Differential Late Paleozoic active margin evolution in South-Central Chile (37°S–40°S) – the Lanalhue Fault Zone

The N–S oriented Coastal Cordillera of South Central Chile shows marked lithological contrasts along strike at 38°S. Here, the sinistral NW–SE-striking Lanalhue Fault Zone (nomen novum) juxtaposes Permo-Carboniferous magmatic arc granitoids and associated, frontally accreted metasediments (Eastern Series) in the northeast with a Late Carboniferous to Triassic basal-accretionary forearc wedge complex (Western Series) in the southwest. The fault is interpreted as an initially ductile deformation zone with divergent character, located in the eastern flank of the basally growing, upwarping, and exhuming Western Series. It was later transformed and reactivated as a semiductile to brittle sinistral transform fault. Rb–Sr data and fluid inclusion studies of late-stage fault-related mineralizations revealed Early Permian ages between 280 and 270 Ma for fault activity, with subsequent minor erosion. Regionally, crystallization of arc intrusives and related metamorphism occurred between 306 and 286 Ma, preceded by early increments of convergence-related deformation. Basal Western Series accretion started at >290 Ma and lasted to 250 Ma. North of the Lanalhue fault, Late Paleozoic magmatic arc granitoids are nearly 100 km closer to the present day Andean trench than further south. We hypothesize that this marked difference in paleo-forearc width is due to an Early Permian period of subduction erosion north of 38°S, contrasting with ongoing accretion further south, which kinematically triggered the evolution of the Lanalhue Fault Zone. Permo-Triassic margin segmentation was due to differential forearc accretion and denudation characteristics, and is now expressed in contrasting lithologies and metamorphic signatures in todays Andean forearc region north and south of the Lanalhue Fault Zone.     

Spatial and temporal patterns of iceberg rafting (IRD) along the East Greenland margin,ca. 68°N,over the last 14 cal.ka

Twelve 1–2 m, 10-cm-diameter gravity cores collected in 1988 and 1991, from the continental shelf and fjords of East Greenland near Kangerlussuaq Fjord/Trough (ca. 68°N, 32°W), have distinct changes in lithofacies and in the quantity of iceberg rafted (IRD) sediments. These changes are readily observed in X-radiographs of the split cores. We quantify the IRD contribution through grain-size analyses and counting the number of clasts >2 mm from the X-radiographs. Chronological control is provided by acclerated mass spectroscopy ¹⁴C dates on foraminifera. During deglaciation, after 14 cal.ka there was one interval of IRD accumulation ca. 12–13 cal.ka, followed by a brief return to IRD conditions centred at 9 cal.ka. Thereafter, a prominent feature of most cores on the shelf is an increase in IRD accumulation that started ca. 5–6 cal.ka, and which has increased toward the present. Indicators of iceberg rafting, such as the net sand flux and numbers of clasts >2 mm ka⁻¹, follow a power law distribution when graphed against distance from the present East Greenland coast, a measure of the position of the glacier margins. The form of the relationship indicates that there is a dramatic decrease in the supply of sediment from the fjords to the shelf. These relationships are used to estimate changes in the location of the ice margin during the late Quaternary based on a site on the East Greenland slope, Denmark Strait, and to discuss factors that can negate such a simple transfer function. © 1997 by John Wiley & Sons Ltd.     

Talc friction in the temperature range 25°–400 °C: Relevance for Fault-Zone Weakening

Talc is one of the weakest minerals that is associated with fault zones. Triaxial friction experiments conducted on water-saturated talc gouge at room temperature yield values of the coefficient of friction, μ (shear stress, τ/effective normal stress, σ′_N) in the range 0.16–0.23, and μ increases with increasing σ′_N. Talc gouge heated to temperatures of 100°–400 °C is consistently weaker than at room temperature, and μ < 0.1 at slow strain rates in some heated experiments. Talc also is characterized by inherently stable, velocity-strengthening behavior (strength increases with increasing shear rate) at all conditions tested. The low strength of talc is a consequence of its layered crystal structure and, in particular, its very weak interlayer bond. Its hydrophobic character may be responsible for the relatively small increase in μ with increasing σ′_N at room temperature compared to other sheet silicates.Talc has a temperature–pressure range of stability that extends from surficial to eclogite-facies conditions, making it of potential significance in a variety of faulting environments. Talc has been identified in exhumed subduction zone thrusts, in fault gouge collected from oceanic transform and detachment faults associated with rift systems, and recently in serpentinite from the central creeping section of the San Andreas fault. Typically, talc crystallized in the active fault zones as a result of the reaction of ultramafic rocks with silica-saturated hydrothermal fluids. This mode of formation of talc is a prime example of a fault-zone weakening process. Because of its velocity-strengthening behavior, talc may play a role in stabilizing slip at depth in subduction zones and in the creeping faults of central and northern California that are associated with ophiolitic rocks.     

Chemistry of the 350°C hot springs on the crest of the East Pacific rise at 21°N

Hydrothermal fields on submarine spreading centres were first studied systematically during dives of the deep submersible ALVIN on the crest of the Galapagos Ridge in 86°W in the spring of 1977. While the exiting waters had temperatures only about 20°C above that of the ambient water column detailed analysis of their chemistry showed them to be formed by mixing of cold sea water (as “ground-water”) with a hydrothermal endmember of approximate temperature 350°C. Subsequently fields of hot springs with this temperature were found on the crest of the East Pacific Rise at 21°N by ALVIN in 2 600 metres water depth. Reconnaissance water sampling of these systems was made in November 1979 and a detailed study has just been completed (November 1981).The 350°C solutions are completely depleted of their original sea-water concentrations of Mg and SO₄. They are acid with a pH (25°C, 1 atmos) of 3.6 and an acidity of 400 μeq/kg. They contain about 7 mmol/kg of H₂S. The isotopic composition of this sulphur and the arsenic to sulphur ratio in the solutions indicate that about 85% of it is of igneous origin. The “soluble elements” Li, K and Rb are strongly enriched over the sea-water values, as are Ca and Ba. Sr is present at close to the sea-water concentrations however the isotopic compositon is identical to that of the basalts. The exiting solutions are clear and homogeneous super-critical fluids of in situ density approximately 0.65 g/cm³. Velocities in the throat of the orifices are around 1.5 m/sec. The iron concentrations are 1.8 mmol/kg and the Fe/Mn ratio is about 3. The reconnaissance samples gave Zn of 120 μol/kg and Cu and Ni of about 15 μol/kg.Upon mixing with sea-water the hot springs precipitate a voluminous black “smoke” predominantly composed of fine-grained FeS. Anhydrite is precipitated around the throat of the orifice producing chimney-like constructional features up to 10-m high. As these grow vertically the anydrite is replaced by sulphide minerals. The outer surface of the chimneys is colonized by several species of worms that secrete mats of tubes, up to several centimetres in diameter, composed of a tough organic material. Lateral growth of the chimneys via leaks in their walls leads to precipitation of sulphide minerals in a morphology controlled by the organic mats. All the numerous extinct sulphide deposits in the area have this characteristic surface texture.The active deposits on the EPR are unlike ophiolite type massive sulphides chemically, mineralogically and texturally. However, they do represent the primary precipitate. It appears that during lateral growth and coalescence of the chimneys in a given field the original deposit is reworked chemically as the 350°C solutions stream through the disequilibrium rapidly precipitated material. A “zone refined” substrate results consisting of coarsely crystalline, permeable relatively pure pyrite. This secondary deposit is, of course, capped with juvenile chimneys. It is these that probably constitute the ochres, the oxidized surficial zones of massive sulphides historically worked for silver and other elements present at only trace levels in the bulk deposit.     

Crustal attenuation in the Southern Andean retroarc (38°–39°30′ S) determined from tectonic and gravimetric studies: The Lonco-Luán asthenospheric anomaly

The western retroarc of the Southern Andes between 38° and 40° S is formed by a NNW-elongated ridge not associated with stacked thrust sheets. On the contrary, during the last 4–3 Ma this ridge was affected by extensional deformation, regional uplift and related folding on a very broad scale. Receiver function analysis shows that the drainage divide area and adjacent retroarc lie over an attenuated crust. Expected crustal thickness at these latitudes is around 38 km, whereas in this part of the retroarc the thickness is less than 32 km. The causes for such attenuation have been linked to a moderate steepening of the subducted Nazca plate beneath the South American plate, which is suggested by a westward shift and narrowing of the magmatic arc during the last 4 to 5 Ma. Gravimetric studies show that the upper plate did not react homogeneously to slab steepening, but ancient sutures and lithospheric discontinuities deeply buried under Mesozoic to Cenozoic sequences in the retroarc were locally reactivated. These processes resulted in an asthenospheric anomaly that correlates at the surface with the area of Pliocene to Quaternary doming, widespread extension and three radial troughs. Two of the troughs have accommodated substantial amounts of extension, but the third was probably aborted at an early stage. Moreover, the presence of an anomalous concentration of calderas and large volcanic centers over the proposed asthenospheric anomaly, and their age distribution, may indicate minor migration of the asthenospheric anomaly between 4 and 2 Ma through the western South American plate.     

Macrobioerosion and Microbioerosion in Marine Molluscan Shells from Holocene and Modern Beaches (39°−40°S, South of Buenos Aires Province, Argentina)

The marine sediments of the area of Verde Peninsula-Jabali Island(39°28′S/62°19′W-40°28′S/62°11′W) Holocene in age(3-2 ky),and modern beaches contain a significant amount of bioeroded mollusc shells.Fifteen sites were analyzed,in which 20.11%of the mollusc shells(2168 valves) presented bioerosion traces,in 54 species(30 bivalves and 24 gastropods).Fourteen ichnogenera were reported:Entobia,Maeandropoiydora,Iramena,Caulostrepsis,Pennatichnus,Pinaceocladichnus,Trypanites,and Gastrochaenolites(Domichnia),Gnathichnus and Radulichnus(Pascichnia),Finichnus and Centrichnus(Fixichnia),Oichnus(Praedicnia)(macrobioerosion),y Semidendrina(microbioerosion),the latter is first reported in mollusc shells in Argentina.Eleven ichnospecies were identified Finichnus peristroma,Maeandropoiydora sulcans,Gnathichnus pentax,Pinaceocladichnus onubensis,Caulostrepsis taeniola,Centrichnus eccentricus,Radulichnus inopinatus,Oichnus simplex,Oichnus paraboloides,Oichnus gradatus,and Gastrochaenolites torpedo(lithic remains).The dominant ichnogenera in the Holocene deposits are Iramena,Entobia and Oichnus.The same ichnogenera are constant with different abundance in the modern beaches,and increasing representation of Pinaceocladichnus and Pennatichnus.The dominant ichnofacies in the Holocene deposits is Trypanites,revealing a benthonic marine community composed of cheilostome bryzoans,clionaid sponges,predator gastropods,regular echinoids,polychaete annelids,bivalves,thallophytas and fungi.Generally,the area was described as a sublittoral,low-energy,stable environment with high rate of oxygenation,and sandy bottoms,with rocky bottoms at Villalonga locality.     

Exploration for scheelite in East Greenland — A case study

An intensive scheelite exploration program was carried out in Precambrian crystalline and sedimentary rocks intruded by granites of Precambrian and Caledonian ages in East Greenland (70–74°30′N). Previous heavy-mineral panning (2100 samples within an area of 100,000 km²) formed the basis for selection of scheelite-anomalous subregions (1550 km²)In the subregions, pan-concentrate samples were taken from first- and second-order rivers and from mid and side moraines of active glaciers. All samples were studied in the field under UV light, and scheelite grains were counted. A consideration of the distribution of scheelite in the samples together with the river and glacial drainage systems, led to definition of the potential source areas of the scheelite within localities of 2–5 km².Within the localities, panning of scree fines (samples every 100–200 m along talus slopes) and UV-light traverses at night led to the finding of outcropping or sub-outcropping scheelite mineralisation. Scheelite was observed associated with granite-carbonate contact zones, quartz vein stockworks, and fault zones in limestones, at nine localities within the 300-km-long zone of investigation.The heavy-mineral panning method with the counting of scheelite grains in the field and the subsequent definition of potential scheelite-bearing areas has the advantage that it is possible to execute a program from the subregional to the outcropping mineralisation stage in one field season. The investigation in this case was performed by five geologists during the 1979 2.5-month field season.     

Mechanism of chalcopyrite formation from iron monosulphides in aqueous solutions (< 100°C, pH 2–4.5)

In low-temperature aqueous solutions (< 100°C, pH 2–4.5), chalcopyrite (CuFeS₂) does not form through direct precipitation from solution. The pathway is exclusively via precursor iron sulphides and dissolved Cu salts. The reaction of dissolved Cu (II) salts with natural hexagonal pyrrhotite (Fe_0.9S) is diffusion controlled. The initial stage has an apparent activation energy of 11.4 ± 1.8 kJ mol⁻¹ and the rate (in units of mol dm⁻³s⁻¹ cm⁻²) is independent of the solid reactant surface area. The reaction proceeds through a series of metastable Cu-Fe-sulphide intermediaries. These phases form a series of ephemeral layers penetrating into the pyrrhotite surface. The first phase formed has the stoichiometry Cu_0.1Fe_0.9S. No Fe is released into the solution during its formation and this, together with the extremely low apparent activation energy and the stoichiometry, suggest that it is formed by stuffing of electron holes in the pyrrhotite structure with Cu ions. The transformation from the hexagonal close-packed arrangement of the pyrrhotite structure to the essentially cubic packing in chalcopyrite proceeds through a series of intermediaries, approximating in composition to members of the cubanite group. The rate of formation of these phases is controlled by the coupled diffusion of Fe (II), Fe (III), Cu (I) and Cu (II) species through the surface reaction zone, although the process as a whole can be approximated by steady-state diffusion of total Cu into a semi-infinite medium. Experiments with metastable precursor iron monosulphide phases, including amorphous FeS and synthetic mackinawite indicate similar reaction pathways.

The results suggest that chalcopyrite formation in low-temperature natural systems may be significantly constrained by kinetic factors. Chalcopyrite is, at least, a diagenetic mineral since its formation requires the prior formation of iron sulphides. However, at ambient temperatures its formation is probably limited to very early diagenesis.