ALBERT

Description: North American and Pacific spore-pollen records show a major extinction event at the Cretaceous-Tertiary (K-T) boundary, and abrupt changes are similarly found in many marine organisms world-wide. In contrast, records from the Old World reveal little evidence of terrestrial vegetational change across the boundary. In order to improve the characterization of changes across the K-T boundary, palynological assemblages from two sections in the southern Pyrenees have been evaluated. The abundance and diversity of trilete fern spores are high in Maastrichtian samples and show a statistically significant decrease during the Danian. The fern spike' of low-diversity spores found elsewhere is not recorded in the Pyrenean region. Minor replacements of taxa across the K-T boundary are also noted, as well as an increase in inaperturate gymnosperm pollen in the Danian. Comparing our two examined sections with one another reveals important differences in angiosperm pollen composition.

Description: This work analyses the present-day principal strain orientation on the downgoing slab of the South America Plate (SAM) beneath the Sandwich Plate (SAND). The strain regime was deduced from the study of 331 earthquake focal mechanism solutions examined by fault population analysis methods. In the slab, the maximum horizontal shortening direction (ey) rotates in trend in a clockwise direction from NE in the north, to SE in the south. Based on this rotation, three different areas were defined according to the prevailing focal mechanism type: (1) the North Zone, with ey oriented N058{degrees}E and reverse and strike-slip focal mechanisms; (2) the Central Zone, with only reverse focal mechanisms and ey striking N080{degrees}E; and (3) the South Zone, with ey oriented N106{degrees}E and reverse and strike-slip focal mechanisms. The strain field in the North Zone of the SAND involves decoupling of the slab at approximately 70 km depth. In contrast, the South Zone edge slab exhibits no decoupling and it exhibits different geometry (hook-like shaped) from the North Zone. Finally, we define the dextral strike-slip component acting at the South Sandwich Fracture Zone (SSFZ), according to focal mechanism solutions and the regional tectonic configuration.

Description: New structural, geochronological, and petrological data highlight which crustal sections of the North American-Caribbean Plate boundary in Guatemala and Honduras accommodated the large-scale sinistral offset. We develop the chronological and kinematic framework for these interactions and test for Palaeozoic to Recent geological correlations among the Maya Block, the Chortis Block, and the terranes of southern Mexico and the northern Caribbean. Our principal findings relate to how the North American-Caribbean Plate boundary partitioned deformation; whereas the southern Maya Block and the southern Chortis Block record the Late Cretaceous-Early Cenozoic collision and eastward sinistral translation of the Greater Antilles arc, the northern Chortis Block preserves evidence for northward stepping of the plate boundary with the translation of this block to its present position since the Late Eocene. Collision and translation are recorded in the ophiolite and subduction-accretion complex (North El Tambor complex), the continental margin (Rabinal and Chuacus complexes), and the Laramide foreland fold-thrust belt of the Maya Block as well as the overriding Greater Antilles arc complex. The Las Ovejas complex of the northern Chortis Block contains a significant part of the history of the eastward migration of the Chortis Block; it constitutes the southern part of the arc that facilitated the breakaway of the Chortis Block from the Xolapa complex of southern Mexico. While the Late Cretaceous collision is spectacularly sinistral transpressional, the Eocene-Recent translation of the Chortis Block is by sinistral wrenching with transtensional and transpressional episodes. Our reconstruction of the Late Mesozoic-Cenozoic evolution of the North American-Caribbean Plate boundary identified Proterozoic to Mesozoic connections among the southern Maya Block, the Chortis Block, and the terranes of southern Mexico: (i) in the Early-Middle Palaeozoic, the Acatlan complex of the southern Mexican Mixteca terrane, the Rabinal complex of the southern Maya Block, the Chuacus complex, and the Chortis Block were part of the Taconic-Acadian orogen along the northern margin of South America; (ii) after final amalgamation of Pangaea, an arc developed along its western margin, causing magmatism and regional amphibolite-facies metamorphism in southern Mexico, the Maya Block (including Rabinal complex), the Chuacus complex and the Chortis Block. The separation of North and South America also rifted the Chortis Block from southern Mexico. Rifting ultimately resulted in the formation of the Late Jurassic-Early Cretaceous oceanic crust of the South El Tambor complex; rifting and spreading terminated before the Hauterivian (c. 135 Ma). Remnants of the southwestern Mexican Guerrero complex, which also rifted from southern Mexico, remain in the Chortis Block (Sanarate complex); these complexes share Jurassic metamorphism. The South El Tambor subduction-accretion complex was emplaced onto the Chortis Block probably in the late Early Cretaceous and the Chortis Block collided with southern Mexico. Related arc magmatism and high-T/low-P metamorphism (Taxco-Viejo-Xolapa arc) of the Mixteca terrane spans all of southern Mexico. The Chortis Block shows continuous Early Cretaceous-Recent arc magmatism.

Description: Monitoring of real-time magnetic signals at Popocatepetl during 2006 has allowed discrimination of magma injection and dome growth. Magnetic signals correlated with seismic, volcanotectonic events and harmonic tremor, as well as number of small emissions, spring water pH, ash components and dome evolution helped define upward magma transport and yield a better understanding of the volcanic plumbing system. Magma ascent occurs mostly in periods of 7{+/-}3 days associated with harmonic tremor and decreasing magnetic signals between -1.1 and -15 nT, followed by increasing signals linked to cooling of the domes and increased seismicity over periods of 1 to more than 3 months. The dome clogs the vent after the negative magnetic anomaly-harmonic tremor period associated with magma ascent and forces an explosive crater-reopening explosion. Larger negative changes in the magnetic signals occurred in April (-6 nT), August (-3 to -6 nT) and October to December (-5 to -15 nT), associated with dome formation and growth. Negative magnetic anomalies preceded eruptions by 3 days in 2006.

Description: This work describes the coseismic ceiling block collapse within Benis Cave (-213 m; Murcia, SE Spain), associated with the 1999 Mula earthquake (mb=4.8, MSK VII). The collapse occurred at -156 m into the Earthquake Hall, and as a consequence one small gallery became blind. We studied the geology, topography and active tectonic structures relevant to the cave. In addition, we carried out a seismotectonic analysis of the focal mechanism solutions, and also a fault population analysis on slickensides measured in fault planes in the cave. The stress and strain regime is interpreted as being congruent with the palaeoseismic evidence, and agrees with the fault kinematics established for cave galleries developed within fault planes and growth anomalies of coral flowstone. Our analysis suggests that one active segment (NNE-SSW) determined the morphology and topography of the Benis Cave, where strong to moderate palaeoearthquakes (6[≤]M[≤]7) took place. As a consequence of this intense seismic activity a small gallery collapsed. A new palaeoseismic structure, or seismothem, has been recognized, namely the effect of palaeoearthquakes affecting the pattern of development of the spatial coral flowstone distribution located at the bottom of the cave.

Description: The ophiolites preserved in the Variscan suture of NW Iberia (Galicia) show a broad variability in lithology, geochemistry and chronology. This wide variety rules out the simplest plate tectonic scenario in which these ophiolites would have been exclusively related to the oceanic domain closed during the final Pangaea assembly, that is the Rheic Ocean. The ophiolitic units from Galicia also provide important data about the palaeogeography immediately preceding the opening of this ocean, and some information about pre-Gondwanan supercontinent cycles. Six different ophiolites can be distinguished in the allochthonous complexes of Galicia: the Purrido, Somozas, Bazar, Vila de Cruces, Moeche and Careon units. The Purrido Ophiolite is constituted by metagabbroic amphibolites with igneous protoliths dated at 1159{+/-}39 Ma (Mesoproterozoic), and geochemical affinities typical of island-arc tholeiites. These mafic rocks can be interpreted as one of the scarce members of the pre-Rodinian ophiolites, and they were probably generated in a back-arc setting in the periphery of the West African Craton. The Somozas Ophiolitic Melange consists of a mixing of submarine volcanic rocks (pillow-lavas, submarine breccias, pillow-breccias, hyaloclastites), diabases, gabbros, microgabbros, diorites and granitoids, surrounded by a matrix of serpentinites or, less frequently, phyllites. Two granitic samples from this melange yield U-Pb ages ranging between c. 527 and 503 Ma (Cambrian), which together with the characteristic arc signatures obtained in all the studied igneous rocks suggest that this ophiolite was generated in a peri-Gondwanan volcanic arc. The Bazar Ophiolite is formed by different tectonic slices with high temperature amphibolites, granulites, metagabbros and ultramafic rocks. The amphibolites are the most abundant rock type and show typical N-MORB compositions with igneous protoliths dated at 498{+/-}2 Ma (Cambrian). The high-temperature metamorphism affecting some parts of the unit has been dated at c. 480 Ma (lower Ordovician), and it is considered to be related to the development of an oceanic accretionary complex under the volcanic arc represented by the upper units of the allochthonous complexes of Galicia. Considering the most common palaeogeographic reconstructions for the Cambrian period, it is suggested that the oceanic lithosphere represented by the Bazar Ophiolite was formed into the peri-Gondwanan oceanic domain prior to the rifting of the Avalonian microcontinent, that is the Iapetus-Tornquist Ocean. According to current data about the Vila de Cruces Unit, it can be interpreted as a composite terrane, whose lithologies have U-Pb ages ranging from 1176-497 Ma, but constituted by metaigneous rocks with arc signatures. This dataset has been interpreted in relation to the development of a back-arc basin around the Cambrian-Ordovician limit, involving a Mesoproterozoic basement and the reactivation of a former suture. The opening of this back-arc basin can also be identified as the birth of the Rheic Ocean, and probably it would also include the lithological succession belonging to the Moeche Unit, although its basic rocks exhibit compositions with more oceanic character. Finally, the Careon Ophiolite includes remnants of an oceanic lithosphere generated in a supra-subduction zone setting at 395{+/-}2 Ma (middle Devonian). This ophiolite was formed in a contractive Rheic Ocean, shortly preceding the closure of this ocean. This is the only ophiolite in Galicia that can be related to mature stages of the Rheic Ocean, although as it is commonly observed in other regions the N-MORB crust is not preserved. This common oceanic crust has disappeared during subduction, probably in an intra-oceanic setting and during the generation of the igneous section preserved in the Careon Ophiolite.

Description: An early Turonian (c. 93 Ma) anoxic, cyclic marine deposition is registered in the unfolded outcrops from the Tarfaya coastal basin, where very high sedimentation rates enable the investigation of past geomagnetic field record at high temporal resolution. One hundred and fourteen samples have been sampled along a 10.5 m vertical profile (c. 200–500 ka) of orbital-scale forced sedimentation. Rock magnetic investigations reveal mineralogy principally controlled by diamagnetic and paramagnetic behaviour, along with very low concentrations of low-coercivity ferromagnetic material which is probably magnetite. A well-defined magnetic fabric can be seen with the minimum susceptibility axis perpendicular to the foliation plane, and magnetic lineation compatible with NW African palaeostress since sedimentation times and/or the palaeocurrent associated with upwelling system deposition. Magnetic signature has the potential for performing reliability checks of reversed tiny wiggles, which were found in four samples not considered for the tectonic analysis. Alternating field demagnetization shows a single, stable, low-coercivity directional component. The new palaeopole (N=88; PLat=64.3°, PLon=256.3°, A95=2.5°; K=38.7), obtained after moderate (f=0.8) inclination flattening correction, is the first early Turonian palaeopole for the NW African Craton. It can contribute to the 90 Ma-centred sliding window of the different proposed synthetic Apparent Polar Wander Paths.

Description: The Etive Dyke Swarm is the largest Caledonian dyke swarm in Britain and Ireland. Field and petrophysical (AMS-anisotropy of magnetic susceptibility) data are presented that indicate the dykes resulted from passive upwelling of magmas into fractures created by regional tectonic stresses. During the period of dyke intrusion (around 415 Ma) periodic episodes of sinistral transpression affected the region and were probably associated with sinistral movement on the Great Glen and other regional faults that lie subparallel (approximately 5{degrees} clockwise) from the long axis of the swarm. AMS data show that magma upwelled subvertically across the swarm and suggests that the dykes fed a volcanic field much larger than the now preserved Glencoe Volcano complex.

Description: Bacteria have contributed to the formation of minerals since the advent of life on Earth. Bacterial biomineralization plays a critical role on biogeochemical cycles and has important technological and environmental applications. Despite the numerous efforts to better understand how bacteria induce/mediate or control mineralization, our current knowledge is far from complete. Considering that the number of recent publications on bacterial biomineralization has been overwhelming, here we attempt to show the importance of bacteria-mineral interactions by focusing in a single bacterial genus, Myxococcus, which displays an unusual capacity of producing minerals of varying compositions and morphologies. First, an overview of the recent history of bacterial mineralization, the most common bacteriogenic minerals and current models on bacterial biomineralization is presented. Afterwards a description of myxobacteria is presented, followed by a section where Myxococcus-induced precipitation of a number of phosphates, carbonates, sulphates, chlorides, oxalates and silicates is described and discussed in lieu of the information presented in the first part. As concluding remarks, implications of bacterial mineralization and perspectives for future research are outlined. This review strives to show that the mechanisms which control bacterial biomineralization are not mineral- or bacterial-specific. On the contrary, they appear to be universal and depend on the environment in which bacteria dwell.

Description: The Cretaceous-Tertiary (K-T) boundary interval recovered by the ODP Leg 171 at Site 1049 (Blake Nose, NW Atlantic) contains a thick (9-17 cm) spherule bed marking the boundary. The spherules are mainly perfect spheres with a lesser proportion of oval spherules. They usually range from 100 to 1000 {micro}m. This bed represent the diagenetically altered impact ejecta from Chicxulub and further supports this structure as the site of the K-T impact. Mineralogical and geochemical investigations indicate that impact-generated glass was altered to smectite. Transmission electron microscopy observations revealed in some spherules that smectite is forming from a Si-rich or Ca-rich material, which could suggest a precursor similar to Haitian glasses. The variable thickness and the presence of some Cretaceous planktonic foraminifera and clasts of Cretaceous chalk suggest reworking of the ejecta material. However, the spherule bed confirms that a large volume of the Chicxulub ejecta material reached the Blake Nose Plateau.