Age of amphibolites associated with alpine peridotites in the Dinaride ophiolite zone,Yugoslavia

Amphibolites associated with alpine peridotites in the Central Ophiolite zone in Yugoslavia have K-Ar ages of 160–170 m.y. These amphibolites and associated peridotites underwent deep-seated metamorphism prior to tectonic emplacement into the sedimentary-volcanic assemblage of the Dinarides. The alpine peridotites and associated local rocks of the ophiolite suite are interpreted as Jurassic oceanic crust and upper mantle.     

The origin of high-amplitude magnetic anomalies at the intersection of the Juan de Fuca ridge and Blanco fracture zone

The intersection of the Juan de Fuca ridge and Blanco fracture zone is characterized by unusually high amplitude magnetic anomalies (over 1500 nT) which appear to be associated with a body roughly 50 km in length and 20 km in width aligned along the fracture zone. Simple three-dimensional magnetic models indicate that this anomaly is probably caused by a highly magnetized block of material situated in the western end of the Blanco fracture zone near its intersection with the Juan de Fuca ridge. Rock magnetization studies of tholeiitic basalts dredged from this area confirm the presence of highly magnetized basalts near the ridge crest/transform fault intersection. These tholeiitic basalts are enriched in iron and titanium relative to “normal” oceanic tholeiites, apparently the result of extensive shallow fractionation involving olivine, plagioclase, and clinopyroxene. Magnetic model studies indicate that an average thickness of no more than 500 m of these iron-rich basalts is necessary to produce the observed anomaly pattern. Comparison of these basalts with samples previously dredged from the Juan de Fuca ridge crest suggests that these Fe-rich, highly magnetized basalts probably “leaked” out of the southernmost portion of the Juan de Fuca ridge.     

Trace element evaluation of a suite of rocks from Reunion Island,Indian Ocean

Reunion Island consists of an olivine-basalt shield capped by a series of flows and intrusives ranging from hawaiite through trachyte. Eleven rocks representing the total compositional sequence have been analyzed for U, Th and REE.Eight of the rocks (group 1) have positive-slope, parallel, chondrite-normalized REE fractionation patterns. Using a computer model, the major element compositions of group 1 whole rocks and observed phenocrysts were used to predict the crystallization histories of increasingly residual liquids, and allowed semi-quantitative verification of origin by fractional crystallization of the olivine-basalt parent magma. Results were combined with mineral-liquid distribution coefficient data to predict trace element abundances, and existing data on Cr, Ni, Sr and Ba were also successfully incorporated in the model.The remaining three rocks (group 2) have nonuniform positive-slope REE fractionation patterns not parallel to group 1 patterns. Rare earth fractionation in a syenite is explained by partial melting of a source rich in clinopyroxene and/or hornblende. The other two rocks of group 2 are explained as hybrids resulting from mixing of syenite and magmas of group 1.     

The amino acid composition of a deep-water marine sediment from the upwelling region northwest of Africa

The distribution of amino acids with depth have been described for three samples of a core of recent sediment (< 1000–8000 yr) from the continental slope off N.W. Africa. Basic amino acids are more abundant than acidic amino acids in all samples. The amino acid composition of the oldest sample resembles most closely the amino acid composition of planktonic protein. It is suggested that the differences in amino acid composition of the three samples reflect diagenetic changes which are probably controlled by the environment of deposition.     

Biostratigraphy and depositional environment of algal stromatolites from the Mescal Limestone (Proterozoic) of central Arizona

The Apache Group of central Arizona is subdivided into, from base upward, the Pioneer Formation, the Dripping Spring Quartzite and the Mescal Limestone. Radiometric age determinations by Silver, and Livingston and Damon indicate an age of 1.2–1.4 billion years. Within the Mescal Limestone, algal stromatolites form a conspicuous biostrome, commonly 20–25 m thick. The basal 1–5 m of the biostrome consists of up to three zones of digitate stromatolites, which often form discrete bush-like bioherms. These forms are interpreted as Baicalia baicalica, Parmites sp. and Tungussia sp.; the latter form previously reported by Cloud and Semikhatov (1969). The form Parmites is interpreted as a modification of digitate stromatolites probably by decrease in current velocity within the shallow marine environment, which allowed discrete heads to coalesce. Basal digitate forms are replaced upward in the biostrome by domal and undulatory laminated (stratiform) stromatolites, interpreted to represent gradual upward shoaling, with lower intertidal and subtidal forms (digitate morphology) being replaced by upper intertidal and possibly supratidal forms (stratiform types).The digitate form B. baicalica is suggested by Soviet workers to be indicative of Middle Riphean time (1350-950 m.y.). While many empirical data suggest the possibility of gross subdivision of Late Proterozoic time on the basis of algal stromatolite “zones”, the intercontinental applicability and the ultimate validity of this concept in unresolved.     

Model systems for anatexis of pelitic rocks

Subsolidus and melting reactions involving calcic plagioclase in pelitic assemblages in the K-Na-Ca model system occur at higher temperatures than their K-Na counterparts. For the most calcic plagioclase compositions observed in high-grade pelitic rocks (An₂₅-An₄₀) the equilibria are rarely extended by more than 30 ° C above those in KA1O₂-NaAlO₂-Al₂O₃-SiO₂-H₂O, although all discontinuities in facies inferred for the K-Na system are continuously displaced when they involve Ca-bearing plagioclase. The maximum pressure-temperature overlap between muscovite dehydration and initial melting reactions occurs in the pressure range of 4–6 kbar between about 640 ° and 720 ° C. This provides optimum conditions for anatectic melt generation in felsic rocks of the appropriate compositions progressively metamorphosed in kyanite-sillimanite facies series. Progressive regional metamorphism at pressures of 2–4 kbar, corresponding to andalusite-sillimanite facies series, shows little overlap between muscovite dehydration and initial melting reactions. Consequently anatectic melt generation in andalusite-sillimanite facies series would require the participation of biotite in dehydration-melting reactions. Felsic intrusive rock in andalusite-sillimanite terranes could have risen upward from their anatectic sites in high grade kyanite-sillimanite facies series at depth. Many andalusite-sillimanite facies series terranes culminating in migmatites could represent upward movement of kyanite-sillimanite facies series rocks to shallower depths with uplift rates faster than cooling rates.     

Freshwater reserves of mid-atlantic coast barrier Islands

A multiple regression model was constructed for the purpose of predicting barrier island hydrology from easily measureable island characteristics. The model was developed using data obtained from 17 sites on the Outer Banks of North Carolina. The accuracy of the model for predicting key hydrologic variables was evaluated by statistical and graphic procedures. In general, agreement between observed and predicted values of the hydrologic variables was very good, suggesting that the quantity of potable water at various island sites can be estimated without resorting to extensive field investigations. The model was then applied to Assateague Island, a barrier island located off the coasts of Maryland and Virginia. Results indicate that the original model developed for the Outer Banks may be applied to other barrier islands but that errors involved may necessitate corrections in detailed studies. Correction for bias in predictions for Assateague was shown to be possible with limited field data from surface resistivity surveys.     

Earthquake recurrence on the Calaveras fault east of San Jose, California

Occurrence of small (3 M_L < 4) earthquakes on two 10-km segments of the Calaveras fault between Calaveras and Anderson reservoirs follows a simple linear pattern of elastic strain accumulation and release. The centers of these independent patches of earthquake activity are 20 km apart. Each region is characterized by a constant rate of seismic slip as computed from earthquake magnitudes, and is assumed to be an isolated locked patch on a creeping fault surface. By calculating seismic slip rates and the amount of seismic slip since the time of the last significant (M 3) earthquake, it is possible to estimate the most likely date of the next (M - 3) event on each patch. The larger the last significant event, the longer the time until the next one. The recurrence time also appears to be increased according to the moment of smaller (2 < M_L < 3) events in the interim. The anticipated times of future larger events on each patch, on the basis of preliminary location data through May 1977 and estimates of interim activity, are tabulated below with standard errors. The occurrence time for the southern zone is based on eight recurrent events since 1969, the northern zone on only three. The 95% confidence limits can be estimated as twice the standard error of the projected least-squares line. Events of M 3 should not occur in the specified zones at times outside these limits. The central region between the two zones was the locus of two events (M = 3.6, 3.3) on July 3, 1977. These events occurred prior to a window based on the three point, post-1969 slip-time line for the central region.

Latitude	Longitude	Depth	Mag.	Target date	Standard error (days)
37°17′± 2′N	121°39′±2′W	5.0 ±2 km	3.0–4.0	7-22-77	22.3
37°26′± 2′N	121°47′±2′W	6.0 ± 2 km	3.0–4.0	9-02-77	8.0

Full-size table