Neotectonics and morphotectonics of the southern Almería region (Betic Cordillera-Spain) kinematic implications

Most previous studies related to the recent tectonic development of southeastern Betic Cordillera, and in particular the south of Almería, agree with the kinematic variety that can be observed in the post-Miocene structures at different scales. This variety is difficult to explain if we consider only the regional stress field related to the convergence of the Eurasian and African plates that remained relatively steady over the last 9 Ma. This work proposes a block tectonics with horizontal and vertical relative movements driven by strike-slip movements of major faults to explain the variety of kinematics observed in the area from Middle Miocene to present. The combined activity of the Alpujarras fault zone (dextral strike-slip) and the Carboneras fault zone (sinistral strike-slip) induces, within the major block bounded by them, the squeezing of minor pre-existing fault bounded crustal blocks. This process generates traction areas inside the escaping block, which are linked with the existence of gradients of westward movement rate. This tectonics can explain some of the structures related with the extensional tectonics predominant in the south of Almería province and the development of the minor and major morphostructures driven by faults with different kinematics. The tectonics of blocks proposed is useful to give a coherent interpretation of apparently incoherent local structural data into a regional and homogeneous compressional stress field.Sadly, Professor José L. Hernández Enrile died in April 2003, and thus this paper represents his last thoughts on the topic. Reviews were addressed by the first author; however, some of the suggested changes were discussed to preserve Professor Hernández Enriless thoughts.     

Pre-Middle Jurassic accretionary metamorphism in the southern Klamath Mountains of northern California, USA

Abstract High- P/T metamorphic parageneses are preserved within two late Palaeozoic to early Mesozoic assemblages of the southern Klamath Mountains that show contrasting structural styles and mineral parageneses reflecting formation in different parts of a subduction-zone regime. Blueschist facies tectonites of the Stuart Fork terrane represent a coherent subduction complex formed at relatively deep crustal levels, whereas the chaotic metasedimentary mélange of the eastern Hayfork terrane contains a diverse range of metamorphic parageneses reflecting complex structural mixing of metamorphic components at shallower levels. The convergent-margin-type accretionary metamorphism evident in both terranes pre-dates Middle Jurassic low- P/T metamorphism resulting from regional tectonic contraction and magmatism.
The epidote- to lawsonite-zone Stuart Fork blueschists (and eclogites locally) formed at pressures of about 6-11 kbar and temperatures of 250-400° C. Deformed matrix material of the eastern Hayfork mélange formed at similar temperatures but lower pressures, on the order of 3-6 kbar. The mélange contains a diverse assemblage of tectonic blocks that formed under a range of P-T conditions, including those of the blueschist, pumpellyite-actinolite, greenschist and upper greenschist to amphibolite facies.
The succession of mineral parageneses and inferred P-T conditions of the eastern Hayfork blocks reflect those of igneous protolith formation, structural mixing, subduction-zone metamorphism, olistolith transport, and tectonic and erosional denudation. Although temporal relations are not well constrained, the evolution of these terranes is consistent with formation within a single convergent-margin system.     

40Ar/39Ar geochronology and P-T-t paths from the Cordillera Darwin metamorphic complex, Tierra del Fuego, Chile

Abstract ⁴⁰Ar/³⁹Ar data collected from hornblende, muscovite, biotite and K-feldspar constrain the P-T-t history of the Cordillera Darwin metamorphic complex, Tierra del Fuego, Chile. These data show two periods of rapid cooling, the first between c. 500 and c. 325° C at rates ≥25° C Ma^-1, and the second between c. 250 and c. 200°C. For high-T cooling, ⁴⁰Ar/³⁹Ar ages are spatially disparate and depend on metamorphic grade: rocks that record deeper and hotter peak metamorphic conditions have younger ⁴⁰Ar/³⁹Ar ages. Sillimanite- and kyanite-grade rocks in the south-central part of the complex cooled latest: ⁴⁰Ar/³⁹Ar Hbl = 73–77 Ma, Ms = 67–70 Ma, Bt = 68 Ma, and oldest Kfs = 65 Ma. Thermobarometry and P-T path studies of these rocks indicate that maximum burial of 26–30 km at 575–625° C may have been followed by as much as 10 km of exhumation with heating of 25–50° C. Staurolite-grade rocks have intermediate ⁴⁰Ar/³⁹Ar ages: Hbl = 84–86 Ma, Ms = 71 Ma, Bt = 72–75 Ma, and oldest Kfs = 80 Ma. Thermobarometry on these rocks indicates maximum burial of 19–26 km at temperatures of 550–580° C. Garnet-grade rocks have the oldest ages: Ms = 72 Ma and oldest Kfs = 91 Ma; peak P-T conditions were 525–550° C and 5–7 kbar. Regional metamorphic temperatures for greenschist facies rocks south of the Beagle Channel did not exceed c. 300–325° C from 110 Ma to the present, although the rocks are only 2 km from kyanite-bearing rocks to the north. One-dimensional thermal models allow limits to be placed on exhumation rates. Assuming a stable geothermal gradient of 20–25° C km^-1, the maximum exhumation rate for the St-grade rocks is c. 2.5 mm yr^-1, whereas the minimum exhumation rate for the Ky + Sil-grade rocks is c. 1.0 mm yr^-1. Uniform exhumation rates cannot explain the disparity in cooling histories for rocks at different grades, and so early differential exhumation is inferred to have occurred. Petrological and geochronological comparisons with other metamorphic complexes suggest that single exhumation events typically remove less than c. 20 km of overburden. This behaviour can be explained in terms of a continental deformation model in which brittle extensional faults in the upper crust are rooted to shallowly dipping ductile shear zones or regions of homogeneous thinning at mid- to deep-crustal levels. The P-T-t data from Cordillera Darwin (1) are best explained by a ‘wedge extrusion’model, in which extensional exhumation in the southern rear of the complex was coeval with thrusting in the north along the margin of the complex and into the Magallanes sedimentary basin, (2) suggest that differential exhumation occurred initially, with St-grade rocks exhuming faster than Ky + Sil-grade rocks, and (3) show variations in cooling rate through time that correlate both with local deformation events and with changes in plate motions and interactions.     

Evaluation and validation of landslide-susceptibility maps obtained by a GIS matrix method: examples from the Betic Cordillera (southern Spain)

This work presents the results of applying the matrix method in a Geographic Information System (GIS) to the drawing of maps of susceptibility to slope movements in different sectors of the Betic Cordillera (southern Spain). In addition, the susceptibility models built by the matrix method were compared with a multivariate statistical method, and the first method gave the best results. The susceptibility maps drawn by the GIS matrix method were validated by calculating the coefficients of association with the degree of fit between recent slope movements registered in 1997 and the different levels of susceptibility of previously drawn maps (1995–1996) in different representative zones of the Betic Cordillera (southern Spain). The first sector studied showed excellent degrees of fit, with an error of less than 10% for all the slope failures and 3% when considering only failures of natural origin. In the second sector, the relative errors were less than 5%. In the third sector, the error hardly exceeded 6%. The results are discussed in the different zones and for each type of slope movement. In any case, these results evidence the predictive capacity of susceptibility maps drawn in GIS by the matrix method, for a great number of slope movements.     

Structure and tectonics of the central segment of the Eastern Cordillera of Colombia

In the Eastern Cordillera of Colombia, a new structural model constrained by field data, paleontologic determinations, and interpretations of seismic reflection profiles is proposed. The model implies 70 km of shortening, including reactivation of basement structures as inverse faults in both flanks of the chain. These faults propagated within the lower Cretaceous strata, inducing passively rooted and transported thrust sheets as the successive basement faults were reactivated. Two structural styles are identified in the western flank: (1) positive flower structures in a transpressive regime, which affected rocks older than upper Paleocene and were unconformably covered by post–late Paleocene sediments, and (2) compressive structures during the late Miocene–Recent Andean phase. Presently, WNW-ESE compression reactivates Late Paleocene structures, which locally affect Andean trends. In the western margin of the Eastern Cordillera, the Cambao thrust takes up most displacement, whereas the Bituima fault takes only a minor part. To the south, this relationship reverses, suggesting complementary behavior by the Bituima and Cambao faults, as well as a transfer zone. This suggestion explains the southward termination of the Guaduas syncline as a structure related to the Cambao fault, whereas the Bituima fault increases its displacement southward, generating the Girardot foldbelt that takes over the structural position of the Guaduas syncline.     

Palaeomagnetic,geochronological and geological constraints on the tectonic evolution of the Mejillones Peninsula,northern Chile

Palaeomagnetic and geochronological data from an Early Palaeozoic high grade metamorphic complex (Jorgina Formation) and Jurassic layered basic intrusion (Moreno Complex) are reported from the Mejillones Peninsula of northern Chile (23–23°30'S). ⁴⁰Ar–³⁹Ar dates from the Lower Palaeozoic Jorgina Formation and the Moreno Complex are between 170 and 158 Ma, coincident with a phase of emplacement of the north Chilean coastal batholith. This suggests that intrusion and magnetization of the Moreno Complex and the metamorphism and remagnetization of the Jorgina Formation were related to batholith emplacement. Extracted stable components of magnetization from all units (17 sites) define site-mean directions with a scattered distribution. The scatter in site-mean directions is interpreted as being due to minor, localized, non-uniform, block-fault related (normal or strike-slip, or both) rotation after 158 Ma. The palaeomagnetic and geochronological data indicate that no significant large-scale latitudinal translation of crustal blocks has taken place in this part of northern Chile since the Late Jurassic. In addition, they indicate that the uniform clockwise rotation after the mid-Cretaceous which affected the adjacent Cordillera de la Costa either did not extend into the Mejillones Peninsula or took the form of localized block-fault rotations. The restriction of palaeomagnetically defined styles of rotation to discrete areas within the north Chilean forearc indicates that forearc wide block-fault rotation models are not applicable to the Pacific margin of northern Chile.