ISSN:0148-0227    

DOI:10.1029/91JB02943

年代:1992

数据来源: WILEY

摘要:

Geophysical evidence precludes the existence of a large, mainly molten magma chamber beneath portions of the East Pacific Rise (EPR). A reasonable model, consistent with these data, involves a thin (tens to hundreds of meters high), narrow (<1–2 km wide) melt lens overlying a zone of crystal mush that is in turn surrounded by a transition zone of mostly solidified crust with isolated pockets of magma. Evidence from the superfast spreading portion of the EPR suggests that the composition of the melt lens is mainly moderately fractionated ferrobasalt. These results have important implications for magmatic processes occurring beneath mid‐ocean ridges and are consistent with a model that effectively separates the processes of magma mixing and fractionation into different parts of a composite magma chamber. Magma mixing, as evidenced by disequilibrium relations between host liquids and included phenocrysts, is especially apparent in samples from low magma supply ridges and probably mainly arises from interactions between crystals of the mush zone and new injections of primitive magma rising out of the mantle. Magmatic differentiation beneath mid‐ocean ridges occurs in two parts. Migration of melts through the transition and mush zones can produce chemical trends consistent with in situ fractionation processes. Segregation of melt into molten horizons near the top of a composite magma chamber promotes the more extensive differentiation characteristic of fast spreading ridges. The optimum conditions for the formation of highly differentiated abyssal lavas is where small, discontinuous melt lenses occur, such as at intermediate spreading rates, in the vicinity of propagating rifts, and near ridge offsets at fast spreading ridges. Along‐axis homogenization of subaxial magma is inhibited by the thin, high aspect ratio of the melt lens and by the high viscosities expected in the mush and transition zones. Low magma supply ridges are unlikely to be underlain by eruptable magma in a steady state sense, and eruptions at slow spreading ridges are likely to be closely coupled in time to injection events of new magma from the mantle. Extensional events at high magma supply ridges, which are more likely to be underlain by significant volumes of low‐viscosity melt, can produce eruptions without requiring associated injection events. The critical magma supply necessary for the development of a melt lens near the top of a composite magma chamber is similar to that of normal ridges spreading at rates of about 50–70 mm/yr, a rate approximately corresponding to that marking an abrupt change in the morphology and gravity signal at the ridge axis. A composite magma chamber model can explain several previous enigmas concerning mid‐ocean ridge basalts, including why slow spreading ridges dominantly erupt a narrow range of relatively undifferentiated lavas, why magma mixing is most evident in lavas erupted from slow spreading ridges, why fast spreading ridges erupt a wide range of generally more differentiated compositions, why bimodal lava populations occur in the vicinity of some propagating rifts, and how along‐axis geochemical segmentation can occur at a scale shorter than the major tectonic segmentatio

ISSN:0148-0227    

DOI:10.1029/91JB02508

年代:1992

数据来源: WILEY

摘要:

We propose in this paper an original method for quantifying the material losses at subduction zones mainly based on vertical movements of the active margins. We first test the method, using isostatic and elastic models, applying it to 15 different well‐constrained convergent margins. The main conclusion is that most of them are presently close to the isostatic equilibrium. Consequently, we consider as a first hypothesis that such was also the case in the past and that erosion or accretion are natural mechanisms restoring the equilibrium in response to a perturbation. Then, we apply this model to compute the geometry of the paleoslab knowing the paleotopography of the margin. The comparison between present and past ocean margin lithospheric wedge geometries provides estimates of the removed material that constituted the initial wedge. Two cases were studied for paleoreconstructions: Japan and Peru. The results show that a considerable amount of material was removed during the Neogene along both margins. A detailed interpretation of the Neogene history of the Japan margin is proposed on the basis of our result

ISSN:0148-0227    

DOI:10.1029/91JB02342

年代:1992

数据来源: WILEY

摘要:

Paleomagnetic and paleontologic data from the Valle della Contessa (Umbrian Apennines, Italy) span a 14 m.y. gap in previous magnetostratigraphic sections and reveal several intervals of reversed characteristic remanent magnetization (ChRM) in limestones and marls of the Albian Marne a Fucoidi. Rock magnetic data indicate that the reversed ChRM is carried by hematite having high unblocking temperatures (∼ 625–690° C). Hematite with these unblocking temperatures also carries a normal polarity ChRM in other intervals. A potential bias towards the identification of normal polarity is present since at every horizon measured a normal polarity component, carried by magnetite and hematite, is isolated at lower unblocking temperatures (∼300–600°C). Bulk magnetic properties vary throughout the measured section. In the portion containing most of the reversed ChRM intervals, hematite is the dominant carrier of natural remanent magnetization, while magnetite is the dominant carrier stratigraphically above and below. These changes correspond approximately with lithology. The zone of dominant hematite magnetization is marked by reddish bands (pigmentary hematite) which can be correlated throughout Umbria and are thought to record changes in the oxidation state of the seafloor during deposition. Two models may account for the observed magnetization directions. The reversed magnetizations may record a remagnetization which occurred at least 18 m.y. after deposition, while the beds were still flat‐lying, in reversed polarity chron 33R (83–79 Ma) or later. If the reversed ChRM represents a remagnetization, the process is potentially of great importance since it can produce magnetization patterns which resemble polarity intervals. Alternatively, the reversed magnetizations may have been acquired during intense seafloor oxidation episodes during the mid‐Albian (107–104 Ma) and may record unrecognized intervals of reversed geomagnetic field polarity. Milankovitch‐like bedding cyclicity can be used to tune the sedimentation rate and obtain estimates of the duration of the potential reversed intervals. Using these estimates, two of the potential reversed polarity intervals are of sufficient duration (>100 kyr) to be recognizable in both detailed stratigraphic sections and marine magnetic anomaly surveys. A primary magnetization model predicts that several intervals of reversed magnetization should be found near the boundary of theBiticinella breggiensisandTicinella primulaforaminiferal zones and within thePrediscosphaeracretacea nannofossil zone o

ISSN:0148-0227    

DOI:10.1029/91JB02257

年代:1992

数据来源: WILEY

摘要:

Rupture tests on internally pressurized, thin‐walled hollow cylinders of Westerly granite with impermeable inner membranes suggest that the conventional, or Terzaghi, effective stress law does not describe tensile failure at high internal pressurization rates near 6 MPa/s. Unjacketed and saturated samples, with an initial pore pressure and for which the inner cavity pressure was increased rapidly with respect to the diffusivity, display substantially increased apparent tensile strengths and deformational moduli much higher than similarly configured but more slowly pressurized tests. Alternatively, the properties of completely dry test pieces with no pore pressure show little, if any, dependence on pressurization rate. Further, the behavior of the rapid unjacketed tests was similar to that for completely dry samples. These observations cannot be explained by the predicted undrained response, but they provide indirect evidence for diminished pore pressure effects reminiscent of dilatant hardening observed in compressive failure experiments. Calculated pore pressure diffusion rates support this suggestion as pore pressure perturbations cannot be damped out on the time scale of the rapidly pressurized tests. It is not clear if these effects are produced by elastic microcrack dilatancy, of which the nonlinear stress‐strain curve of granites is symptomatic, or the irreversible production of new porosity as in compressive shear failure te

ISSN:0148-0227    

DOI:10.1029/91JB02256

年代:1992

数据来源: WILEY

摘要:

Although uniaxial consolidation of sediments in basins defines one of the most common and simple of geologic stress paths, there is remarkably little information concerning lateral stresses and elastic moduli associated with this process. Short‐term relationships explored by geotechnical engineers show that ratios of effective horizontal stress (σh′) to effective vertical stress (σν′), orKo, are constant for a given sediment over the low‐stress range, but there is no consensus as to whether these ratios remain constant at the higher stresses and longer durations of geologic conditions. A series of uniaxial consolidation tests on a silty clay and a fine sand, including several elastic unload‐reload cycles, were carried out to explore consolidation and elastic relationships to a σν′ of 35 MPa. Over this stress range,Koremained constant for the clay (0.62) but increased slightly for the sand (0.44 to 0.53). The unload‐reload cycles showed that the vertical Young's modulus (Eν) increases markedly with consolidation stress (σc′). These cycles also demonstrated that σh′/σν′ (elastic) is greater for clay (0.35) than for sand (0.21), but this ratio shows no variation with σc′. This implies that Poisson's ratios (v) did not vary significantly with σν′ or σc′ over the stress range of the tests. Comparison with natural sediments suggests that cementation results in increases ofEof up to an order of magnitude above that of an analogous uncemented sediment, but does not seem to affectv.The effect of geologic time onKowas explored using results of sparse applicable in situ stress measurements; these suggest not only that clay‐rich sediments can support significant differential stresses, but also that stress ratios may even retain

ISSN:0148-0227    

DOI:10.1029/91JB02247

年代:1992

数据来源: WILEY

摘要:

Synthetic quartz aggregates of low porosity have been fabricated from natural quartz powder, impure silica gel, and high‐purity silicic acid by hydrothermal isostatic pressing at 300 MPa. The resulting specimens have water contents comparable to those of natural quartzites and have been used for deformation tests. The flow stress at 1200–1300 K was found to vary in an inverse relationship to the water pressure estimated under the assumption that the measured water content homogeneously filled the measured porosity. It is questionable that the natural‐quartz‐origin specimens were fully equilibrated in respect to the activity of the water, but equilibration has probably been achieved in the amorphous‐silica‐origin specimens, which crystallized under the experimental conditions. In the deformation of the latter materials, the experimental activation energy at water pressures approaching the confining pressure was found to be about 150 kJ mol−1for both. However, the impure gel‐origin material gave a stress exponent of about 2.3, whereas the high‐purity silicic‐acid‐origin material gave a stress exponent of about 4, in spite of the grain sizes being about 90μm and 20 μm, respectively. It is concluded that while the impure specimens have higher intragranular strengths, there is also a significant contribution of grain boundary processes to the strain in them which is absent in th

ISSN:0148-0227    

DOI:10.1029/91JB01748

年代:1992

数据来源: WILEY

摘要:

Okmok and Recheshnoi are adjacent volcanoes on the island of Umnak in the Aleutian Arc. Ninety‐five new chemical analyses of lavas from the two volcanoes show that Okmok exhibits a classical tholeiitic and Recheshnoi a calc‐alkaline differentiation trend. Both volcanoes have erupted lavas that range in composition from basalt to rhyolite. This allows investigation of differences in both the differentiation systematics and the parental magma compositions. In contrast to the predictions of many recent models for calc‐alkaline and tholeiitic volcanism, the major and trace element data show that the parental magmas for the two volcanoes have different compositions. These different parental compositions might themselves be produced by in situ differentiation or other complex fractionation processes from a very magnesian parental magma (16% MgO) with Okmok being derived by low‐pressure fractionation and Recheshnoi by in situ fractionation at higher pressures. An alternative and simpler explanation is that the inferred high‐MgO Okmok parent and the Recheshnoi parent are derived by different extents of melting in the mantle wedge. Modelling based on the rare earth elements suggests approximately 7% melting for the calc‐alkaline parent and 20% for the tholeiitic parent. For these two volcanoes therefore, there may be a correlation between extent of melting and the tendency to follow a calc‐alkaline or tholeiitic differentiation trend. Larger enrichments of highly fluid‐mobile elements such as boron and cesium in the tholeiitic Okmok source suggest that variability in the volume of fluid flux from the slab may be responsible for the different extents of melting. If the partial melting model is applied generally to the Aleutian Arc, it provides an alternative explanation for the volcanic regularities of the arc described by Kay et al. (1982). Smaller extents of melting lead to less melt, fractionation at higher pressures, often including hydrous phases, a preponderance of plutonic rather than volcanic rocks, and smaller calc‐alkaline volcanoes. Large tholeiitic volcanoes are often associated with fracture zones on the subducting Pacific plate (Kay et al., 1982; Marsh, 1982). Since fracture zones are more extensively altered than average oceanic crust and might also serve as sediment traps, they could serve as sources of a larger volatile flux from the slab, leading to greater extents of melting and large, tholeiitic volcanoes. If this explanation is correct, then the origin of the volcanic segmentation of the arc may be found within the subducted slab. This contrasts with the alternative model of control by the stress regime of the overlying plate (Kay et al., 1982; Singer and Myers, 1990; Kay and Kay, 1991). Inferences from Okmok and Recheshnoi may also apply to global variations in convergent margin chemistry. In general, arcs built On thick crust tend to be more calc‐alkaline in character. On the Basis of the negative correlation between convergent margin crustal thickness and inferred extent of melting (Plank and Langmuir, 1988), lower extents of melting may contribute to a tendency towards calc‐alkaline differentiat

ISSN:0148-0227    

DOI:10.1029/91JB02150

年代:1992

数据来源: WILEY

摘要:

Birimian (∼2.1 Ga) terranes in the West African craton are a mixture of highly metamorphosed volcanic, sedimentary and plutonic rocks and low grade metavolcanics and metasediments. The volcanic units contain thick, commonly pillowed, tholeiitic basalts overlain by pelagic sediments cherts and black shales; The sedimentary units are characterized by an abundance of clastic turbiditic sediments. Andesites and calc‐alkaline felsic volcanics occur at uppermost stratigraphic levels and as dykes. Field relationships between the volcanic and sedimentary units remain a matter of debate. Calc‐alkaline and local alkaline granites, which intruded in distinct pulses and occasionally are related to transcurrent tectonics, represent almost half of the Birimian terranes. New isotopic work on the highly metamorphosed units greatly improved the chronology for the Birimian crust. The age of the early Dabakalian event is precisely defined by a U‐Pb zircon age at 2186 ± 19 Ma, while Rb‐Sr and Sm‐Nd methods give ages of 2162 ± 19 Ma and 2141 ± 24 Ma, respectively. A Sm‐Nd garnet‐whole rock age of 2153 ± 13 Ma suggests that metamorphism culminated at about the same time. In contrast, the most precise zircon U‐Pb and Sm‐Nd data for the more widespread Birimian terranes (sensu stricto), from this study and from the literature, cluster between 2.12 and 2.07 Ga. The major evolution of the Birimian crust apparently lasted less than 50 Ma. Isotopic evidence indicates that Birimian granitoids contain a negligible component of Archean crust: εNd(2.1‐Ga) values are positive and similar to those of Birimian basalts, crustal residence times are shorter than 200 Ma, U‐Pb ages for detrital zircons from clastic sediments range from 2098 ±11 Ma to 2125 ± 17 Ma, while granite chemistry and Nd isotopic characteristics are unrelated. Only very locally in Guinea is there isotopic evidence of interaction between Birimian felsic magmas and the Archean rocks from the Man craton. In accord with Abouchami et al.'s (1990) suggestion that Birimian basalts represent oceanic plateaus, the present data argue that the protolith of much of the West African continent was created around 2.1 Ga in an environment remote from Archean crust. Intrusion of calc‐alkaline magmas into the tholeiitic units suggests that island arcs formed on top of the assumed oceanic plateaus which then collided with the Man Archean craton. Taking the Birimian formations from the Guyana shield into account, the minimum crustal growth rate at 2.1 Ga is about 1.6 km3/a, some ∼60% higher than the present growth rate. Birimian crust growth at 2.1 Ga is reminiscent of Archean processes but contrasts with 1.7 – 1.9 Ga crust formation in the North Atlantic continent which generally involved significantly more interaction with older continental crust. A comparison of the Birimian crustal growth rate with the average crustal growth rate over the Earth history implies that a large part of the Birimian crust has been recycled into the mantle or incorporated into younger orogenic segments. This apparent deficit in the crustal budget is even

ISSN:0148-0227    

DOI:10.1029/91JB01640

年代:1992

数据来源: WILEY

摘要:

We use τ(p) data extracted from a small number of wide‐angle recordings of quarry blasts to construct averaged, one‐dimensional velocity models of the crust beneath portions of the Great Valley, Newark Basin, Valley and Ridge, and Allegheny Plateau of the central Appalachians of Pennsylvania. Using the linear form of the equations for τ(p) in terms of slowness‐depth structure, we compare inversion results for the extremal, Backus‐Gilbert, and generalized least squares methods. Although the uncertainties are large, the models do show a well‐constrained increase in midcrustal velocities and crustal thickness beneath the Allegheny Plateau. Inversion of precritical reflections from the Moho suggests bounds of 7.1–7.6 km/s on P wave velocity and 3.9–4.2 km/s on S wave velocity near the base of the crust. P wave velocity models for the Great Valley show a crustal thickness between 40 and 45 km, with an average velocity between 6.4 and 6.6 km/s. Models for the Allegheny Plateau show a larger crustal thickness (47–52 km) and a much higher average velocity (6.8–6.9 km/s). Estimates of average shear wave velocities for the Great Valley range from 3.6 to 3.8 km/s, with crustal thickness estimates between 37 and 44 km. For the relatively small number of singular values retained, standard errors in depth for models derived by generalized least squares range from ±1 to 2 km for the P wave slowness models and from ±2 to 4 km for the S wave models; extremal depth bounds in general are 2 to 3 times as wide. Corresponding uncertainties in interval velocity, estimated from resolving kernels in slowness, range from 0.15 to 0.45 km/s for P wave models and from 0.15 to 0.40 km/s for S wave models. T2–X2inversion of PmP data gives similar estimates for total crustal thickness and average velocity after correction for refraction effects. T2–X2inversion of PmP data for a fourth profile suggests the possibility of a slight thickening of the crust (47–48 km) directly beneath the axis of the Great Valley gravity low. Estimates of average VP/VSfor the crust based on average velocities for models derived by generalized least squares inversion range from 1.73 to 1.77. Estimates based on travel time ratios for events interpreted as P wave and S wave reflections from the Moho lie between 1.75 and 1.79. For a crust in which the effects of velocity anisotropy can be neglected, these estimates correspond to crustal averages between 0.25 and 0.27 for Poisson's ratio. The one‐dimensional velocity models derived here provide estimates of the long‐wavelength component of velocity structure. Besides demonstrating the maximum resolving power inherent in a limited τ(p) data set, these averaged models can be used for migrating reflection data and as starting models for determining tw

ISSN:0148-0227    

DOI:10.1029/91JB02230

年代:1992

数据来源: WILEY