摘要:

Basalts recovered during leg 15 of the Deep Sea Drilling Project in the central Caribbean are finegrained plagioclase tholeiites which were erupted in the Late Cretaceous, at a time when North America was shifting from South America and Africa, and emplaced as thin flows and sills near the water‐sediment interface. The topmost of these units is responsible for seismic horizon B″, and gradually increasingPwave velocities with depth are due to increasing proportions of basaltic rocks relative to interlayered sediments. Basalts from these sites have similar subalkaline characteristics and contain normative hypersthene and 5–15% normative olivine. The major element character of the basalts indicates that they have evolved from more primitive compositions, probably through olivine fractionation. Intersite variations can be explained largely through both olivine and plagioclase fractionation from similar parental liquids. However, olivine is probably not on the liquidus at any pressure for anhydrous melts of these compositions. A small amount of water in the peridotitic source regions may result in olivine being a near‐liquidus phase at pressures of 5–8 kbar. Minor element (e.g., TiO2) variations which cannot be accounted for through crystal fractionation reflect variations in the primary liquid compositions which are probably a consequence of degree of partial melting. Plagioclase was the low‐pressure liquidus phase in all the samples examined but was quickly followed in the paragenetic sequence by calcic clinopyroxene and finally titanomagnetite. Olivine or pseudomorphs after olivine are not observed. Intratelluric crystals, predominantly calcic plagioclase but more rarely clinopyroxene, can be distinguished by their minor element characteristics. Metastable crystallization trends, the lack of pyroxene exsolution, and the presence of interstitial glass (now altered) attest to the rapid cooling of these liquids. Two distinct REE patterns (LREE (light REE) enriched and LREE depleted, the latter being typical of the majority of the samples) require chemically contrasting upper mantle source regions during the Late Cretaceous in the Caribbean Basin. Restrictions on the permissible degrees of partial melting of a garnet‐bearing source preclude derivation of the LREE‐enriched basalts in a one‐stage process, and it is suggested that the source region acquired a LREE‐enriched character prior to the generation of the liquids through 20–30% partial melting. The LREE‐depleted liquids were derived from slightly deeper source regions (70–100 km) previously depleted in the LREE. Contrasting tectonic regimes required to produce these REE characteristics conflict with present thinking on the style of Cari

ISSN:0148-0227    

DOI:10.1029/JB080i035p04775

年代:1975

数据来源: WILEY

摘要:

The anisotropy of magnetic susceptibility has been measured in three suites of basaltic specimens: (1) 312 specimens from known intrusives and extrusives, (2) 22 samples from a sill and underlying lava recovered from the 580‐m depth in the Bermuda deep drill 1972 hole, and (3) 43 samples from hole 319A and 321 of the Deep‐Sea Drilling Project. A new anisotropy of magnetic susceptibility parameter (F) is proposed, and it is shown, by using sample suites 1 and 2, that diagnosis of emplacement modes in basaltic materials can be made with 80% confidence. Measurements ofFin samples from Deep‐Sea Drilling Project leg 34 suggest that at least one intrusive and two extrusive units have been sampled between the 95‐ and 155‐m depths in hole 319A and that material recovered between the 125‐ and 130‐m depths in hole 321 is of an intrusive nature. Contrasts in morphology of the titanomagnetites, as observed in polished thin sections, support these in

ISSN:0148-0227    

DOI:10.1029/JB080i035p04805

年代:1975

数据来源: WILEY

摘要:

A hydrodynamic model of the slip zone between the descending oceanic crust and the overriding continents is used to explain the observed constancy of the dip angle for most island arc‐trench systems. By allowing the thickness of the slip zone to vary with depth, the descending plate is acted upon by a nonhydrostatic pressure force and a viscous shear force of a hydrodynamic origin which can balance the gravitational force acting on the plate. In this way the plate adheres to the overriding continent, maintaining an angle of descent dictated by the geometry of the overriding continent. An estimate of the slip zone thickness at the earth's surface is derived that is consistent with observations. Estimates of the slip zone viscosity, shear stress, and dissipation are also obtained that are in agreement with accepted value

ISSN:0148-0227    

DOI:10.1029/JB080i035p04809

年代:1975

数据来源: WILEY

摘要:

By using magnetic anomaly data from a detailed geophysical survey west of the Juan de Fuca rise between longitudes 143°W–134°W and latitudes 42°N–48°N the history of spreading at the Pacific‐Farallon spreading center in this region can be reconstructed for the period 35–20 m.y. ago (anomalies 12–5E). During this time period, relative migration of spreading axes separated by transform faults resulted in the elimination of the offset represented by the Surveyor fracture zone. Magnetic anomalies in the southern part of the region require eastward jumps of spreading centers of between 40 and 50 km, and those in the northern part imply westward jumps of up to 70 km. The locations of the spreading center jumps migrate along spreading axes with time, concurrently with northward or southward jumps of transform faults, and leave zones of extensively sheared crust with unidentifiable magnetic anomaly patterns in the crust between old and new spreading centers. Such a process may account for the disturbed zone of magnetic anomalies between the Murray and Molokai fracture zones and could be common to all ridge jumps. If so, it suggests that the new spreading centers do not begin simultaneously over long lengths but instead develop in a manner somewhat similar, but not identical, to a crack propagating th

ISSN:0148-0227    

DOI:10.1029/JB080i035p04815

年代:1975

数据来源: WILEY

摘要:

Jurdy and Van der Voo's (1974) definition of polar wander is shown to be related to equations which Lliboutry (1914a,b) and Solomon and Sleep (1974) use for calculating absolute plate motions. A restatement of their polar wander criterion in terms of the net angular momentum of infinitely thin lithospheric plates is compared by a suggestive but nonrigorous argument with Solomon and Sleep's (1974) condition that the net torque acting on the lithosphere be zero, The small amount of polar wander calculated by Jurdy and Van der Voo (1974, 1975) since Early Cretaceous suggests that the abovementioned conditions for absolute plate motion have some validity.

ISSN:0148-0227    

DOI:10.1029/JB080i035p04823

年代:1975

数据来源: WILEY

摘要:

A simple mathematical model of conductive heat flow from a hot spot in a moving medium shows that as the speed increases, the surface heat flow anomalies become weaker, narrower transverse to the motion, steeper upstream, and flatter downstream. The peak of the anomaly shifts downstream. Detectable heat flow anomalies, in this model, extensively melt the lower lithosphere.

ISSN:0148-0227    

DOI:10.1029/JB080i035p04825

年代:1975

数据来源: WILEY

摘要:

The extensional velocity and internal friction are measured in a series of partially melted alloys having different volume fractions of fluid and different dihedral angles. The acoustic properties change discontinuously when melt forms: the velocity decreases whileQ−1usually, but not invariably, increases. The present results, in conjunction with previous studies, form a broadly consistent picture of acoustic properties in partial melts. The dihedral angle has a major effect; the extensional velocity decreases much more rapidly with increasing melt content when the liquid wets the grain boundaries. The data set an approximate upper limit to the volume of fluid which can be present in the low‐velocity zone (LVZ) of the upper mantle and be consistent with seismic data, assuming that the LVZ is due to partial melting and the seismic frequencies are much less than the peak frequency for fluid motion. If the dihedral angle of the melt in the LVZ is zero, only several tenths of 1 vol % of fluid can be present, and less than 5 vol % if the dihedral angle is

ISSN:0148-0227    

DOI:10.1029/JB080i035p04828

年代:1975

数据来源: WILEY

摘要:

A previous study of time‐dependent crack growth in single‐crystal quartz has been expanded to examine the possibility of microfracturing events during stable crack growth, to look for evidence of plastic deformation associated with crack propagation, and to determine the dependence of crack growth on crystallographic orientation. No discernible effect of orientation on the temperature or change in applied stress or partial pressure of water dependencies during sequential crack growth episodes was observed, and no correlation was found between observed microfracturing events and the rate of crack propagation. However, the magnitude of the applied stress to achieve the desired rates of crack extension did vary with orientation. No evidence of plastic deformation has been found in samples of quartz undergoing time‐dependent crack growth at temperatures up to 250°C. Some Dauphiné twins have been observed at temperatures above 125°C. The fact that the stress, temperature, and water dependencies are independent of orientation is interpreted to suggest that the observed time‐dependent cracking is controlled by the transport of water to the

ISSN:0148-0227    

DOI:10.1029/JB080i035p04837

年代:1975

数据来源: WILEY

摘要:

In triaxial laboratory tests, variation of circumferential strain in dilatant granite may reach 100% or more at high stresses. Pseudoelastic compliancesS13andS23in the plane perpendicular to the maximum stress may differ by a factor of 2–10. Given the magnitude of these differences, in situ azimuthal variation of dilatancy should be the observed rule rather than the exception. Such anisotropy might be exploited to determine the strike of a future fault break or the next site of fault motio

ISSN:0148-0227    

DOI:10.1029/JB080i035p04845

年代:1975

数据来源: WILEY

摘要:

We calculate the effective elastic properties of various polycrystals and rocks using a self‐consistent integral equation technique developed in the theory of disordered alloys. This technique includes but is more general than the self‐consistent approximation developed by Hershey, Kröner (1958), and others. In practice the technique is easily adaptable to computer analysis for modest cost. Our computed results compare well with experimental data. Also we point out that an approximation for the perfectly disordered aggregate developed by Kröner (1967) is not exact as claimed but is an approximation to the selfconsistent approxim

ISSN:0148-0227    

DOI:10.1029/JB080i035p04851

年代:1975

数据来源: WILEY