Amphibole-rich xenoliths from Devonian igneous rocks of the Pripyat rift, Southeastern Belarus: a window into cratonic lower-crust–upper-mantle boundary

2021 ◽  
Author(s):  
Galina D. Volkova ◽  
Anna A. Nosova ◽  
Alexey A. Voznyak ◽  
Liudmila V. Sazonova ◽  
Evgenia V. Yutkina ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Lower Crust ◽  
Igneous Rocks ◽  
Mantle Boundary

Preliminary report on some aftershocks of the December 28, 1966 earthquake in Northern Chile

1968 ◽  
Vol 58 (3) ◽  
pp. 843-850 ◽  
Author(s):  
Andrew M. Pitt ◽  
James O. Ellis
Keyword(s):  
Upper Mantle ◽  
Lower Crust ◽  
Preliminary Report ◽  
Main Shock ◽  
Geodetic Survey ◽  
Northern Chile ◽  
The U.S

abstract Epicenters of aftershocks of the December 28, 1966 earthquake in northern Chile lie in a 75-km north-trending zone 20 to 30 km off the coastline. The epicenter for the main shock, as determined by the U.S. Coast and Geodetic Survey, is about 10 km north of the southern end of the aftershock zone. The aftershocks are about 30 km deep in the model used for locations; this places them in the lower crust or upper mantle. The aftershocks have no apparent relation to any surface faults.

Internal lithospheric rotation at the initiation of intra-oceanic rift-drift: An example of proto-transform tectonics from the Vourinos Ophiolite, Greece

2021 ◽  
Author(s):  
Anne Ewing Rassios ◽  
Dina Ghikas ◽  
Anna Batsi ◽  
Petros Koutsovitis ◽  
Evangelos Tzamos ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Oceanic Lithosphere ◽  
Ductile Deformation ◽  
New Paradigm ◽  
Crustal Rocks ◽  
Mantle Boundary ◽  
Ridge Crest ◽  
Transition Area ◽  
The 1960S

ABSTRACT The “petrological Moho” recognized in the Jurassic Vourinos Ophiolite (northern Greece) was the first “crust-mantle” boundary described within a fossil oceanic lithosphere. Early observations suggested a Cenozoic brittle-field block rotation of the petrological Moho transition area resulting in an oblique clockwise rotation of ∼100°, but a brittle fault system responsible for the mechanism of this rotation was never located. A modern interpretation of research dating from the 1960s to the present documents the occurrence of a diverse set of ductile structures overprinting this primary intra-oceanic feature. The following observations from our original “Moho” studies in the Vourinos complex are still pertinent: the contact between the upper mantle units and the magmatic crustal sequence is in situ and intrusional in nature; high-temperature intragranular ductile deformation (mantle creep at temperatures from around 1200 °C down to ∼900 °C) fabrics terminate at the crust-mantle boundary; the overlying oceanic crustal rocks display geochemical fractionation patterns analogous to crustal rocks in the in situ oceanic lithosphere. Since these original studies, however, understanding the mechanisms of ductile deformation and ridge crest processes have advanced, and hence we can now interpret the older data and recent observations in a new paradigm of oceanic lithosphere formation. Our major interpretational breakthrough includes the following phenomena: lower temperature, intergranular deformation of ∼900 °C to 700 °C extends from the upper mantle tectonites up into the lower crustal cumulate section; the origin of mineral lineations within adcumulate crustal rocks as remnants of ductile deformation during early phases of magmatic crystallization; syn-magmatic folding and rotation of the cumulate section; the tectonic significance of flaser gabbro and late gabbroic intrusions in the crustal sequence; and the relevance and significance of a cumulate troctolite unit within the crustal sequence. These observations collectively point to an important process of a ductile-field, syn-magmatic rotation of the Moho transition area. The most plausible mechanism explaining such a rotation is proto-transform faulting deformation near the ridge crest. By recognizing and distinguishing structures that resulted from such initial rotational deformation in the upper mantle peridotites of ophiolites, future field-based structural, petrographic, and petrological studies can better document the mode of the initiation of oceanic transform faults.

scholarly journals Constraints on the rheology of the lower crust in a strike-slip plate boundary: evidence from the San Quintín xenoliths, Baja California, Mexico

Solid Earth ◽  
2017 ◽  
Vol 8 (6) ◽  
pp. 1211-1239 ◽  
Author(s):  
Thomas van der Werf ◽  
Vasileios Chatzaras ◽  
Leo Marcel Kriegsman ◽  
Andreas Kronenberg ◽  
Basil Tikoff ◽  
...  
Keyword(s):  
Grain Size ◽  
Upper Mantle ◽  
Lower Crust ◽  
Baja California ◽  
Plate Boundary ◽  
Volcanic Field ◽  
Strike Slip ◽  
Low Viscosity ◽  
The Pacific ◽  
Flow Laws

Abstract. The rheology of lower crust and its transient behavior in active strike-slip plate boundaries remain poorly understood. To address this issue, we analyzed a suite of granulite and lherzolite xenoliths from the upper Pleistocene–Holocene San Quintín volcanic field of northern Baja California, Mexico. The San Quintín volcanic field is located 20 km east of the Baja California shear zone, which accommodates the relative movement between the Pacific plate and Baja California microplate. The development of a strong foliation in both the mafic granulites and lherzolites, suggests that a lithospheric-scale shear zone exists beneath the San Quintín volcanic field. Combining microstructural observations, geothermometry, and phase equilibria modeling, we estimated that crystal-plastic deformation took place at temperatures of 750–890 °C and pressures of 400–560 MPa, corresponding to 15–22 km depth. A hot crustal geotherm of 40 ° C km−1 is required to explain the estimated deformation conditions. Infrared spectroscopy shows that plagioclase in the mafic granulites is relatively dry. Microstructures are interpreted to show that deformation in both the uppermost lower crust and upper mantle was accommodated by a combination of dislocation creep and grain-size-sensitive creep. Recrystallized grain size paleopiezometry yields low differential stresses of 12–33 and 17 MPa for plagioclase and olivine, respectively. The lower range of stresses (12–17 MPa) in the mafic granulite and lherzolite xenoliths is interpreted to be associated with transient deformation under decreasing stress conditions, following an event of stress increase. Using flow laws for dry plagioclase, we estimated a low viscosity of 1.1–1.3×1020 Pa ⋅ s for the high temperature conditions (890 °C) in the lower crust. Significantly lower viscosities in the range of 1016–1019 Pa ⋅ s, were estimated using flow laws for wet plagioclase. The shallow upper mantle has a low viscosity of 5.7×1019 Pa ⋅ s, which indicates the lack of an upper-mantle lid beneath northern Baja California. Our data show that during post-seismic transients, the upper mantle and the lower crust in the Pacific–Baja California plate boundary are characterized by similar and low differential stress. Transient viscosity of the lower crust is similar to the viscosity of the upper mantle.

scholarly journals O, H and C isotopic systematics of Icelandic groundwater

2019 ◽  
Vol 98 ◽  
pp. 07031
Author(s):  
Arny E. Sveinbjörnsdóttir ◽  
Andri Stefánsson ◽  
Jan Heinemeier
Keyword(s):  
Isotopic Composition ◽  
Carbon Isotopes ◽  
Upper Mantle ◽  
Lower Crust ◽  
Natural Waters ◽  
Large Range ◽  
Carbon Sources ◽  
Water Isotopes ◽  
Stable Water Isotopes ◽  
Range Of Values

Stable water isotopes of oxygen and hydrogen have been studied in Icelandic natural waters since 1960 for hydrological and geothermal research. All the waters are of meteoric and seawater origin. The measured range in δD and δ18O is large -131 to +3.3‰ and -20.8 to +2.3‰ respectively. Some of the waters are more depleted than any present-day precipitation suggesting a pre-Holocene component in the groundwater. Carbon isotopes of streams, rivers, soil and groundwater have been studied since 1990 in order to evaluate the carbon sources and reactions that possibly influence the carbon systematics of the water. Results show large range of values, for δ13CDIC -27.4 to +4.5‰ and for 14CDIC +0.6 to +118 pMC. Apart from atmospheric, organic and rock leaching, input of gas at depth with similar isotopic composition as the pre-erupted melt of the upper mantle and lower crust beneath Iceland have been identified as sources for carbon in the deeper groundwater.

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