Role of strike-slip faults in the Betic-Rifian orogeny

A new model for the Betic-Rifian orogeny of the Western Mediterranean (Spain and North Africa) is proposed in which four strike-slip faults play an important role; the faults are not of the same age. Two faults, the left-lateral Jebha fault to the south (in Morocco and principally in the Mediterranean Sea) and the right-lateral North Betic fault (southern Spain) to the north, define the boundaries of the Alboran block (Betic and Rifian internal zones). Final movement along these faults was during the Burdigalian time. Two other faults, the left-lateral Nekor fault (North Africa) to the south of the Jebha fault and the right-lateral Crevillente fault, somewhat to the north of the North Betic fault, define a larger Alboran block (including part of the Betic and Rifian external zones) that was present during the Tortonian.The following sequence of events is proposed:

1. (a) During the Eocene and Oligocene, the African and European plates converged in a N-S sense causing the breakup and overthrusting of the Betic, Rifian and Kabyle internal zones and then the movement towards the WSW of the Alboran block by slip along the Jebha and North Betic faults.

2. (b) By the end of Burdigalian time, movement along the Jebha and North Betic faults ceased.

3. (c) With continued N-S convergence, the Nekor and Crevillente faults, which bound a larger Alboran block, were formed during the mid- and late Miocene. The Arc of Gibraltar (the zone lying between the four major faults) seems to be a result of WSW motion of a crustal block being thrust over external zones.

The model proposed adds to the earlier idea that tectogenesis proceeds from the interior to the exterior of an erogenic belt. In the Betic-Rifian orogeny major strike-slip fracture zones shifted to the exterior of the orogenic belt as the orogeny progressed in order to relieve the stress caused by locking of the more internal faults.     

A new engineering-geological rock durability classification

The construction of the 5.3-m diameter, 82-km long Orange—Fish Tunnel in the Republic of South Africa, in sensibly horizontal beds of mudrocks, siltstones and sandstones of the Karoo Supergroup (Upper Carboniferous to Triassic age), revealed the paramount importance of rock durability. The presence of non-durable rock types was found to be one of the causes of the rock distress problems encountered in some tunnel sections during construction.

Geomechanical tests conducted on representative samples of potentially difficult rock types from the Orange—Fish Tunnel, indicated important limitations of international rock material classifications, such as the Deere—Miller Modulus—Strength and the Deere—Gamble Durability—Plasticity systems. The inadequacy of these classifications in the case of the Karoo sedimentary rocks, underlined the need for a new rock durability classification which could be used for the timely indication of the presence of non-durable rock material sections at tunnel level.

An account is given of the lithology, mineralogy and the postulated weathering mechanics of the Karoo rocks encountered, as well as the application of these parameters to the numerical methods used for the quantitative assessment of rock durability. It is considered that a reliable appraisal of this time-dependent parameter can be provided by the measurement of the strength and free swelling properties of the intact rock material.

A new rock durability classification, styled the Geodurability Classification, is discussed. This empirical system appears to be practical and simple as it depends on the minimum number of index rock properties, which can be measured fairly rapidly in a field laboratory by means of simple test apparatus and semi-skilled labour. It is based on different ranges of ratios of the uniaxial compressive strength, σ_c, and the “Duncan” free swelling coefficient, ε_D, as index parameters. The intact rock material is classified according to a rating system which varies from “excellent” (Class A) to “very poor” (Class F).

The extensive use of the Geodurability Classification indicated such a system to be particularly relevant to compacted and weakly cemented rocks, where the tendency of the rock material to deteriorate with time (and not the presence of rock discontinuities such as joints and bedding planes) dominates the time-dependent behaviour of tunnel rock masses.     

Textures in deforming forsterite aggregates up to 8 GPa and 1673 K

We report results from axisymmetric deformation experiments carried out on forsterite aggregates in the deformation-DIA apparatus, at upper mantle pressures and temperatures (3.1–8.1 GPa, 1373–1673 K). We quantified the resulting lattice preferred orientations (LPO) and compare experimental observations with results from micromechanical modeling (viscoplastic second-order self-consistent model—SO). Up to 6 GPa (~185-km depth in the Earth), we observe a marked LPO consistent with a dominant slip in the (010) plane with one observation of a dominant [100] direction, suggesting that [100](010) slip system was strongly activated. At higher pressures (deeper depth), the LPO becomes less marked and more complex with no evidence of a dominant slip system, which we attribute to the activation of several concurrent slip systems. These results are consistent with the pressure-induced transition in the dominant slip system previously reported for olivine and forsterite. They are also consistent with the decrease in the seismic anisotropy amplitude observed in the Earth’s mantle at depth greater than ~200 km.     

Ice-free glacial northern Asia due to dust deposition on snow

During the Last Glacial Maximum (LGM, 21 kyr BP), no large ice sheets were present in northern Asia, while northern Europe and North America (except Alaska) were heavily glaciated. We use a general circulation model with high regional resolution and a new parameterization of snow albedo to show that the ice-free conditions in northern Asia during the LGM are favoured by strong glacial dust deposition on the seasonal snow cover. Our climate model simulations indicate that mineral dust deposition on the snow surface leads to low snow albedo during the melt season. This, in turn, caused enhanced snow melt and therefore favoured snow-free peak summer conditions over almost the entire Asian continent during the LGM, whereas perennial snow cover is simulated over a large part of eastern Siberia when glacial dust deposition is not taken into account.     

Potential minerals for determining U–Th–Pb chemical age using electron microprobe

After a decade of studies and development, it is now accepted that reliable U–Th–total Pb isochron ages can be calculated for monazite using an electron microprobe at μm scale, either directly on thin sections or on separated grains mounted in polished section. The potential for determining U–Th–Pb chemical ages from other U- and Th-enriched phases has been investigated compared to chemical monazite-dating results for which individual spot-age precisions of 20 to 100 Ma can be achieved from individual spot analyses. Using isochron plots for monazite, the age homogeneity of a given population of data can be assessed and, depending upon the number of analyses (n  50), a precision of 5 to 10 Ma can be obtained. The U content in xenotime widely varies from less than 0.1 wt.% up to 3 wt.%, but Th rarely exceeds 1 wt.%. As a consequence, the amount of radiogenic Pb produced during a given period remains significantly lower for xenotime than for monazite, leading to a lower precision (± 20 Ma) on the mean ages. Xenotime, however, appears to remain as a closed system, but common Pb must be carefully checked. Furthermore, the electron-microprobe technique (EPMA) allows controlling any age discrepancy on xenotime grains as small as 10–20 μm that cannot be dated by other isotopic methods. Such xenotime ages can be useful when studying the monazite–xenotime equilibrium. The electron microprobe is not the most reliable method for dating zircon since U and Th concentrations are generally low and common Pb is not negligible. Nevertheless, the spatial resolution of EPMA coupled with isotope methods allows conclusive in situ studies about radiogenic Pb mobility and metamictization. Thorite does not seem suitable for dating with either isotope methods or EPMA because of continuous radiogenic Pb loss. Conversely, the oxide phases, thorianite and baddeleyite are robust minerals with closed systems. They are rather rare and seem to incorporate negligible common Pb, making EPMA a method of choice for dating them. For thorianite, the precision on the mean age can be similar as that obtained for monazite, or even better, while the precision for baddeleyite cannot be significantly better than 20 to 50 Ma due to the limited amount of U ( 0.1%) and the lack of Th.