Exhumation of metamorphic terranes: introduction

2002 ◽  
Vol 66 (1) ◽  
pp. 1-3 ◽  
Author(s):  
S. J. Cuthbert ◽  
M. Balle̒vre
Keyword(s):  
Driving Forces ◽  
Gravity Currents ◽  
Metamorphic Rocks ◽  
Petrophysical Properties ◽  
Brittle Deformation ◽  
Extensional Tectonics ◽  
Zone Development ◽  
Metamorphic Terranes ◽  
Tectonic Extrusion ◽  
Weak Zones

Ever since the landmark paper by England and Richardson (1977) it has been clear that metamorphic rocks are not merely passively advected towards the surface during unroofing, but respond in a dynamic and interactive way to the mechanisms and rates of exhumation. England and Richardson based their modelling upon erosion-driven exhumation dominated by fluvial processes in mid-latitude climates. Since then a variety of mechanisms has been identified, mostly emphasizing tectonic mechanisms including extensional tectonics (e.g. Platt, 1996), tectonic extrusion (Chemenda et al., 1996; Hynes et al., 1996; Thompson et al., 1997) and ‘gravity currents’ (Wernicke and Getty, 1997). It has also become apparent that exhumation itself may be strongly influenced by metamorphic processes. For example, changes in petrophysical properties such as density may dramatically affect the buoyancy of rock masses and hence alter the driving forces for exhumation (Bousquet et al., 1997). The evolution of fluids or melts may create weak zones in the crust and enhance ductile or brittle deformation and shear zone development (e.g. Austrheim, 1998).

scholarly journals Renewed tectonic extrusion of high-grade metamorphic rocks in the MCT footwall since Late Miocene (Sutlej Valley, India)

2008 ◽  
Vol 2 (4) ◽  
pp. 102-103 ◽  
Author(s):  
Vincent Baudraz ◽  
Jean-Claude Vannay ◽  
Elizabeth Catlos ◽  
Mike Cosca ◽  
Torsten Vennemann
Keyword(s):  
Late Miocene ◽  
Metamorphic Rocks ◽  
High Grade ◽  
Special Issue ◽  
Tectonic Extrusion

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp. 102-3

Even the low-T garnet from the iconic Barrow’s zone is tetragonal

2021 ◽  
Author(s):  
Bernardo Cesare ◽  
Fabrizio Nestola
Keyword(s):  
Diffusion Length ◽  
Metamorphic Rocks ◽  
Unit Cell Parameters ◽  
Rock Composition ◽  
Cell Parameters ◽  
Scottish Highlands ◽  
Biotite Schist ◽  
Metamorphic Terranes ◽  
Garnet Zone ◽  
Tetragonal Cell

<p>Common (anhydrous) Fe-Mg-Ca-Mn garnet, the archetypal cubic mineral, has been recently discovered to be tetragonal in metapelites and metabasites from low-temperature regional metamorphic terranes (Cesare et al., 2018).</p><p>Despite the differences in bulk rock composition and pressure conditions, such low-T tetragonal garnets share common chemical features, namely high grossular (>25 mol%) and low pyrope (<7 mol%) contents. Similar compositions are documented in other contexts worldwide, both in blueschists-eclogites and in phyllites, including the metapelites from the garnet zone of the iconic Barrovian metamorphism of the Scottish highlands (Viete et al., 2011).</p><p>We have analysed a garnet crystal from a chlorite-biotite schist collected at the Barrow’s garnet zone in Glen Esk. The unit cell parameters were refined using diffraction reflections between 1.20 and 0.55 Å providing a tetragonal cell with a = 11.5731(5) Å and c = 11.5887(8) Å and volume V = 1552.15(15) Å3. Systematic absences analysis on complete intensity data collected up to 2theta = 80° indicated I41/acd space group confirming the cell parameters refinement.</p><p>Therefore, the garnet is tetragonal and not cubic, as suggested by its weak birefringence under crossed polarizers.</p><p>These results show that the tetragonal structure of common Fe-Mg-Ca-Mn garnet is verified whenever this mineral displays the Ca-rich, Mg-poor composition often observed in low-T metamorphic rocks. And support the hypothesis that the lowering of symmetry is composition-dependent.</p><p> </p><p>References</p><p>Cesare, B., et al. Garnet, the archetypal cubic mineral, grows tetragonal. Sci Rep <strong>9</strong>, 14672 (2019).</p><p>Viete, D.R., et al. The nature and origin of the Barrovian metamorphism, Scotland: Diffusion length scales in garnet and inferred thermal time scales. J. Geol. Soc. London <strong>168</strong>, 115–132 (2011).</p><p> </p>

scholarly journals Insights from elastic thermobarometry into exhumation of high-pressure metamorphic rocks from Syros, Greece

Solid Earth ◽  
2021 ◽  
Vol 12 (6) ◽  
pp. 1335-1355
Author(s):  
Miguel Cisneros ◽  
Jaime D. Barnes ◽  
Whitney M. Behr ◽  
Alissa J. Kotowski ◽  
Daniel F. Stockli ◽  
...  
Keyword(s):  
High Pressure ◽  
Low Temperature ◽  
Oxygen Isotope ◽  
Thermal Conditions ◽  
Garnet Growth ◽  
Metamorphic Rocks ◽  
Core Complex ◽  
Pressure Groups ◽  
Mineral Growth ◽  
Metamorphic Terranes

Abstract. Retrograde metamorphic rocks provide key insights into the pressure–temperature (P–T) evolution of exhumed material, and resultant P–T constraints have direct implications for the mechanical and thermal conditions of subduction interfaces. However, constraining P–T conditions of retrograde metamorphic rocks has historically been challenging and has resulted in debate about the conditions experienced by these rocks. In this work, we combine elastic thermobarometry with oxygen isotope thermometry to quantify the P–T evolution of retrograde metamorphic rocks of the Cycladic Blueschist Unit (CBU), an exhumed subduction complex exposed on Syros, Greece. We employ quartz-in-garnet and quartz-in-epidote barometry to constrain pressures of garnet and epidote growth near peak subduction conditions and during exhumation, respectively. Oxygen isotope thermometry of quartz and calcite within boudin necks was used to estimate temperatures during exhumation and to refine pressure estimates. Three distinct pressure groups are related to different metamorphic events and fabrics: high-pressure garnet growth at ∼1.4–1.7 GPa between 500–550 ∘C, retrograde epidote growth at ∼1.3–1.5 GPa between 400–500 ∘C, and a second stage of retrograde epidote growth at ∼1.0 GPa and 400 ∘C. These results are consistent with different stages of deformation inferred from field and microstructural observations, recording prograde subduction to blueschist–eclogite facies and subsequent retrogression under blueschist–greenschist facies conditions. Our new results indicate that the CBU experienced cooling during decompression after reaching maximum high-pressure–low-temperature conditions. These P–T conditions and structural observations are consistent with exhumation and cooling within the subduction channel in proximity to the refrigerating subducting plate, prior to Miocene core-complex formation. This study also illustrates the potential of using elastic thermobarometry in combination with structural and microstructural constraints, to better understand the P–T-deformation conditions of retrograde mineral growth in high-pressure–low-temperature (HP/LT) metamorphic terranes.

scholarly journals Characteristics of a fracture network surrounding a hydrothermally altered shear zone from geophysical borehole logs

Solid Earth ◽  
2020 ◽  
Vol 11 (3) ◽  
pp. 829-854
Author(s):  
Eva Caspari ◽  
Andrew Greenwood ◽  
Ludovic Baron ◽  
Daniel Egli ◽  
Enea Toschini ◽  
...  
Keyword(s):  
Shear Zone ◽  
Damage Zone ◽  
Fracture Network ◽  
Core Material ◽  
Petrophysical Properties ◽  
Brittle Deformation ◽  
Borehole Radar ◽  
Borehole Logs ◽  
Self Potential

Abstract. Hydrothermally active and altered fault/shear zones in crystalline rocks are of practical importance because of their potential similarities with petrothermal reservoirs and exploitable natural hydrothermal systems. The petrophysical and hydraulic characterization of such structures is therefore of significant interest. Here, we report the results of corresponding investigations on a prominent shear zone of this type located in the crystalline Aar massif of the central Swiss Alps. A shallow borehole was drilled, which acutely intersects the core of the shear zone and is entirely situated in its surrounding damage zone. The focus of this study is a detailed characterization of this damage zone based on geophysical borehole measurements. For this purpose, a comprehensive suite of borehole logs, comprising passive and active nuclear, full-waveform sonic, resistivity, self-potential, optical televiewer, and borehole radar data, was collected. The migrated images of the borehole radar reflection data together with the optical televiewer data reveal a complicated network of intersecting fractures in the damage zone. Consequently, the associated petrophysical properties, notably the sonic velocities and porosities, are distinctly different from intact granitic formations. Cluster analyses of the borehole logs in combination with the structural interpretations of the optical televiewer data illustrate that the variations in the petrophysical properties are predominantly governed by the intense brittle deformation. The imaged fracture network and the high-porosity zones associated with brittle deformation represent the main flow pathways. This interpretation is consistent with the available geophysical measurements as well as the analyses of the retrieved core material. Furthermore, the interpretation of the self-potential and fluid resistivity log data suggests a compartmentalized hydraulic behavior, as evidenced by inflows of water into the borehole from different sources, which is likely to be governed by the steeply dipping structures.

scholarly journals Characteristics of a fracture network surrounding a hydrothermally altered shear zone from geophysical borehole logs

2019 ◽  
Author(s):  
Eva Caspari ◽  
Andrew Greenwood ◽  
Ludovic Baron ◽  
Daniel Egli ◽  
Enea Toschini ◽  
...  
Keyword(s):  
Shear Zone ◽  
Damage Zone ◽  
Fracture Network ◽  
Core Material ◽  
Petrophysical Properties ◽  
Brittle Deformation ◽  
Borehole Radar ◽  
Borehole Logs ◽  
Self Potential

Abstract. Hydrothermally active and altered fault/shear zones in crystalline rocks are of practical importance because of their potential similarities with petrothermal reservoirs and exploitable natural hydrothermal systems. The petrophysical and hydraulic characterization of such structures is therefore of significant interest. Here, we report the results of corresponding investigations on a prominent shear zone of this type located in the crystalline Aar massif of the central Swiss Alps. A shallow borehole was drilled, which acutely intersects the core of the shear zone and is entirely situated in its surrounding damage zone. The focus of this study is a detailed characterization of this damage zone based on geophysical borehole measurements. For this purpose, a comprehensive suite of borehole logs, comprising passive and active nuclear, full-waveform sonic, resistivity, self-potential, optical televiewer, and borehole radar data, was collected. The migrated images of the borehole radar reflection data together with the optical televiewer data reveal a complicated network of intersecting fractures in the damage zone. Consequently, the associated petrophysical properties, notably the sonic velocities and porosities, are distinctly different from intact granitic formations. Cluster analyses of the borehole logs in combination with the structural interpretations of the optical televiewer data illustrate that the variations in the petrophysical properties are predominantly governed by the intense brittle deformation. The imaged fracture network and the high-porosity zones associated with brittle deformation represent the main flow pathways. This interpretation is consistent with the available geophysical measurements as well as the analyses of the retrieved core material. Furthermore, the interpretation of the self-potential and fluid resistivity log data suggests a compartmentalized hydraulic behavior, as evidenced by inflows of water into the borehole from different sources, which is likely to be governed by the steeply dipping structures.

scholarly journals Insights from elastic thermobarometry into exhumation of high-pressure metamorphic rocks from Syros, Greece

2020 ◽  
Author(s):  
Miguel Cisneros ◽  
Jaime D. Barnes ◽  
Whitney M. Behr ◽  
Alissa J. Kotowski ◽  
Daniel F. Stockli ◽  
...  
Keyword(s):  
High Pressure ◽  
Oxygen Isotope ◽  
Garnet Growth ◽  
Metamorphic Rocks ◽  
Core Complex ◽  
Subduction Channel ◽  
Pressure Groups ◽  
Second Stage ◽  
Mineral Growth ◽  
Metamorphic Terranes

Abstract. We combine elastic thermobarometry with oxygen isotope thermometry to quantify the pressure-temperature (P-T) evolution of retrograde metamorphic rocks of the Cycladic Blueschist Unit (CBU), an exhumed subduction complex exposed on Syros, Greece. We employ quartz-in-garnet and quartz-in-epidote barometry to constrain pressures of garnet and epidote growth near peak subduction conditions and during exhumation, respectively. Oxygen isotope thermometry of quartz and calcite within boudin necks was used to estimate temperatures during exhumation and to refine pressure estimates. Three distinct pressure groups are related to different metamorphic events and fabrics: high-pressure garnet growth at ~1.4–1.7 GPa between 500–1550 °C, retrograde epidote growth at ~1.3–1.5 GPa between 400–500 °C, and a second stage of retrograde epidote growth at ~1.0 GPa and 400 °C. These results are consistent with different stages of deformation inferred from field and microstructural observations, recording prograde subduction to blueschist-eclogite facies and subsequent retrogression under blueschist-greenschist facies conditions. Our new results indicate that the CBU experienced cooling during decompression after reaching maximum high-pressure/low-temperature conditions. These P-T conditions and structural observations are consistent with exhumation and cooling within the subduction channel in proximity to the refrigerating subducting plate, prior to Miocene core-complex formation. This study also illustrates the potential of using elastic thermobarometry in combination with structural and microstructural constraints, to better understand the P-T-deformation conditions of retrograde mineral growth in HP/LT metamorphic terranes.

Spatio-Temporal Shifts in Magmatism and Mineralization in Northern Colorado Beginning in the Late Eocene

2021 ◽  
Author(s):  
Joshua M. Rosera ◽  
Sean P. Gaynor ◽  
Drew S. Coleman
Keyword(s):  
Late Eocene ◽  
Low Grade ◽  
Extensional Tectonics ◽  
Porphyry Deposits ◽  
Higher Power ◽  
Northern Colorado ◽  
Melting Event ◽  
Spatio Temporal ◽  
Spatiotemporal Analyses ◽  
Weak Zones

Abstract Magmatism in northern Colorado beginning in the late Eocene is associated with the formation of Pb-Zn-Ag carbonate-replacement and polymetallic vein deposits, the onset of caldera-forming magmatism, and eventually, the formation of rift-related, F-rich Mo porphyries (“Climax-type” intrusions). We use high-precision U/Pb zircon geochronology to better evaluate the temporal framework of magmatism and mineralization in the region. Our results demonstrate that mineralization in the Leadville area occurred between 43.5 and 39.7 Ma and was followed by mesothermal mineralization in the Montezuma area at approximately 38.7 Ma. Mineralization is associated with a suite of approximately 43 to 39 Ma intermediate magmatic centers that extended from Twin Lakes through Montezuma. The oldest porphyries associated with F-rich Mo prospects and deposits (Middle Mountain; 36.45 Ma) intruded 900 kyr after the start of the ignimbrite flare-up in the region. Spatiotemporal analyses reveal that the pattern of magmatism shifted in orientation between 40 and 35 Ma. We propose a model wherein magmatism before 39 Ma was the result of fluids evolved from the subducted Farallon slab being focused through weak zones in the lithospheric mantle and into the lower crust. This was followed by a more diffuse and higher power melting event that corresponds to a distinct change in the spatial patterns of magmatism. Our data suggest that low-grade Mo porphyry deposits can form close in time to calderas. We hypothesize that the transition from subduction to extensional tectonics in the region was responsible for this more widespread melting and a distinct shift in the style of magmatic-hydrothermal mineralization.

scholarly journals Extensional unroofing of the Veliki Jastrebac dome (Serbia)

2007 ◽  
pp. 21-27 ◽  
Author(s):  
Milun Marovic ◽  
Ilija Djokovic ◽  
Marinko Toljic ◽  
Darko Spahic ◽  
Jelena Milivojevic
Keyword(s):  
Late Cretaceous ◽  
Vertical Section ◽  
Metamorphic Core Complex ◽  
Metamorphic Rocks ◽  
Low Grade ◽  
Structural Elements ◽  
Extensional Tectonics ◽  
Core Complex ◽  
Vardar Zone ◽  
Metamorphic Core

This paper presents the basic structural elements of the dome of Veliki Jastrebac, as well as the chronology and mechanisms of the deformational events responsible for its formation. It was determined that the dome of Veliki Jastrebac consists of two large sequences which are, in the vertical section, in the inverse position. The lower part is made of Late Cretaceous and Cretaceous-Palaeogene low-grade to medium-grade metamorphic rocks, which are intruded by Paleogene granitoid (probably the Vardar Zone), which are covered with a large overthrust consisting metamorphics of the Serbian-Macedonian Mass. The low-grade to medium-grade metamorphosed complex of Veliki Jastrebac, with the granitoid, represents a metamorphic core complex, exhumed by mechanisms of extensional tectonics in the Paleogene.

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