Neogene palaeoceanographic changes recorded in a carbonate contourite drift (Santaren Channel,Bahamas)

The Santaren Drift between the Great Bahama Bank and Cay Sal Bank (Bahamas) is closely linked to the development of the Gulf Stream and its shape and geometry record the local to global oceanographic, climatic and tectonic events since the Miocene. High‐resolution multichannel seismic data from the Santaren Channel allow detailed insight into the growth phases of the contourite drift, and by using the stratigraphic information from Ocean Drilling Program Site 1006 to infer its sedimentation rates. The results bring new understanding to this region and to interpretation of carbonate drifts. The data document that the signatures of a bottom current flow in the Santaren Channel initiated about 12·3 Ma, as indicated by the first occurrence of sheeted drifts and moat development at the northern part of the Santaren Channel. Narrowing and steepening of moat flanks as well as the pronounced upslope migration of the moat reflects a sustained current acceleration of the bottom currents until 5·5 Ma, associated with a transformation into mounded elongated drifts. Between 5·5 Ma and 3·1 Ma, bottom current intensity reached its maximum probably caused by the final closure of the Central American Seaway. The last 3·1 Myr were characterized by a marked increase in volume through flow reaching a maximum during the past 900 kyr. Drift growth was driven by the combined sources of export from the shallow‐water carbonate factory and by pelagic rain. The Middle Miocene channel‐related sheeted drift of the inner Santaren Channel is characterized by low accumulation rates, but a rapid increase of accumulation rates occurred during the Early Pliocene. The contourite drift buildup was disturbed by minor erosional phases with narrow moats in the Late Pliocene due to increasing bottom‐current velocities forced by strengthened Atlantic Ocean ventilation. The Early Pleistocene was dominated by increased periplatform sedimentation and margin progradation facilitated by a reduction in along‐slope current flow speed and a concurrent widening and flattening of the moats.     

Formation of the Wiesloch Mississippi Valley-type Zn-Pb-Ag deposit in the extensional setting of the Upper Rhinegraben, SW Germany

The Mississippi Valley-type (MVT) Zn–Pb–Ag deposit in the Wiesloch area, Southwest Germany, is controlled by graben-related faults of the Upper Rhinegraben. Mineralization occurs as vein fillings and irregular replacement ore bodies consisting of sphalerite, banded sphalerite, galena, pyrite, sulfosalts (jordanite and geocronite), barite, and calcite in the Middle Triassic carbonate host rock. Combining paragenetic information, fluid inclusion investigations, stable isotope and mineral chemistry with thermodynamic modeling, we have derived a model for the formation of the Wiesloch deposit. This model involves fluid mixing between ascending hot brines (originating in the crystalline basement) with sedimentary formation waters. The ascending brines originally had a near-neutral pH (around 6) and intermediate oxidation state, reflecting equilibrium with granites and gneisses in the basement. During fluid ascent and cooling, the pH of the brine shifted towards more acidic (around 4) and the oxidation state increased to conditions above the hematite-magnetite buffer. These chemical characteristics contrast strongly with those of the pore and fracture fluid residing in the limestone aquifer, which had a pH between 8 and 9 in equilibrium with calcite and was rather reduced due to the presence of organic matter in the limestone. Mixing between these two fluids resulted in a strong decrease in the solubility of silver-bearing sphalerite and galena, and calcite. Besides Wiesloch, several Pb–Zn deposits are known along the Upper Rhinegraben, including hydrothermal vein-type deposits like Badenweiler and the Michael mine near Lahr. They all share the same fluid origin and formation process and only differ in details of their host rock and fluid cooling paths. The mechanism of fluid mixing also seems to be responsible for the formation of other MVT deposits in Europe (e.g., Réocin, Northern Spain; Trèves, Southern France; and Cracow-Silesia, Poland), which show notable similarities in terms of their age, mineralogy. and mineral chemistry to the MVT deposit near Wiesloch.     

REE systematics of fluorides,calcite and siderite in peralkaline plutonic rocks from the Gardar Province,South Greenland

The Gardar failed-rift Province is world-famous for its (per-)alkaline plutonic rocks. Elevated contents of F in the mantle source and F-enrichment in the parental melts have been suggested to account for the peculiarities of the Gardar rocks (e.g. their rare mineralogy, extreme enrichment of HFSE elements, Be or REE in the Ilímaussaq agpaites, and the formation of the unique Ivigtut cryolite deposit). To constrain the formation and chemical evolution of F-bearing melts and fluids, fluorides (fluorite, cryolite, villiaumite, cryolithionite), calcite and siderite from the Ilímaussaq, Motzfeldt and Ivigtut complexes were analysed for their trace element content focusing on the rare earth elements and yttrium (REE).The various generations of fluorite occurring in the granitic Ivigtut, agpaitic Ilímaussaq and miaskitic to agpaitic Motzfeldt intrusions all share a negative Eu anomaly which is attributed to (earlier) feldspar fractionation in the parental alkali basaltic melts. This interpretation is supported by the abundance of anorthositic xenoliths in many Gardar plutonic rocks.The primary magmatic fluorites from Ilímaussaq and Motzfeldt display very similar REE patterns suggesting a formation from closely related parental melts under similar conditions. Hydrothermal fluorites from these intrusions were used to constrain the multiple effects responsible for the incorporation of trace elements into fluorides: temperature dependence, fluid migration/interaction and complexation resulting in REE fractionation. Generally, the REE patterns of Gardar fluorides reflect the evolution and migration of a F/CO₂-rich fluid leading to the formation of fluorite and fluorite/calcite veins. In certain units, this fluid inherited the REE patterns of altered host rocks. In addition, there is evidence of an even younger fluid of high REE abundance which resulted in highly variable REE concentrations (up to three orders of magnitude) within one sample of hydrothermal fluorite.The REE patterns of the granitic Ivigtut intrusion show flat to slightly heavy-REE-enriched patterns characterised by a strong tetrad effect. This effect is interpreted to record extensive fluid–rock interaction in highly fractionated, Si-rich systems.Interestingly, the fluorides appear to record different source REE patterns, as the spatially close Motzfeldt and Ilímaussaq intrusions show strong similarities and contrast with the Ivigtut intrusion located 100 km NE. These variations may be attributed to differences in the tectonic position of the intrusions or mantle heterogeneities.     

Controls on sonic velocity in carbonates

Compressional and shear-wave velocities (V _p andV _s) of 210 minicores of carbonates from different areas and ages were measured under variable confining and pore-fluid pressures. The lithologies of the samples range from unconsolidated carbonate mud to completely lithified limestones. The velocity measurements enable us to relate velocity variations in carbonates to factors such as mineralogy, porosity, pore types and density and to quantify the velocity effects of compaction and other diagenetic alterations.Pure carbonate rocks show, unlike siliciclastic or shaly sediments, little direct correlation between acoustic properties (V _p andV _s) with age or burial depth of the sediments so that velocity inversions with increasing depth are common. Rather, sonic velocity in carbonates is controlled by the combined effect of depositional lithology and several post-depositional processes, such as cementation or dissolution, which results in fabrics specific to carbonates. These diagenetic fabrics can be directly correlated to the sonic velocity of the rocks.At 8 MPa effective pressureV _p ranges from 1700 to 6500 m/s, andV _s ranges from 800 to 3400 m/s. This range is mainly caused by variations in the amount and type of porosity and not by variations in mineralogy. In general, the measured velocities show a positive correlation with density and an inverse correlation with porosity, but departures from the general trends of correlation can be as high as 2500 m/s. These deviations can be explained by the occurrence of different pore types that form during specific diagenetic phases. Our data set further suggests that commonly used correlations like Gardner's Law (V _p-density) or the time-average-equation (V _p-porosity) should be significantly modified towards higher velocities before being applied to carbonates.The velocity measurements of unconsolidated carbonate mud at different stages of experimental compaction show that the velocity increase due to compaction is lower than the observed velocity increase at decreasing porosities in natural rocks. This discrepancy shows that diagenetic changes that accompany compaction influence velocity more than solely compaction at increasing overburden pressure.The susceptibility of carbonates to diagenetic changes, that occur far more quickly than compaction, causes a special velocity distribution in carbonates and complicates velocity estimations. By assigning characteristic velocity patterns to the observed diagenetic processes, we are able to link sonic velocity to the diagenetic stage of the rock.     

Mullite-corundum-spinel-cordierite-plagioclase xenoliths in the Skaergaard Marginal Border Group: multi-stage interaction between metasediments and basaltic magma

Metapelitic country rocks were contact- and pyro-metamorphosed by the Tertiary Skaergaard Intrusion, East Greenland. In an initial stage of heating, while they were probably still in place within the host rock contact aureole, they overstepped a range of equilibrium and disequilibrium melting reactions and produced both a granitic melt and very refractory spinel+cordierite+plagioclase±corundum residuals. Parts of these refractory rocks were then subjected to another melting event after being entrained as xenoliths into the Skaergaard Marginal Border Group, where they experienced a temperature of about 1,000°C at a pressure of about 650 bars and at an oxygen fugacity about 0.2–0.5 log units below the FMQ buffer. Here, they underwent bulk melting, but did not mix with the Skaergaard magma, presumably because of the high viscosity contrast. The Al-rich melts crystallized to an assemblage of corundum+mullite+sillimanite+ plagioclase+spinel+rutile±tridymite±cordierite and they reacted with the surrounding basalt producing a strongly cryptically zoned rim of plagioclase (An₅₅ close to the basalt to An₉₀ close to the Al-rich melt). The assemblage in the inner parts of the xenoliths provides textural evidence for disequilibrium growth due to slow diffusivities in the highly viscous, probably water-free Al-rich melt. Later interaction of lower temperature ferrobasaltic to granophyric melts with the xenoliths along their margins and along cracks led to consumption of corundum and mullite and to the stable assemblage of spinel+cordierite+plagioclase+quartz+K-feldspar +magnetite+ilmenite at about 800°C.     

A Cretaceous carbonate delta drift in the Montagna della Maiella,Italy

The Upper Cretaceous (Campanian–Maastrichtian) bioclastic wedge of the Orfento Formation in the Montagna della Maiella, Italy, is compared to newly discovered contourite drifts in the Maldives. Like the drift deposits in the Maldives, the Orfento Formation fills a channel and builds a Miocene delta‐shaped and mounded sedimentary body in the basin that is similar in size to the approximately 350 km² large coarse‐grained bioclastic Miocene delta drifts in the Maldives. The composition of the bioclastic wedge of the Orfento Formation is also exclusively bioclastic debris sourced from the shallow‐water areas and reworked clasts of the Orfento Formation itself. In the near mud‐free succession, age‐diagnostic fossils are sparse. The depositional textures vary from wackestone to float‐rudstone and breccia/conglomerates, but rocks with grainstone and rudstone textures are the most common facies. In the channel, lensoid convex‐upward breccias, cross‐cutting channelized beds and thick grainstone lobes with abundant scours indicate alternating erosion and deposition from a high‐energy current. In the basin, the mounded sedimentary body contains lobes with a divergent progradational geometry. The lobes are built by decametre thick composite megabeds consisting of sigmoidal clinoforms that typically have a channelized topset, a grainy foreset and a fine‐grained bottomset with abundant irregular angular clasts. Up to 30 m thick channels filled with intraformational breccias and coarse grainstones pinch out downslope between the megabeds. In the distal portion of the wedge, stacked grainstone beds with foresets and reworked intraclasts document continuous sediment reworking and migration. The bioclastic wedge of the Orfento Formation has been variously interpreted as a succession of sea‐level controlled slope deposits, a shoaling shoreface complex, or a carbonate tidal delta. Current‐controlled delta drifts in the Maldives, however, offer a new interpretation because of their similarity in architecture and composition. These similarities include: (i) a feeder channel opening into the basin; (ii) an excavation moat at the exit of the channel; (iii) an overall mounded geometry with an apex that is in shallower water depth than the source channel; (iv) progradation of stacked lobes; (v) channels that pinch out in a basinward direction; and (vi) smaller channelized intervals that are arranged in a radial pattern. As a result, the Upper Cretaceous (Campanian–Maastrichtian) bioclastic wedge of the Orfento Formation in the Montagna della Maiella, Italy, is here interpreted as a carbonate delta drift.     

Discrimination of a seismic source doublet in the Northridge, California earthquake of 17 January 1994

The January 17, 1994 Northridge earthquake (Mw = 6.7, 34.213° N, 118.537° W, depth = 18.4 km) was recorded extensively in the immediate source region by strong, ground motion accelerometers. The resulting seismograms show complex S wave patterns. Nevertheless, visual correlations of the strong-ground-motion velocity and displacement time-histories clearly identify two significant wave pulses: a secondary S pulse (called S2) arriving 3–5 seconds after the initial S wave pulse (called S1). A plausible assumption is that these phases are generated at areas on the rupturing thrust fault that experienced especially large slip. Conventional travel-time computations, relating the relative arrival times between the onsets of the primary S1 and secondary S2 phases, yield a hypocenter of the initiation point, constrained to a independently etimated fault plane, of the secondary wave source (called H2) at 34.26°N, 118.54° W, with a depth of 14.1 km; the 68% confidence error in depth is 1.3 km. This location is about 6 km up-dip and north from the estimated hypocenter, on the fault plane of the initial principal seismic source (called H1). The seismic moment for both the initial H1 and secondary source H2 was estimated from the SH displacement pulse. Values averaged over eight stations were 8.61 ± 9.56 × 10²⁴ dyne-cm and 2.49 ± 2.31 × 10²⁵ dyne-cm respectively. Reasons why the sum of the two seismic moments is smaller than the total estimated seismic moment of 1.2 × 10²⁶ dyne-cm for the Northridge earthquake are discussed. The location of the initiation point of a second source H2 in the Northridge thrust faulting is consistent with independent computations of the fault slip pattern. The estimated stress drop for the initial and secondary sources are 1 = 150 ± 15 bars and 2 = 110 ± 11 bars, respectively.     

Non-destructive damage evaluation of a structure from limited modal parameters

A method to predict structural damage in its location and severity from modal characteristics of the damaged structure is proposed. No a priori knowledge of the modal characteristics of a corresponding baseline structure is required in the proposed formulation. Instead, information on the geometry of the structure which is reflected in its mass and stiffness distribution is needed. From matrix structural analysis, a system of equations is generated which relates the relative change of stiffness of structural members to a load vector generated from modal parameters of the damaged structure. Different solution techniques are suggested to determine the damage from the generated equations. The feasibility of the proposed formulation is demonstrated via a numerical example of a 10-storey building. Further, an error investigation on the error in the damage predictions due to uncertainties in the input data is carried out.