Carlos Sanz de Galdeano

Autopsy on a dead spreading center: The Phoenix Ridge, Drake Passage, Antarctica

New bathymetric and magnetic anomaly data from the Phoenix Ridge, Antarctica, show that extinction of all three remaining segments occurred at the time of magnetic chron C2A (3.3 ± 0.2 Ma), synchronous with a ridge-trench collision south of the Hero Fracture Zone. This implies that the ultimate cause of extinction was a change in plate boundary forces occasioned by this collision. Spreading rates slowed abruptly at the time of chron C4 (7.8 ± 0.3 Ma), probably as a result of extinction of the West Scotia Ridge, which would have led to an increase in slip rate and transpressional stress across the Shackleton Fracture Zone. Spectacular, high-relief ridges flanking the extinct spreading center, mapped for the first time using multibeam swath bathymetry, are interpreted as a consequence of a reduction in spreading rate, involving a temporary magma oversupply immediately prior to extinction.

Nature and impact of the Neotectonic deformation in the western Sierra Nevada (Spain)

Abstract The current high relief of the western sector of the Sierra Nevada was practically nonexistent before the Tortonian, as attested by the marine Late Neogene outcrops that reach a height of 1830 m, which would easily surpass 2000 m if not for erosion. The significant uplift produced, over 3500 m, has caused considerable thinning, detachments and gravitational slides in the Alpujarride units. The general displacements of these movements are towards the WNW to SSW. On the whole, the morphology is fan-like, adapting itself to the border of the Sierra Nevada. This border contains normal conjugate NW–SE and N–S to NNE–SSW faults, the latter also with sinistral strike–slip character. These faults have accommodated the uplift of the Sierra Nevada within an almost N–S compressive context and linked E–W extension. This sector coincides with the western boundary of the Nevado-Filabride complex and, in its southern continuation, with the western limit of the lower Alpujarride units, seeming to indicate a fault (or faults) reaching deep into the basement. Uplift has taken place unevenly over the last few million years, with three noteworthy periods: during the deposition of the Block Formation in the Late Tortonian, towards the Early?–Middle Pleistocene, and in the Middle?–Late Pleistocene. This has resulted in considerable deepening of the fluvial net surrounding Sierra Nevada at the present time.

Tectonic evolution of the Málaga Basin (Betic Cordillera). Regional implications

AbstractDuring the Neogene (uppermost Aquitanian-Lower Burdigalian, Tortonian and Pliocene), three successive marine episodes took place in the present-day Malaga Basin. The first of these affected a wide area of the Belic Internal Zones and was brought to an abrupt conclusion by the westward displacement of these Zones, together with important horizontal movements associated with N70-100 direction strike-slip faults and the superposition of materials from the Campo de Gibraltar. The two other marine episodes were clearly controlled by vertical movements of NW-SE and NK-SW faults, caused by a clear E-W distension which, according to regional data, was associated with some compression in an approximately N-S direction. The area has also been affected, although to a lesser extent, by the uplift of the Betic Cordillera from the Upper Miocene to the present day.

Isotopic identification of CO2 from a deep origin in thermomineral waters of southeastern Spain

Abstract The overexploitation of the Alto Guadalentin detrital aquifer has led to an unusually high gas content in the waters, which is released as the hydrostatic charge decreases. The predominant gas proved to be CO 2 . Isotopic studies show that δ 18 O values are in the range −8.8 to −6.5‰ SMOW, δ D ranges between −45 and −40‰ SMOW, excess deuterium lies between +8.0 and +27.4, and δ 13 C values lie between −8.1 and −3.8‰ PDB. This reveals that there is an enrichment in 18 O as well as mixing processes between meteoric and deep circulating waters. Tectonic and seismotectonic studies indicate crustal thinning in the studied region and numerous normal, reverse and strike-slip faults marked by positive thermal anomalies. There is also evidence of present day tectonic activity.

Palaeogeography of the late aquitanian-early Burdigalian basin in the western Betic Internal Zone

Abstract The discovery of new outcrops of transgressive sediments in the western Betic Internal Zone belonging to the UpperAquitanian-Lower Burdigalian (Blows N.4?, N.5 and N.6 zones of planktonic foraminifera and CN-1C calcareous nannoplankton of Okada & Bukry) shows that the basin was very large and was not restricted to the frontal part of the Internal Zone. This basin was formed in a distensive environment where marine sediments rich in siliceous materials accumulated at a depth of several hundred metres. It was tectonically controlled by faults at present showing an approximately N90 direction, which delimited several subsiding areas, between which some reliefs emerged. Sedimentation was interrupted by the tectonic superposition of the Campo de Gibraltar Complex during the Early Burdigalian. The evolution of the basin took place in the context of the important events occurring at this time in the western Mediterranean, particularly the uppermost Aquitanian-Burdigalian distension and the westward displacement of the Betic-Rifian Internal zone.

The transition from an active to a passive margin (SW end of the South Shetland Trench, Antarctic Peninsula)

Abstract The lateral ending of the South Shetland Trench is analysed on the basis of swath bathymetry and multichannel seismic profiles in order to establish the tectonic and stratigraphic features of the transition from an northeastward active to a southwestward passive margin style. This trench is associated with a lithospheric-scale thrust accommodating the internal deformation in the Antarctic Plate and its lateral end represents the tip-line of this thrust. The evolutionary model deduced from the structures and the stratigraphic record includes a first stage with a compressional deformation, predating the end of the subduction in the southwestern part of the study area that produced reverse faults in the oceanic crust during the Tortonian. The second stage occurred during the Messinian and includes distributed compressional deformation around the tip-line of the basal detachment, originating a high at the base of the slope and the collapse of the now inactive accretionary prism of the passive margin. The initial subduction of the high at the base of the slope induced the deformation of the accretionary prism and the formation of another high in the shelf—the Shelf Transition High. The third stage, from the Early Pliocene to the present-day, includes the active compressional deformation of the shelf and the base-of-slope around the tip-line of the basal detachment, while extensional deformations are active in the outer swell of the trench.

Garnet variety and zircon ages in UHP meta-sedimentary rocks from the Jubrique zone (Alpujárride Complex, Betic Cordillera, Spain): evidence for a pre-Alpine emplacement of the Ronda peridotite

The Ronda peridotite is a group of lherzolite slabs (1.5 to 2 km thick) in southern Spain. Despite clear evidence that pre-Alpine events affected pre-Permo-Triassic units from the Alborán domain (internal zone of the Betic-Rif Cordillera, Spain, and Morocco), numerous papers continue to emphasize Alpine metamorphic and structural evolution. Here, we evaluate the pre-Cenozoic evolution of the Ronda peridotite by reporting new petrographic and U–Pb SHRIMP zircon dating of meta-sedimentary rocks from the Jubrique zone (Alpujárride Complex, Betic Cordillera, Spain) directly overlying the Ronda peridotite. Field inspection and petrographical study revealed generalized migmatitic textures and a gradual transition mainly defined by garnet content (from ~30 to <3 wt.%) and size (from 1.5 cm to <0.5 mm) in the overlying granulite-gneiss sequence, suggesting that most garnet grew as a consequence of the peridotite emplacement. Garnet shows notable variations in composition and inclusion types, which are interpreted as reflecting different stages of garnet growth. Diamond-bearing garnets are only well-preserved in gneisses from the uppermost part of the sequence, whereas the large garnets from rocks overlying the peridotite mainly record later thermal events. SHRIMP zircon dating indicates two age peaks at 330 ± 9 and 265 ± 4 Ma. The oldest age characterizes rims overgrowing detrital cores and reflects an early Hercynian metamorphism; the younger age characterizes zircon with magmatic oscillatory zoning, reflecting anatexis. On the basis of these data and of previous dating of monazite included in the large garnets, we conclude that the peridotite was emplaced either shortly before or during early Hercynian times, ~330 Ma.

Metamorphic evolution of Triassic rocks from the transition zone between the Maláguide and Alpujárride complexes (Betic Cordilleras, Spain).

A complete sequence of mineral assemblages ranging from late diagenesis to the epizone has been identified in Triassic rocks from “intermediate units” between the Alpujarride and the Malaguide complexes (Betic Cordillera, Spain). These units appear as a set of tectonic slices, the uppermost showing lithological characteristics similar to the Malaguide complex, which change as depth increases, towards lithologies typical of the Alpujarride complex. The study was carried out by X-ray diffraction, optical microscopy, electron microprobe and X-ray fluorescence. The mineralogical composition shows a clear vertical evolution: A dickite-bearing assemblage, characteristic of the upper slices is replaced by sudoite ± pyrophyllite-bearing assemblages, these by trioctahedral chlorite-rich assemblages, and finally by paragonite-bearing assemblages. Examination of bulk-rock chemistries indicates that most of the mineralogical changes are the result of the increasing metamorphic grade, with the exception of chloritoid; its growth does appear to be partly controlled by the rocks chemistry (it is Al 2 O 3 dependent). The evolution of the metamorphic assemblages occurred prior to emplacement of the slices by thrusting and reveals a progressive transition in metamorphic grade between both complexes, although some discontinuities in the KI values between consecutive slices are observed.

COMPOSITIONAL AND STRUCTURAL VARIATION OF SUDOITE FROM THE BETIC CORDILLERA (SPAIN): A TEM/AEM STUDY

Sudoite from diagenetic to very low-grade metaclastites of the Betic Cordillera was studied by X-ray diffraction and transmission/analytical electron microscopy. Sudoite formed directly from dickite, the assemblage dickite + sudoite + illite being replaced at increasing metamorphic grade by the assemblage pyrophyllite + sudoite + illite. Sudoite ranges in composition from Mg-rich to Fe-rich chemistries. In addition, a wide variety of mixed-layered structures (illite-sudoite, pyrophyllite-sudoite, and dickite-sudoite) was also identified. Mg-rich sudoite shows a mean chemical composition of (Al2.91Fe0.252+Mg1.80)(Si3.10Al0.90)O10(OH)8, and a IIb ordered structure with b = 9.055 Å. Intermediate Fe-Mg sudoite exhibits a very variable composition, the Fe-rich phases having a mean composition of (Al2.09Fe0.613+Fe0.872+Mg1.44)(Si3.31Al0.69O10(OH)8. These are disordered polytypes with b values ranging from 9.070 to 9.101 Å. Fe occurs in both octahedral sheets, according to two types of substitutions: Fe3+ for Al in the dioctahedral sheet and Fe2+ for Mg in the trioctahedral sheet. Sudoite with such a composition has not been described previously.

Suhailite, a new ammonium trioctahedral mica

A new ammonium-bearing trioctahedral mica (suhailite) has been found in gneisses from the Betic Cordillera (Spain). Suhailite appears as aggregates of golden grains unoriented with respect to the main foliation. It shows weak pleochroism from light to darker yellow and low birefringence (0.028). X-ray data indicate the presence of two compositional variations: a NH4-rich phase (with a basal spacing of 10.40–10.44 Å) and a NH4-K intermediate phase (with basal spacing of 10.20–10.26 Å). Average composition, as deduced from analyses obtained at the scale of the scanning electron microscope is [Ca0.04Na0.07K0.35(NH4)0.55](Al0.42Ti0.22Fe1.33Mn0.01Mg0.71)Σ=2.70(Si2.67Al1.33)O10(OH)2. Thermal data indicate that maximum NH4 detachment occurs at 502 °C, suggesting a thermal stability similar to tobelite. Textural data indicate that unoriented golden grains consist of fine intergrowths of annite and suhailite and suggest that suhailite formed from primary red annite during the annite to fibrolite transformation.