Granite on the a trace of the El Bajío Fault System. Furthermore, the high exhumation rate in the Comanja Granite suggests that gravitational collapse played an important role during exhumation.     

龙里高山草原形成机理与旅游资源初评

龙里高山草原是一个待开发的风景旅游区,它由海拔1578～ 1674m的山巅坪台及深切到海拔1100m左右的岩溶峡谷组合而成,包括无边无际的平坦草原、险而奇的岩溶森林峡谷,及残丘、岩溶洼地、峰林等多种类型的地貌。草原坪台上覆盖着草甸,岩溶峡谷内长满竹林及阔叶混合林,是开展多种旅游项目不可多得的绿色旅游资源。该景区的形成与特殊的地质地貌演化历史有关,即新构造运动使得区内地壳不断抬升,分布在平缓的箱状背斜轴部的石炭系石灰岩上的石英砂岩,由于其抗风化能力强,而残留在一级剥夷面上成为山巅坪台,即方山地貌。该旅游区具有“奇、新、特”及距贵阳近的得天独厚的旅游优势。      

A sedimentary facies model for stepped,fluvial tufa systems in the Iberian Range (Spain): the Quaternary Piedra and Mesa valleys

Stratigraphic and sedimentological analyses of the Quaternary tufa and associated deposits in the Piedra and Mesa river valleys allowed a number of stages of their sedimentary evolution to be characterized, and a depositional sedimentary model for this north‐central sector of the Iberian Range (Spain) to be established. The proposed sedimentary facies model may explain tufa arrangements in other medium to high gradient, stepped, fluvial tufa systems with narrow transverse profiles occurring in temperate, semi‐arid areas, in both recent and past scenarios. There are several tufa deposits within the Piedra and Mesa river valleys that, over a maximum thickness of about 90 m, record one or more stages of tufa deposition produced following the fluvial incision of the bedrock or previous tufa deposits. Each depositional stage begins with coarse detrital sedimentation. Six fundamental, vertical sequences of tufa facies with small amounts of detrital material reveal the sedimentary processes that occurred in different fluvial environments: channel areas with: (i) free‐flowing water; (ii) barrages and/or cascades; and (iii) dammed water and palustrine floodplains. The proposed sedimentary model involves narrow, stepped, fluvial valleys in which tufa cascades were common. Alternating intervals of bryophyte and stromatolite facies commonly formed at some cascades. Many of these represented barrage‐cascade structures that consisted of phytoclast rudstones, thick phytoherms of mosses and associated stromatolites, and curtain‐shaped phytoherms of stems. Upstream of these structures, dammed areas with bioclastic sands and silts developed and palustrine vegetation grew. The channel stretches between barrages and/or cascades were loci for extensive stromatolite growth in fast flowing water. The palustrine floodplain was home to pools and drainage channels. The model also explains the growth of some barrages in the River Piedra that surpassed the height of the divide, with the diffluence of the main channel into a secondary course forming other tufa deposits in the area. The distribution and abundance of certain types of tufa facies in fluvial basins may be an indicator of differences in their gradients. The facies studied in this work suggest that the gradient of the ancient River Piedra was steeper than that of the ancient River Mesa. Assuming similar scenarios for climate and hydrology, the depositional settings mentioned above and their dimensions would have been determined mainly by the gradient and width of the associated river valleys. This sedimentary model may also be useful for inferring variations in other river basin slopes, as well as accounting for the presence of tufas in areas that normally have no permanent water input.     

Syn-extensional intra-plate trachydacite-rhyolitic dome volcanism of the Mesa Central, southern Sierra Madre Occidental volcanic province, Mexico

Oligocene dome complexes of trachydacitic to rhyolitic composition are common in the southern portion of the Mesa Central physiographic province, which forms part of the southern Basin and Range extensional province as well as of the southern Sierra Madre Occidental volcanic province. Generally, dome complexes occur aligned with regional fault systems, mostly associated with the southern Basin and Range province, and thus suggesting that faults controlled the felsic magmas that formed these domes. Two distribution patterns are evident, one aligned NE–SW and another aligned NNE. The set of domes were emplaced at 33–28 Ma. Emplacement of domes occurred in three continuous phases starting with those of trachydacite affinity at 33–32 Ma, to trachydacite–rhyolitic at 32–31 Ma, and finally to those with rhyolitic composition at 31–28 Ma. Felsic magmas that originated the domes were apparently generated by partial melting at the base of the continental crust. Contrary to previous hypothesis, our evidence suggest that these magmas in these particular areas of the Mesa Central were not accumulated in large magma reservoirs emplaced at shallow levels in the crust, but crossed the continental crust directly. Since continental crust in this region is relatively thin (30–33 km), we propose that an intense extensional episode favored the direct ascension of these magmas through the brittle crust, with little interaction with the country rock during ascent to the surface, to end up forming aligned dome chains or complexes. Geochemical data favors this model, as the felsic rocks show no depletions in Nb and Th but instead relatively enrichment in these elements. REE show flat or concave up patterns, suggesting that the magmas involved enriched (fertile), metasomatized lithospheric fluids that generated partial melting at the base of the continental crust. Based upon these data, we infer an intra-plate tectonic setting for these rocks.     

Sediment storage and transport in Pancho Rico Valley during and after the Pleistocene‐Holocene transition,Coast Ranges of central California (Monterey County)

Factors influencing sediment transport and storage within the 156·6 km² drainage basin of Pancho Rico Creek (PRC), and sediment transport from the PRC drainage basin to its c. 11 000 km² mainstem drainage (Salinas River) are investigated. Numeric age estimates are determined by optically stimulated luminescence (OSL) dating on quartz grains from three sediment samples collected from a ‘quaternary terrace a (Qta)’ PRC terrace/PRC‐tributary fan sequence, which consists dominantly of debris flow deposits overlying fluvial sediments. OSL dating results, morphometric analyses of topography, and field results indicate that the stormy climate of the Pleistocene‐Holocene transition caused intense debris‐flow erosion of PRC‐tributary valleys. However, during that time, the PRC channel was backfilled by Qta sediment, which indicates that there was insufficient discharge in PRC to transport the sediment load produced by tributary‐valley denudation. Locally, Salinas Valley alluvial stratigraphy lacks any record of hillslope erosion occurring during the Pleistocene‐Holocene transition, in that the alluvial fan formed where PRC enters the Salinas Valley lacks lobes correlative to Qta. This indicates that sediment stripped from PRC tributaries was mostly trapped in Pancho Rico Valley despite the relatively moist climate of the Pleistocene‐Holocene transition. Incision into Qta did not occur until PRC enlarged its drainage basin by c. 50% through capture of the upper part of San Lorenzo Creek, which occurred some time after the Pleistocene‐Holocene transition. During the relatively dry Holocene, PRC incision through Qta and into bedrock, as well as delivery of sediment to the San Ardo Fan, were facilitated by the discharge increase associated with stream‐capture. The influence of multiple mechanisms on sediment storage and transport in the Pancho Rico Valley‐Salinas Valley system exemplifies the complexity that (in some instances) must be recognized in order to correctly interpret terrestrial sedimentary sequences in tectonically active areas. Copyright © 2009 John Wiley & Sons, Ltd.     

Geophysical surveys of the Joya Honda maar (México) and surroundings; volcanic implications

Joya Honda (JH) is a Quaternary maar excavated in Mesozoic limestone. It is located in central Mexico and belongs to the Ventura volcanic field (VVF), which is composed by cinder cones and maars made of intraplate-type mafic alkalic rocks. Volcanoes in the region form  N20W lineaments, roughly parallel to a regional set of normal faults, but there is no obvious relation between these faults and vent distribution in the exposed geology around the maar. The volcanic rock volume is small in the VVF, and most volcanoes and their products are scattered in a region where outcrops are dominated by limestone.The near-vent tephra associated to the JH maar lies north of the crater. This relation suggests that the crater was formed by directed hydromagmatic explosions and may indicate an inclined volcanic conduit near the surface. The tephra stratigraphy suggests that the initial explosions were relatively dry and the amount of water increased during the maar forming eruption. Therefore, the existing model of the maar–diatreme formation may not be applicable to Joya Honda as it requires the formation of a cone of depression in the aquifer and deepening of the focii of the explosions as the crater and underlying diatreme grew. Thus, it is unlikely that there is a diatreme below Joya Honda.Aeromagnetic data shows a boundary between two regional magnetic domains near the elongated volcanic cluster of the VVF. The boundary is straight, with a distinct kink, from NE- to NW-trend, near JH. The limit between the domains is interpreted as fault contacts between mid-Tertiary volcanic rocks and marine Mesozoic sedimentary rocks. Hence, magma ascent in the area may have been facilitated by fractures near the surface.Magnetic and gravimetric ground surveys show that the anomalies associated with the maar are not centered in the crater, which could be consistent with an inclined volcanic conduit. A magnetic profile measured on exposed limestone across the volcanic lineament failed to show an anomaly such as that caused by a connecting dike between the volcanoes. Therefore, either the dike does not exist or it is so deep or so thin that it is beyond the limit of detection of the method and/or equipment used. Thus, the volcanic conduit immediately below Joya Honda can be reasonably modeled in the shape of a plug. A 2-D model of the crater anomaly is consistent with a roughly tabular deposit formed by fall-back pyroclasts and slump deposits near the surface. Based on this result we propose an alternative model for the formation of maar-type volcanoes excavated in hard rock, where there is no evidence of a gradual decrease of the water/magma ratio as the eruption advanced.     

山东岱崮地貌研究综述

岱崮地貌是继岩溶地貌、丹霞地貌、张家界地貌、嶂石岩地貌之后的中国第5种岩石地貌,作为一种新命名的地貌类型,岱崮地貌具有极大的美学价值和科研价值。但对其的研究和开发却相对滞后。该文介绍了岱崮地貌的研究现状、分布及其形成过程、发育阶段等,岱崮地貌的分布规律、形态特征、组成结构,其演变趋势与模式及其形成机制等,是其学术研究的核心问题。加强对岱崮地貌的研究,不仅能够改变地貌学中关于方山研究的薄弱局面,丰富大陆风化及陆地碳循环研究,而且可深化对沂蒙山区地质地貌条件、生态环境特点的认识,也可为申报国家地质公园和世界自然遗产提供科学支撑。     

新生代玄武岩地区找水中遥感技术应用研究——以阿巴嘎玄武岩为例

利用TM图像对火山口开口方向和玄武岩流向进行解译分析,圈定出玄武岩台地上的汇水盆地,从而确定了新生代玄武岩强富水区———找水目标区。     

山东岱崮地貌研究综述

岱崮地貌是继岩溶地貌、丹霞地貌、张家界地貌、嶂石岩地貌之后的中国第5种岩石地貌,作为一种新命名的地貌类型,岱崮地貌具有极大的美学价值和科研价值。但对其的研究和开发却相对滞后。该文介绍了岱崮地貌的研究现状、分布及其形成过程、发育阶段等,岱崮地貌的分布规律、形态特征、组成结构,其演变趋势与模式及其形成机制等,是其学术研究的核心问题。加强对岱崮地貌的研究,不仅能够改变地貌学中关于方山研究的薄弱局面,丰富大陆风化及陆地碳循环研究,而且可深化对沂蒙山区地质地貌条件、生态环境特点的认识,也可为申报国家地质公园和世界自然遗产提供科学支撑。     

New optically stimulated luminescence ages provide evidence of MIS3 and MIS2 eolian activity on Black Mesa, northeastern Arizona, USA

Eolian deposition on the semiarid southern Colorado Plateau has been attributed to episodic aridity during the Quaternary Period. However, OSL ages from three topographically controlled (e.g. falling) dunes on Black Mesa in northeastern Arizona indicate that eolian sediments there were deposited in deep tributary valleys as early as 35–30 ka, with most sand deposited before 20 ka. In contrast, the oldest OSL ages for sand sheets fall within the Pleistocene-Holocene climatic transition (~ 12–8 ka). Thus most eolian sediment accumulated on Black Mesa under climatic conditions that were in general cooler, moister, and more variable than today, not more arid, pointing to a considerable increase in sediment supply.