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The recovery of drill cores involves changes in pressure and temperature conditions, which inevitably alter the mechanical properties of unlithified sediments. While expansion from unloading after core recovery is well studied, the effects from cooling on standard geotechnical tests are commonly neglected. Along the central portion of the Nankai margin sediments were recovered from high in-situ temperatures of up to 110 °C during IODP Leg 190. So far, the interpretation of the consolidation state of the Lower Shikoku Basin facies (LSB) entering the accretionary Nankai margin is ambiguous. Results from laboratory consolidation tests at room temperature show high pre-consolidation stresses. These were interpreted as hardening caused by cementation, while the field-based porosity vs. depth trend points towards normal consolidation. As an explanation for this discrepancy, the change of the mechanical properties by cooling from in-situ to laboratory conditions is proposed. In this paper, the results of a thermo-mechanical model are compared to published field data. This comparison suggests that the observed hardening is at least partially an artefact from cooling during core recovery, and that the strata may be considered normally consolidated to slightly overconsolidated. The latter can be explained by minor cementation or the influence of secondary consolidation. The results suggest that cooling from high in-situ temperatures may be important for the interpretation of the consolidation state of other sedimentary successions elsewhere. 相似文献
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N. Stark R. Wilkens V. B. Ernstsen M. Lambers-Huesmann S. Stegmann A. Kopf 《Geotechnical and Geological Engineering》2012,30(1):1-14
The societal usage of coastal zones (including offshore wind energy plants, waterway deepening, beach conservation and restoration)
is of emerging importance. Sediment dynamics in these areas result in sandy deposits due to strong tidal and wave action,
which is difficult to simulate in laboratory geotechnical tests. Here, we present data from in situ penetrometer tests using
the lightweight, free-fall Nimrod penetrometer and complementary laboratory experiments to characterize the key physical properties of sandy seafloors in areas
dominated by quartzose (North Sea, Germany) and calcareous (Hawaii, USA) mineralogy. The carbonate sands have higher friction
angles (carbonate: 31–37°; quartz: 31–32°) and higher void ratios (carbonate: 1.10–1.40; quartz: 0.81–0.93) than their siliceous
counterparts, which have partly been attributed to the higher angularity of the coral-derived particles. During the in situ
tests, we consistently found higher sediment strength (expressed in deceleration as well as in estimated quasi-static bearing
capacity) in the carbonate sand (carbonate: 68–210 g; quartz: 25–85 g), which also showed a greater compressibility. Values
were additionally affected by seafloor inclination (e.g., along a sub-aqueous dune or a channel), or layering in areas of
sediment mobilization (by tides, shorebreak or currents). The study shows that the differences in in situ measured penetration
profiles between carbonate sands and quartz sands are supported by the laboratory results and provide crucial information
on mobile layers overlying sands of various physical properties. 相似文献
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Microstructural tectonometamorphic processes and the development of gneissic layering: a mechanism for metamorphic segregation 总被引：6，自引：1，他引：5
The Mary granite, in the East Athabasca mylonite triangle, northern Saskatchewan, provides an example and a model for the development of non-migmatitic gneissic texture. Gneissic compositional layering developed through the simultaneous evolution of three microdomains corresponding to original plagioclase, orthopyroxene and matrix in the igneous rocks. Plagioclase phenocrysts were progressively deformed and recrystallized, first into core and mantle structures, and ultimately into plagioclase-rich layers or ribbons. Garnet preferentially developed in the outer portions of recrystallized mantles, and, with further deformation, produced garnet-rich sub-layers within the plagioclase-rich gneissic domains. Orthopyroxene was replaced by clinopyroxene and garnet (and hornblende if sufficient water was present), which were, in turn, drawn into layers with new garnet growth along the boundaries. The igneous matrix evolved through a number of transient fabric stages involving S-C fabrics, S-C-C' fabrics, and ultramylonitic domains. In addition, quartz veins were emplaced and subsequently deformed into quartz-rich gneissic layers. Moderate to highly strained samples display extreme mineralogical (compositional) segregation, yet most domains can be directly related to the original igneous precursors. The Mary granite was emplaced at approximately 900 °C and 1.0 GPa and was metamorphosed at approximately 750 °C and 1.0 GPa. The igneous rocks crystallized in the medium-pressure granulite field (Opx–Pl) but were metamorphosed on cooling into the high-pressure (Grt–Cpx–Pl) granulite field. The compositional segregation resulted from a dynamic, mutually reinforcing interaction between deformation, metamorphic and igneous processes in the deep crust. The production of gneissic texture by processes such as these may be the inevitable result of isobaric cooling of igneous rocks within a tectonically active deep crust. 相似文献
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Drilling two mud domes on the Mediterranean Ridge during ODP Leg 160 has demonstrated that the eruption of mud breccia began
at least 1.5 Ma ago. An evolution through extrusive building of a cone, followed by successive eruptions of clast-bearing
mud debris flows and subsequent subsidence can be deduced for both domes. Results from permeability and shear strength tests,
grain size analyses, sedimentary textures, and clast provenance provide clues concerning the mechanism of mud volcanism. The
collision of Africa with Eurasia resulted in backthrusting of the evaporite-dominated accretionary wedge against a rigid backstop.
This allowed egress of overpressured fluid-rich mud of presumed Messinian age from the décollement, although many of the clasts
may have originated from the overlying accretionary wedge. 相似文献
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Kopf A. Klaeschen D. Weinrebe W. Flueh E.R. Grevemeyer I. 《Marine Geophysical Researches》2001,22(3):225-234
The Ninetyeast Ridge is a well-studied hot spot trail in the Indian Ocean. A recent geophysical survey in its central portion
near 17° S included Hydrosweep bathymetric mapping, Parasound echosounder profiles, and high resolution seismic reflection
data. These data reveal a number of small cones of a few hundreds of meters in diameter and up to 200 m height. Seismic evidence
exists regarding a magmatic origin of these features. Different events of basaltic flow and tuff deposition intercalated with
hemipelagic oozes of Eocene to present age, as being known from nearby drilling, allow dating of these latest stages of volcanic
activity. An activity of at least 6 Ma longer than termination of the dominant constructional phase of the ridge can be demonstrated.
These eruptions occur at shallow water depth, and seem to be related to tectonic lineaments in the area. Transtensional stresses
together with a more durable magmatic source beneath this part of the ridge allow magma to ascend along pull-apart structures.
The age discrepancy found calls for special attention when trying to reconstruct global plate motions.
This revised version was published online in November 2006 with corrections to the Cover Date. 相似文献
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Sebastian B. Hammerschmidt Earl E. Davis Andre Hüpers Achim Kopf 《Geo-Marine Letters》2013,33(5):405-418
Along the Nankai Trough megasplay fault off SE Japan, the effect of fluid migration on subduction-related seismogenesis and tsunamigenesis remains unresolved. To investigate the existence and role of fluid flow, a SmartPlug borehole observatory was installed at Site C0010 of the Integrated Ocean Drilling Program NanTroSEIZE Kumano transect, where a shallow branch of the fault was intersected and in situ fluid pressure monitored from August 2009 to November 2010. The tidal signal in the formation showed no phase shift relative to seafloor loading. The attenuation of 0.73 reflects the loading efficiency accurately, and enabled calculation of a formation compressibility of 1.0×10–9 Pa–1 and a hydraulic diffusivity (HD) of 1.5×10–5 m2 s–1. A similar HD is predicted by a tidal response model based on SmartPlug pressure data. By contrast, permeability measurements on intact samples from Site C0004 SE along-strike the splay fault and from Site C0006 in the frontal thrust zone were found to be similar and one magnitude smaller respectively, despite having a higher porosity. This is explained by the presence of fractures, which are covered by the larger scale of in situ measurements at Site C0010. Consequently, HD can be set to be at least 10–5 m2 s–1 for the splay fault and 10–6 m2 s–1 for the frontal thrust fault zone. Considering recent publications makes fluid flow at the splay fault unlikely, despite the presence of fractures. If the influence of fractures is limited, then processes leading to fault weakening may be enhanced. 相似文献
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The Jura-Cretaceous Peninsular Ranges batholith (PRB) of Southern and Baja California contains a remarkable example of variation in crustal composition and structure across a batholith-parallel lithospheric-scale discontinuity. This lithospheric boundary between western oceanic-floored and eastern continental-floored crust influenced contractional deformation, arc magmatism, and differential exhumation of western and eastern zones in the batholith during its evolution.In the Sierra San Pedro Martir of Baja California, Mexico, a ca. 20 km wide, doubly vergent fan structure occurs across the PRB basement transition that consists of inward-dipping mylonite thrust sheets on the sides of the fan that gradually transition to a steeply-dipping tectonized zone in the center. A dramatic inverted metamorphic gradient occurs on the western side of this structure where mid-crustal amphibolite metamorphic grade rocks with peak pressures of 5–6 kbar in the center of the fan were thrust over upper-crustal sub-greenschist grade rocks (peak pressures < 2 kbar) in the western zone footwall. An inverted but smaller gradient occurs on the eastern side of the structure where rocks of the fan interior have been thrust eastwards over amphibolite to upper greenschist grade rocks (peak pressures 4–5 kbar).Gradients in cooling ages determined by 40Ar/39Ar biotite and K-feldspar and apatite fission track methods coupled with U–Pb zircon ages and Al in hornblende thermobarometry studies on plutons across this zone indicate that structures focused along the transition zone between contrasting lithosphere in the PRB accommodated nearly 15 km of the differential exhumation of western and eastern basement in the orogen. The western zone of the batholith was a major forearc depo-center for thick clastic sequences derived from the uplifting eastern PRB and remained at low average elevation during the Late Cretaceous and Paleogene. In contrast the eastern zone experienced dramatic uplift subsequent to achieving a crustal thickness in excess of 55 km by mid-Cretaceous time. This region had the isostatic potential for 4–5 km surface elevations, and likely formed a topographically high orogenic plateau. Exhumation of the fan structure initiated after 100 Ma and was largely complete by 85 Ma. Eastward-migrating unroofing of the rest of the eastern PRB continued into the Paleogene.A variety of factors were responsible for exhumation in this region. Structural thickening of the eastern zone of the orogen resulted from more than 30 million years of episodic contractional deformation in the fan structure, much of which followed island arc accretion of the western zone along this segment of the batholith. An episode of voluminous magmatism involving the intrusion of the 99–92 Ma La Posta-type magmatic suite across the eastern zone of the PRB triggered exhumation in the fan structure. Denudation in this region appears to have been solely by erosion; no evidence has been found for extensional tectonics during this time. This arc orogen demonstrates the important influence of inherited tectonic boundaries in controlling the spatial distribution of structural thickening and magmatism. It also displays the complex interrelationships among structural thickening, exhumation, and the role of magmatism in triggering exhumation episodes within orogens. 相似文献