Cumulate gabbros from the Southwest Indian Ridge, 54°S-7° 16′ E: implications for magmatic processes at a slow spreading ridge

A diverse volcanic and plutonic rock suite was recovered from the center of the 80 km long ridge segment of the Southwest Indian Ridge (54°S, 7°16 E) between the Islas Orcadas and Shaka Fracture Zones. The cumulus nature of the gabbroic rocks in the suite is indicated by phase, modal and cryptic layering, igneous lamination, and low incompatible element abundances. We present a mass-balance model for calculating the proportions and compositions of cumulus phases and crystallized intercumulus liquid from bulk-rock major element compositions. The model is based on the ability to define a compositional array of basaltic liquids and on the assumption that cumulus minerals are initially in equilibrium with trapped liquid. Calculated proportions of trapped liquid range from 3%–15%; values that are characteristic of adcumulates to mesocumulates. Models of postcumulus crystallization indicate significant enrichments of incompatible elements and buffering of compatible elements in residual trapped liquids, thus explaining the high TiO₂ contents observed in magnesian clinopyroxenes. Cumulus phase assemblages and compositions suggest solidification in shallow level magma chambers, but disequilibrium plagioclase compositions suggest some crystallization at greater depth. Furthermore, basalt compositions projected onto the olivine-clinopyroxenequartz pseudoternary suggest magma generation over a range of pressures (from less than 10 to greater than 20 kb) as well as polybaric fractional crystallization. We suggest that the Southwest Indian Ridge is characterized by low magma supply with small batches of melt that either ascend directly to the surface having undergone limited polybaric crystallization or are trapped in shallow crustal magma chambers where they evolve and solidify to form cumulate gabbros. The adcumulus nature of the gabbros investigated here suggests slow cooling rates typical of large intrusions implying relatively large, but ephemeral magma chambers below segments of the Southwest Indian Ridge.     

Habitat partitioning between the xanthid crabsPanopeus herbstii andEurypanopeus depressus on intertidal oyster reefs (Crassostrea virginica) in southeastern North Carolina

The abundances of the xanthid crabsPanopeus herbstii andEurypanopeus depressus were examined relative to surface oyster shell cover, surface oyster cluster volume, subsurface shell content, substrate sand and silt composition, and oyster reef elevation. During August 1986 through July 1987, xanthid crabs were collected monthly from twelve 0.25 m²×15 cm deep quadrats, during low tide, from intertidal oyster reefs in Mill Creek, Pender County, North Carolina, USA, with respective quadrat details recorded. The abundance ofP. herbstii, and to a lesser degreeE. depressus, was positively correlated with surface shell cover. The abundance ofE. depressus, and to a lesser degreeP. herbstii, was positively correlated with surface cluster volume. The majority ofP. herbstii inhabited the subsurface stratum of the oyster reef, whereas the majority ofE. depressus inhabited the cluster stratum. Seasonality (i.e., temperature) appeared to influence the strata habitation of both species, with a higher incidence of cluster habitation during warmer months and a lower incidence during colder months. Crab abundance was not related to other factors examined, such as subsurface shell, substrate sand and silt composition, or elevation within the oyster reef. The analyses show thatP. herbstii andE. depressus have partitioned the intertidal oyster reef habitat, withE. depressus exploiting surface shell clusters andP. herbstii the subsurface stratum.     

Analysis of swarms of high-frequency seismic events at Nevado del Ruiz volcano, Colombia (January 1986–August 1987): Development of a procedure

The determination of signal properties like frequency, amplitude, “signature” of the sequence of arrivals of P- and S-phases and the subsequent computation of hypocenter parameters, are the steps of a procedure in the analysis of swarms of high-frequency seismic events from Nevado del Ruiz volcano, which could be useful in other volcanic areas. The use of this procedure on Ruiz data led to the determination of various seismic source volumes, which together with hypocenters of non-swarmed events and the identification of geological features such as faults which cross the area, alignment of domes and volcanic edifices, and the record of low-frequency seismic events not centered in the area of the active vent, suggest the existence of a magma chamber of a rather complex geometry. This hypothesis would suggest that magma, emplaced in zones of weakness (faults) that affect the area, may act as the source of seismic activity. This demands a revision of the methodology used in the deformation measurements, and in the analysis of released energy and event occurrence too. The relations between hypocenters (sources), location and relative energy of the swarms, as well as the general volcanic activity (tremor, ash and gas emission, deformation) could turn out to be a possible means to assist in the identification of premonitory activity and thus be relevant to the process of monitoring and forecasting.     

Efficiency test of earthquake prediction around Thessaloniki from electrotelluric precursors

Since the completion of the network in January 1983, the electric field of the earth has been continuously monitored at four sites near Thessaloniki, the capital of northern Greece. From the present study and from previous investigations by similar measurements in Greece, it is evident that transient changes of the electrotelluric field occur prior to earthquakes. The analysis of these electric forerunners leads in many cases to a successful prediction of the epicentral area, the magnitude and the time of the impending event. Predictions prior to regional earthquakes are issued and documented with telegrams.From November 1983 until the end of May 1984 twelve earthquakes (M_L > 3.5) occurred in the vicinity of Thessaloniki. Ten of these were predicted and warnings given by telegram, whereas two smaller seismic events were missed. Two additional predictions were unsuccessful. Independent of their magnitudes, predicted events took place within a time window of 6 hrs to 6 days after the observations of the electrotelluric anomalies. The accuracy of the predicted epicenters in eight cases is better than 100 km, which corresponds roughly to the mean distance between the electric stations. Magnitude estimates deviate by less than 0.5 magnitude units from the seismically observed ones.Considering the two largest earthquakes, it is shown that the probability of making each of these predictions by chance is of the order of 10⁻².     

Deglaciation and postglacial timberline in the San Juan Mountains,Colorado

Lake Emma, which no longer exists because of a mining accident, was a tarn in a south-facing cirque near the headwaters of the Animas River in the San Juan Mountains of southwestern Colorado. During the Pinedale glaciation, this area was covered by a large transection glacier centered over the Lake Emma region. Three radiocarbon dates on basal organic sediment from Lake Emma indicate that by ca. 15,000 yr B.P. this glacier, one of the largest in the southern Rocky Mountains, no longer existed. Twenty-two radiocarbon dates on Picea and Abies krummholz fragments in the Lake Emma deposits indicate that from ca. 9600 to 7800 yr B.P., from 6700 to 5600 yr B.P., and at 3100 yr B.P. the krummholz limit was at least 70 m higher than present. These data, in conjunction with Picea:Pinus pollen ratios from both the Lake Emma site and the Hurricane Basin site of J. T. Andrews, P. E. Carrara, F. B. King, and R. Struckenrath (1975, Quaternary Research 5, 173–197) suggest than from ca. 9600 to 3000 yr B.P. timberline in the San Juan Mountains was higher than present. Cooling apparently began ca. 3000 yr B.P. as indicated by decreases in both the percentage of Picea pollen and Picea:Pinus pollen ratios at the Hurricane Basin site (Andrews et al., 1975). Cooling is also suggested by the lack of Picea or Abies fragments younger than 3000 yr B.P. at either the Lake Emma or the Hurricane Basin site.     

Spectra of the earthquake sequence February-March, 1981, in south-central Sweden

On February 13, 1981 a relatively strong earthquake occurred in the Lake Vänern region in south-central Sweden. The shock had a magnitude ofM_L = 3.3 and was followed within three weeks by three aftershocks, with magnitudes 0.5 ≤ M_L ≤ 1.0. The focal mechanism solution of the main shock indicates reverse faulting with a strike in the N-S or NE-SW direction and a nearly horizontal compressional stress. The aftershocks were too small to yield data for a full mechanism solution, but first motions of P-waves, recorded at two stations, are consistent for the aftershocks. Dynamic source parameters, derived from Pg- and Sg-wave spectra, show similar stress drops for the main shock (2 bar) and the aftershocks (1 bar), while the differences in seismic moment (1.5·10²⁰ resp. 4·10¹⁸dyne cm), fault length (0.7 resp. 0.2 km) and relative displacement (0.15 resp. 0.03 cm) are significant.     

GPS network monitors the Western Alps' deformation over a five-year period: 1993–1998

  The Western Alps are among the best studied collisional belts with both detailed structural mapping and also crustal geophysical investigations such as the ECORS and EGT seismic profile. By contrast, the present-day kinematics of the belt is still largely unknown due to small relative motions and the insufficient accuracy of the triangulation data. As a consequence, several tectonic problems still remain to be solved, such as the amount of N–S convergence in the Occidental Alps, the repartition of the deformation between the Alpine tectonic units, and the relation between deformation and rotation across the Alpine arc. In order to address these problems, the GPS ALPES group, made up of French, Swiss and Italian research organizations, has achieved the first large-scale GPS surveys of the Western Alps. More than 60 sites were surveyed in 1993 and 1998 with a minimum observation of 3 days at each site. GPS data processing has been done by three independent teams using different software. The different solutions have horizontal repeatabilities (N–E) of 4–7 mm in 1993 and 2–3 mm in 1998 and compare at the 3–5-mm level in position and 2-mm/yr level in velocity. A comparison of 1993 and 1998 coordinates shows that residual velocities of the GPS marks are generally smaller than 2 mm/yr, precluding a detailed tectonic interpretation of the differential motions. However, these data seem to suggest that the N–S compression of the Western Alps is quite mild (less than 2 mm/yr) compared to the global convergence between the African and Eurasian plate (6 mm/yr). This implies that the shortening must be accomodated elsewhere by the deformation of the Maghrebids and/or by rotations of Mediterranean microplates. Also, E–W velocity components analysis supports the idea that E–W extension exists, as already suggested by recent structural and seismotectonic data interpretation. Received: 27 November 2000 / Accepted: 17 September 2001     

Climate change in Yellowstone National Park: Is the drought-related risk of wildfires increasing?

The increased frequency of wildfires in the United States has become a common prediction associated with the build-up of greenhouse gases. In this investigation, variations in annual wildfire data in Yellowstone National Park are compared to variations in historical climate conditions for the area. Univariate and multivariate analytical techniques reveal that (a) summer temperatures in the Park are increasing, (b) January-June precipitation levels are decreasing, and (c) variations in burn area within the Park are significantly related to the observed variations in climate. Outputs from four different general circulation model simulations for 2 × CO₂ are included in the analyses; model predictions for increasing aridity in the Yellowstone Park area are generally in agreement with observed trends in the historical climate records.     

Glass crust on intratelluric phenocrysts in volcanic ash as a measure of eruptive violence

A comparison of each of the volumes of nine deposits of airborne volcanic ash from the Coatepeque Volcano in El Salvador, Central America to the amount of glass crust adhering to intratelluric phenocrysts shows an inverse relationship. As violence, volume, and eruptive abrasion increase, glass crusts decrease. The eruptive violence of a prehistoric eruption and the original volume of ash can thus be calculated from glass-to-crystal ratios observed in erosional remnants.     

Luna 20: mineralogy and petrology of fragments less than 125 μm size

Two milligrams of less than 125 μm size particles from the Luna 20 soil sample (22001,17) have been examined. The results of the mineral identification and analysis verify earlier assumptions that the nature of the lunar highlands is predominantly anorthositic in composition. The presence of highly magnesian clinopyroxene, orthopyroxene and olivine suggests crystallization of Highland rocks in an ultrabasic environment. No fragments were observed that could be unequivocally assigned to mare basalt types.