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

Angel F. Nieto-Samaniego  María de Jesús Paulina Olmos-Moya  Gilles Levresse  Susana A. Alaniz-Alvarez  Fanis Abdullin  Alexis del Pilar-Martínez 《International Geology Review》2020,62(3):311-319

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.  相似文献   

Structure, ordering and cation interactions in Ca-free P2 1 /c clinopyroxenes     

R. J. Angel  C. McCammon  A. B. Woodland 《Physics and Chemistry of Minerals》1998,25(4):249-258

A suite of 14 synthetic Ca-free P2 ₁ /c low-clinopyroxenes of compositions across the enstatite-ferrosilite (En-Fs) join have been studied by X-ray powder diffraction and Mössbauer spectroscopy. The crystal structure of one sample with composition X _Fs=0.39 was determined by single-crystal X-ray diffraction. The powder diffraction data show that there is no significant (<0.25% at X _Fs=0.5) excess volume of mixing on the enstatite-ferrosilite join. The molar volumes are described by V=31.261(17)+1.677(23) X _Fs cm³.mol(MSiO₃)^–1. The linear behaviour of volume with composition arises from the compensating effects of non-linear changes in the unit-cell parameters with composition. Unweighted fits to the data yielded the relationships: a=9.6100(15)+0.105(2)X _Fs, b=8.8156(28)+0.218(12)X _Fs+0.0481(117)X ² _Fs , c?=5.1702(4)+0.0879(21)X _Fs–0.0214(21)X ² _Fs , β=108.345(8)+0.947(30)X _Fs–0.805(26)X ² _Fs . The strong positive deviation of β from linearity is directly correlated to the difference in site occupancies between M1 and M2 as determined by Mössbauer spectroscopy and arises directly from the expansion of M2-O1 and M2-O3 bonds upon initial substitution of Fe for Mg in enstatite. The hyperfine parameters from Mössbauer spectra are consistent with variations in the average local environment as recorded by the X-ray data. Asymmetric line broadening of the Mössbauer spectra provides evidence for next-nearest neighbour effects, and is consistent with no significant clustering of Fe or Mg within the samples.  相似文献   

A Method for Obtaining the Nuclear Component in Active Galaxies     

Miguel Sanchez Portal  Angeles I Diaz Beltran 《Astrophysics and Space Science》1998,263(1-4):119-122

We present a new method for the obtention of the nuclear component of active galatic nuclei, that relies on the convolution of a two-dimensional model of the host galaxy, based on seeing-free morphological parameters, with the seeing PSF. The procedure overcomes some limitations of the methods currently in use and does not assume that the nucleus is unresolved. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Model for simulating soil-water content considering evapotranspiration — Reply     

Abas Afshar  Miguel A. Marino 《Journal of Hydrology》1979,40(3-4):393-395

  相似文献   

Moderately volatile siderophiles in ordinary chondrites     

Ermanno R. Rambaldi  Miguel Cendales 《Earth and Planetary Science Letters》1979,44(3):397-408

The contents of the moderately volatile elements Ga, Ge, Cu and Sb in ordinary chondrites give us some clues with regard to the metal-silicate fractionation process. Their concentration in coexisting magnetic and non-magnetic portions of members of each ordinary chondrite group will be discussed. Germanium and Sb are mostly siderophilic, but Ga is strongly lithophilic in unequilibrated chondrites; its partition coefficient between magnetic and non-magnetic portions is positively correlated with petrologic type in L and LL chondrites, but not in H4–6 chondrites. From 25 to 50% of the total Cu is found in the non-magnetic fraction of chondrites, but there is no correlation between Cu content and petrologic type. The abundances of Ga, Cu and Sb (relative to Si) are constant in ordinary chondrites, independent of the amount of metal present, indicating that these elements were not in solid solution in the metal phase of chondrites when the metal-silicate fractionation process occurred. Germanium, which is the most volatile among the four elements analyzed, is more abundant in H than in L and LL chondrites, indicating that it was fractionated by this process. Nebular oxidation processes can be responsible for the behavior of Ga if this element was in oxidized form when loss of metal occurred, but cannot explain the results for Cu and Sb which are predicted to condense as metals and accrete mostly in metallic form. It is possible that Cu and Sb, upon condensation, did not form solid solutions with metallic Ni-Fe until after the separation of metal from silicates took place.  相似文献   

The development of folds in bedded chert and related rocks in the Malaguide Complex, southern Spain     

Miguel Orozco  Rafael Galvez 《Tectonophysics》1979,56(3-4)

Folds and folding mechanism in a chert sequence and related rocks of the Maláguide Complex (the uppermost tectonic unit of the Betic Zone) have been investigated. The geometric study shows that folds that developed in the chert sequence are usually angular in shape and asymmetric. Chevron and conjugate folds are common.Folding in bedded chert is explained in terms of a suggested model:

1. (1) Development of folds by kink and conjugate kinking.

2. (2) As the shortening increases, the interlimb angles decrease; in the kink folds this is caused by a reduction of the angle between the layers within the kink and the kink boundaries. There seems to be a relationship between this angle and the asymmetrical thinning-out in the limbs of many folds: the smaller is the angle between the kink boundary and the layers within the kink band, the larger is the reduction of the thickness in these layers.

Single limestone layers embedded in slate deform very probably by a buckling mechanism, implying tangential longitudinal strain and an additional flattening.  相似文献   

An investigation of the small ice cap instability in the Southern Hemisphere with a coupled atmosphere-sea ice-ocean-terrestrial ice model     

M. A. Morales Maqueda  A. J. Willmott  J. L. Bamber  M. S. Darby 《Climate Dynamics》1998,14(5):329-352

 A simple climate model has been developed to investigate the existence of the small ice cap instability in the Southern Hemisphere. The model consists of four coupled components: an atmospheric energy balance model, a thermodynamic snow-sea ice model, an oceanic mixed layer model and a terrestrial ice model. Results from a series of experiments involving different degrees of coupling in the model show that the instability appears only in those cases when an explicit representation of the Antarctic ice sheet is not included in the model. In order to determine which physical processes in the ice sheet model lead to a stabilization of the system we have conducted several sensitivity experiments in each of which a given ice sheet process has been removed from the control formulation of the model. Results from these experiments suggest that the feedback between the elevation of the ice sheet and the snow accumulation-ice ablation balance is responsible for the disappearance of the small ice cap instability in our simulation. In the model, the mass balance of the ice sheet depends on the air temperature at sea level corrected for altitude and it is, therefore, a function of surface elevation. This altitude-mass balance feedback effectively decouples the location of the ice edge from any specific sea level isotherm, thus decreasing the model sensitivity to the albedo-temperature feedback, which is responsible for the appearance of the instability. It is also shown that the elevation-radiative cooling feedback tends to stabilize the ice sheet, although its effect does not seem to be strong enough to remove the instability. Another interesting result is that for those simulations which include the terrestrial ice model with elevation-dependent surface mass balance, hysteresis is exhibited, where for a given level of external forcing, two stable solutions with different, non-zero ice-sheet volume and area and different air and ocean temperature fields occur. However, no unstable transition between the two solutions is ever observed. Our results suggest that the small ice cap instability mechanism could be unsuitable for explaining the inception of glaciation in Antarctica. Received: 14 April 1997 / Accepted: 22 October 1997  相似文献   

Crystal bending in metamorphic calcite,and its relations to associated twinning     

Francis J. Turner  Miguel Orozco 《Contributions to Mineralogy and Petrology》1976,57(1):83-97

This paper deals with the incidence and geometry of lattice bending in calcite of naturally strained marble and possible relations in time to associated {01¯12} twinning, as revealed by microscopic examination with a universal stage. Microscopic technique and graphic treatment of data with a view to identifying the relation in time of bending to twinning, and possible glide mechanisms involved in the bending process are reviewed as a basis for present and subsequent investigation of this general topic.In most instances development of surviving thin e twins postdates bending of the host crystal (such twins everywhere are rationally oriented). Rarely (in contrast with experimental experience) some thin twins are slightly irrational (L_e lamellae) and apparently developed in the final stages of bending. Thick twins (a few tens of microns wide) on the other hand rather commonly predate or are synchronous with bending.The geometry of bending (external rotation) — especially for rotation less than 25° —commonly is compatible with models (based on experimental experience) of simple translation in the strained domains. The commonest translation system so identified is on {10¯11}, with sense of shear, where recognizable, either negative or positive. Translation on {02¯21} is relatively rare. Unexpectedly there is rather strong but not conclusive evidence of translation on {0001} parallel to an a axis. This mechanism, hitherto never identified with certainty, is now here documented unequivocally in a crystal of calcite experimentally strained at 300° C, 5 kb.  相似文献   

Reconnaissance geochronology of basement rocks from the Amazonas and Maranhão Basins in Brazil     

Adam Kovach  Harold W. Fairbairn  Patrick M. Hurley  Miguel A.S. Basei  Umberto G. Cordani 《Precambrian Research》1976,3(5):471-480

Rb/Sr whole rock as well as K/Ar whole rock and mineral age determinations have been carried out on drill core samples obtained from the crystalline basement underlying the Amazonas and Maranhão Basins in Brazil. The data confirm the existence of an ancient basement under the Upper and Middle Amazonas Basin. The primary age of the granitic rocks is estimated at about 1560 Ma which correlates with recent age data obtained on granitic rocks on both sides of the Amazonas syneclise. In some areas the K/Ar ages show a decrease probably due to the effect of the Nickerie thermal episode described previously in the Guayana Shield.In the basement of the interconnected Lower Amazonas and Maranhão Basins Late Precambrian (Caririan) metamorphism and igneous activity seems to have been pervasive. The data allow the continuation of the Brazilian age province known in northeastern Brazil under the Maranhão Basin, linking this province to the Brasilia-Paraguay orogenic belt of similar age.  相似文献   

Location,location, location: changes in the diversity of animal resources exploited by Tardiglacial humans in northern Spain     

Pablo Turrero  Eva García‐Vázquez  Miguel Arbizu  Gema E. Adán 《第四纪科学杂志》2010,25(2):214-221

Prey diversity of Homo sapiens in north Iberia exhibited shifts associated with climate warming after the Last Glacial Maximum, as deduced from animal remains obtained from Upper Palaeolithic caves in Asturias (northern Spain). Significant association of prey diversity and river proximity was found during adverse climate conditions, indicating opportunistic predation. A high increase of prey diversity was found when small animals (including fish) were taken into account in archaeological records, highlighting the importance of this type of prey in Palaeolithic diets. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献