The Athabasca Granulite Terrane and Evidence for Dynamic Behavior of Lower Continental Crust

2018 ◽  
Vol 46 (1) ◽  
pp. 353-386 ◽  
Author(s):  
Gregory Dumond ◽  
Michael L. Williams ◽  
Sean P. Regan
Keyword(s):  
Continental Crust ◽  
Lower Crust ◽  
Tectonic Evolution ◽  
Paradigm Shift ◽  
Lower Continental Crust ◽  
Deep Crust ◽  
And Behavior ◽  
Lower Crustal ◽  
New Evidence ◽  
Lower Crustal Xenolith

Deeply exhumed granulite terranes have long been considered nonrepresentative of lower continental crust largely because their bulk compositions do not match the lower crustal xenolith record. A paradigm shift in our understanding of deep crust has since occurred with new evidence for a more felsic and compositionally heterogeneous lower crust than previously recognized. The >20,000-km2Athabasca granulite terrane locally provides a >700-Myr-old window into this type of lower crust, prior to being exhumed and uplifted to the surface between 1.9 and 1.7 Ga. We review over 20 years of research on this terrane with an emphasis on what these findings may tell us about the origin and behavior of lower continental crust, in general, in addition to placing constraints on the tectonic evolution of the western Canadian Shield between 2.6 and 1.7 Ga. The results reveal a dynamic lower continental crust that evolved compositionally and rheologically with time.

scholarly journals The earthquake cycle in the dry lower continental crust: insights from two deeply exhumed terranes (Musgrave Ranges, Australia and Lofoten, Norway)

2021 ◽  
Vol 379 (2193) ◽  
pp. 20190416 ◽  
Author(s):  
Luca Menegon ◽  
Lucy Campbell ◽  
Neil Mancktelow ◽  
Alfredo Camacho ◽  
Sebastian Wex ◽  
...  
Keyword(s):  
Continental Crust ◽  
Lower Crust ◽  
Shear Zones ◽  
Wall Rock ◽  
Earthquake Cycle ◽  
Geological Time ◽  
Lower Continental Crust ◽  
Seismic Slip ◽  
Crustal Earthquakes ◽  
Deep Source

This paper discusses the results of field-based geological investigations of exhumed rocks exposed in the Musgrave Ranges (Central Australia) and in Nusfjord (Lofoten, Norway) that preserve evidence for lower continental crustal earthquakes with focal depths of approximately 25–40 km. These studies have established that deformation of the dry lower continental crust is characterized by a cyclic interplay between viscous creep (mylonitization) and brittle, seismic slip associated with the formation of pseudotachylytes (a solidified melt produced during seismic slip along a fault in silicate rocks). Seismic slip triggers rheological weakening and a transition to viscous creep, which may be already active during the immediate post-seismic deformation along faults initially characterized by frictional melting and wall-rock damage. The cyclical interplay between seismic slip and viscous creep implies transient oscillations in stress and strain rate, which are preserved in the shear zone microstructure. In both localities, the spatial distribution of pseudotachylytes is consistent with a local (deep) source for the transient high stresses required to generate earthquakes in the lower crust. This deep source is the result of localized stress amplification in dry and strong materials generated at the contacts with ductile shear zones, producing multiple generations of pseudotachylyte over geological time. This implies that both the short- and the long-term rheological evolution of the dry lower crust typical of continental interiors is controlled by earthquake cycle deformation. This article is part of a discussion meeting issue ‘Understanding earthquakes using the geological record’.

Zinc isotopic composition of the lower continental crust estimated from lower crustal xenoliths and granulite terrains

2020 ◽  
Vol 276 ◽  
pp. 92-108
Author(s):  
Ganglan Zhang ◽  
Yongsheng Liu ◽  
Frédéric Moynier ◽  
Yangtao Zhu ◽  
Zaicong Wang ◽  
...  
Keyword(s):  
Isotopic Composition ◽  
Continental Crust ◽  
Lower Continental Crust ◽  
Crustal Xenoliths ◽  
Lower Crustal

Reflection seismic surveys to site the Drilling the Ivrea Verbano zonE (DIVE) proposed drill-holes, Val Sesia and Val d’Ossola, Italy.

2020 ◽  
Author(s):  
Andrew Greenwood ◽  
Ludovic Baron ◽  
Yu Liu ◽  
György Hetényi ◽  
Klaus Holliger ◽  
...  
Keyword(s):  
Continental Crust ◽  
Transition Zone ◽  
Lower Crust ◽  
Seismic Reflection ◽  
Spatial Scales ◽  
Seismic Reflection Data ◽  
Seismic Surveys ◽  
Crustal Rocks ◽  
Drill Holes ◽  
Lower Crustal

<p>The Ivrea-Verbano Zone in the Italian Alps represents one of the most complete and best-studied cross-sections of the continental crust. Here, geological and geophysical observations indicate the presence of the Moho transition zone at shallow depth, possibly as shallow as 3 km in the location of Balmuccia in Val Sesia. Correspondingly, the Ivrea-Verbano Zone is a primary target for assembling data on the deep continental crust as well as for testing several hypotheses regarding its formation and evolution.</p><p>            Within the context of a project submitted to the International Continental Scientific Drilling Program (ICDP), the Drilling the Ivrea-Verbano zonE (DIVE) team proposes to establish three drill holes across pertinent structures within the Ivrea-Verbano Zone. Two of the planned drill holes, each with a length of ~1000 m, are within Val d’Ossola and target the Pre-Permian lower and upper section of the lower crust. The third proposed drill hole, with a length of ~4000 m, is targeting the lower most crust of the Permian magmatic system of the Ivrea-Verbano Zone in the Val Sesia, close to the Insubric Line. Combined, the three drill holes will compose a complete section of the lower crust and the Moho transition zone, and will reveal the associated structural and composition characteristics at different scales.</p><p>To bridge across the range of spatial scales and to support the drilling proposal, we have carried out active seismic surveys using an EnviroVibe source in the Val d’Ossola. These surveys combined 2D transects (in-line) with the simultaneous collection of short cross-lines, and spatially varied source points, to collect sparse 3D data with a preferential CMP coverage across strike. This survey geometry was largely controlled by environmental considerations and access for the vibrator. Accordingly, 2D profiles, both in-line and cross-line, have been processed using crooked-line geometries, which include CMPs from the 3D infill.</p><p>The very high acoustic impedance contrast of the Quaternary valley infill sediments with respect to the predominant metapelitic and gabbroic lower crustal rocks, as well as the highly attenuative nature of the sediments, were both beneficial and problematic. The former enables mapping of the valley structure, while the latter largely prevents the detection of low-amplitude reflections from within the underlying lower crustal rocks.</p><p>Here, we present the latest results of these seismic reflection surveys and discuss the observations with respect to the prevailing structure and the planning of the drilling operations. Beyond the specific objectives pursued in this study, our results have important implications with regard to the acquisition and processing of high-resolution seismic reflection data in crystalline terranes and their capacity for resolving complex, steeply dipping structures.</p>

scholarly journals Fracturing and crystal plastic behaviour of garnet under seismic stress in the dry lower continental crust (Musgrave Ranges, Central Australia)

Solid Earth ◽  
2019 ◽  
Vol 10 (5) ◽  
pp. 1635-1649 ◽  
Author(s):  
Friedrich Hawemann ◽  
Neil Mancktelow ◽  
Sebastian Wex ◽  
Giorgio Pennacchioni ◽  
Alfredo Camacho
Keyword(s):  
Plastic Deformation ◽  
Continental Crust ◽  
High Stress ◽  
Shear Zones ◽  
Granulite Facies ◽  
Lower Continental Crust ◽  
Granulite Facies Metamorphism ◽  
High Calcium ◽  
Central Australia ◽  
Lower Crustal

Abstract. Garnet is a high-strength mineral compared to other common minerals such as quartz and feldspar in the felsic crust. In felsic mylonites, garnet typically occurs as porphyroclasts that mostly evade crystal plastic deformation, except under relatively high-temperature conditions. The microstructure of granulite facies garnet in felsic lower-crustal rocks of the Musgrave Ranges (Central Australia) records both fracturing and crystal plastic deformation. Granulite facies metamorphism at ∼1200 Ma generally dehydrated the rocks and produced millimetre-sized garnets in peraluminous gneisses. A later ∼550 Ma overprint under sub-eclogitic conditions (600–700 ∘C, 1.1–1.3 GPa) developed mylonitic shear zones and abundant pseudotachylyte, coeval with the neocrystallization of fine-grained, high-calcium garnet. In the mylonites, granulite facies garnet porphyroclasts are enriched in calcium along rims and fractures. However, these rims are locally narrower than otherwise comparable rims along original grain boundaries, indicating the contemporaneous diffusion and fracturing of garnet. The fractured garnets exhibit internal crystal plastic deformation, which coincides with areas of enhanced diffusion, usually along zones of crystal lattice distortion and dislocation walls associated with subgrain rotation recrystallization. The fracturing of garnet under dry lower-crustal conditions, in an otherwise viscously flowing matrix, requires transient high differential stress, most likely related to seismic rupture, consistent with the coeval development of abundant pseudotachylyte. Highlights. Garnet is deformed by fracturing and crystal plasticity under dry lower-crustal conditions. Ca diffusion profiles indicate multiple generations of fracturing. Diffusion is promoted along zones of higher dislocation density. Fracturing indicates transient high-stress (seismic) events in the lower continental crust.

scholarly journals A New Magma Type in the Continental Collision Zone. The Case of Capraia Island (Tuscany, Italy)

Geosciences ◽  
2021 ◽  
Vol 11 (2) ◽  
pp. 104 ◽  
Author(s):  
Alba Patrizia Santo
Keyword(s):  
Continental Crust ◽  
Lower Crust ◽  
Distinct Type ◽  
Continental Collision ◽  
Time Interval ◽  
Lower Continental Crust ◽  
Mafic Enclaves ◽  
Magma Type ◽  
Rock Types ◽  
Collision Zone

The Tuscany Magmatic Province consists of a Miocene to Pleistocene association of a wide variety of rock types, including peraluminous crustal anatectic granites and rhyolites, calcalkaline and shoshonitic suites and ultrapotassic lamproites. In addition to the magma types already recognised, the occurrence of a new, distinct magma type at Capraia and Elba islands and in mafic enclaves in the San Vincenzo rhyolites has been suggested by recent studies. This particular type of magma, represented by intermediate to acidic calcalkaline rocks showing high Sr, Ba, and LREE, is restricted to the northwestern sector of the province and to a time interval of about 8 to 4.5 Ma. New data obtained on rocks from Capraia Island have allowed for the verification of the occurrence of this new magma type, the exploration of its origin and a discussion of its possible geodynamic significance. The high-Sr-Ba andesite-dacite rocks occurring in the Laghetto area at Capraia display a composition that is intermediate between adakitic and calcalkaline rocks. It is suggested that they represent a distinct type of magma that originated at mantle pressure by melting of the lower continental crust, followed by mixing with other Capraia magmas. The geodynamic model that best explains the composition of the studied rocks is the thickening of the continental crust during continental collision, followed by extension that favoured melting of the lower crust.

Evolution of crust vs. mantle contributions to continental arc granitoids within a few Myr: evidence from zircon Hf-O isotopes and high-precision U-Pb dating in the Famatinian Arc, Argentina

2020 ◽  
Author(s):  
Julien Cornet ◽  
Oscar Laurent ◽  
Jörn-Frederik Wotzlaw ◽  
Juan Otamendi ◽  
Olivier Bachmann
Keyword(s):  
Continental Crust ◽  
High Precision ◽  
Lower Crust ◽  
Plate Tectonics ◽  
Complex Function ◽  
Mafic Magma ◽  
Isotopic Signatures ◽  
Mafic Intrusions ◽  
Relative Contribution ◽  
Lower Crustal

<p>The presence of a thick continental crust makes Earth a unique planet in the solar system. During post-Archaean times, with the onset of plate tectonics, processes by which continents form is a complex function of juvenile growth and recycling of pre-existing crust. Indeed, post-Archean mantle-derived magmas commonly intrude pre-existing, felsic continental crust. As a result, the origin of upper crustal granitoids, the most accessible products of planetary differentiation, is either accounted for by the melting of the pre-existing mid- to lower crust or the differentiation of mantle-derived mafic magmas. It is therefore critical to identify the relative contribution of these two different granite-forming processes in a given magmatic province, as well as how this relative contribution evolves over time, to assess crustal growth and/or recycling. To shed some light on this question, we used the combination of oxygen, hafnium and uranium-lead isotopic systems in zircons from granitoids of the Ordovician Famatinian Arc (Argentina) representing a typical crust-forming geotectonic setting. While the lower crustal section of Valle Fertíl, representing the basal level of the Famatinian crust, is already well studied, little is known on the timing and nature of igneous processes that built up the mid- and upper crust. </p><p>From our study, we observe a systematic co-variation of the O and Hf isotopic signatures of zircon in the mid- to upper crustal rocks, from a clearly crustal footprint (granodiorites with zircon δ<sup>18</sup>O of ca. +8 ‰; εHf<sub>t</sub> of ca. –3) to a mantle-like signature (granites and rhyolites: zircon δ<sup>18</sup>O of ca. +5 ‰; εHf<sub>t</sub> of ca. +5). Moreover, the high-precision (ID-TIMS) U-Pb dating obtained from the same zircons seem to record a progressive building of the Ordovician continental crust lasting for ca. 13Myrs from 483 to 470 Myrs ago. The results overlap with published ID-TIMS U-Pb data for the Famatinian lower crust, clustering at 470 Myrs, which confirms that the Famatinian Arc was a transcrustal magmatic system ultimately fed by mantle-derived magmas. In details, the oldest granitoids (483 Myrs) show the strongest crustal Hf-O isotopic fingerprint while the younger ones define a continuous range from this end-member towards the mantle signature. These results could be explained by (i) continuous ingrowth and “self-shielding” of lower crustal mafic intrusions progressively decreasing crustal melting or contamination of ascending mafic magma from a homogenous mantle source; (ii) progressive defertilization of an enriched lithospheric mantle or a strongly slab-enriched mantle wedge. The fact that the earliest (483 Myr-old) granitoids also show a more significant crustal contribution (ASI >1.1, inherited zircon cores) supports the first scenario. In this case, the combination of Hf-O isotopic studies as well as high precision U-Pb dating for the Famatinian arc comply with a progressive building of a magmatic column where a certain amount of time is needed for the system to mature and eventually reach mantle dominated processes in the formation of granites and so, new continental crust.</p>

scholarly journals A Reconstitution Approach for Whole Rock Major and Trace Element Compositions of Granulites from the Kapuskasing Structural Zone

Minerals ◽  
2020 ◽  
Vol 10 (6) ◽  
pp. 573
Author(s):  
Robert B. Emo ◽  
Balz S. Kamber
Keyword(s):  
Trace Element ◽  
Continental Crust ◽  
Lower Crust ◽  
Inductively Coupled Plasma ◽  
Bulk Rock ◽  
Lower Continental Crust ◽  
Inductively Coupled ◽  
Incompatible Elements ◽  
Plasma Mass Spectrometry ◽  
Incompatible Trace Elements

Current estimates for the composition of the lower continental crust show significant variation for the concentrations of the highly incompatible elements, including large uncertainties for the heat-producing elements. This has consequences for models of the formation of lower crust. For example, is lower continental crust inherently poor in incompatible elements or has it become so after extraction of partial melts caused by thermal incubation? Answering these questions will require better agreement between estimates for the chemistry of the lower crust. One issue is that granulite samples may have been altered during ascent. Xenoliths often experience contamination from the entraining alkaline magma, potentially resulting in elevated concentrations of incompatible trace elements when analysed by conventional bulk rock techniques. To avoid this, we assessed an in situ approach for reconstructing whole rock compositions with granulites from the Kapuskasing Structural Zone, Superior Province, Canada. As terrain samples, they have not been affected by host magma contamination, and as subrecent glacial exposures, they show minimal modern weathering. We used scanning electron microscope electron dispersive spectroscopy (SEM-EDS) phase mapping to establish the modal mineralogy. Major and trace element concentrations of mineral phases were determined by electron microprobe and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), respectively. These concentrations were combined with the modal mineralogies to obtain reconstructed whole rock compositions, which were compared to conventional bulk rock analyses. The reconstructed data show good reproducibility relative to the conventional analyses for samples with massive textures. However, the conventional bulk rock chemistry systematically yields higher K concentrations, which are hosted in altered feldspars. Thus, even in terrain samples, minor alteration can lead to elevated incompatible element estimates that may not represent genuine lower continental crust.

Petrogenesis of the Higashi-Akaishi Ultramafic Body: Implications for Lower Crustal Foundering and Mantle Wedge Processes

2020 ◽  
Author(s):  
Meghan R Guild ◽  
Christy B Till ◽  
Tomoyuki Mizukami ◽  
Simon Wallis
Keyword(s):  
Continental Crust ◽  
Sierra Nevada ◽  
Lower Crust ◽  
Mantle Wedge ◽  
Metamorphic Belt ◽  
Subduction Channel ◽  
Compositional Evolution ◽  
Garnet Pyroxenite ◽  
Lower Crustal ◽  
Crustal Foundering

Abstract Recycling of ultramafic lower crustal cumulates via delamination or foundering is often invoked as a mechanism to return mafic material to the mantle during continental crust formation. These recycled pieces of the lower crust are rarely sampled but are preserved in several locations including the Kohistan and Talkeetna arc sections, Sierra Nevada and Colorado Plateau pyroxenite xenoliths and, as discussed here for the first time, the exhumed Higashi-Akaishi (HA) ultramafic body in Japan. The HA is located in the Besshi region of the Sanbagawa metamorphic belt in southwestern Japan and is dominantly composed of dunite with lesser garnet pyroxenite and harzburgite lenses. Although the petrogenetic history of the HA body is still debated, our new bulk major and trace element compositions, radiogenic isotope data, as well as petrologic and field observations, are consistent with a lower crustal cumulate origin for the HA dunite and pyroxenite, with a later slab-derived fluid overprint. Clinopyroxene and olivine in the foliated HA dunite have compositions consistent with ultramafic cumulates with high Mg#s (Mg# clinopyroxene = 0·94, Mg# olivine = 0·88), high NiO in olivine (∼0·26 wt %) and low-Al clinopyroxene. In addition, the bulk major element chemistry of the HA dunite and garnet pyroxenite follow systematic behavior in Mg# vs SiO2 wt %, similar to those observed in other lower crustal cumulate lithologies and corresponding intrusive lithologies, pointing to different liquid lines of descent for the corresponding melts. Our new thermobarometric estimates (peak pressure–temperature at 2·6 GPa, 713ºC) are consistent with a hot slab surface subduction path, rather than the lower crustal temperatures recorded in arc sections (Kohistan & Talkeetna: 1 GPa, 800ºC). A pervasive slab-fluid influence is also indicated in the HA lithologies by LREE and Ce enrichments and strong Nb and Zr depletions. The trace elements and the pressure–temperature estimates, as well as the thermodynamic modeling results necessitate removal of the HA body from the lower crust and incorporation into cooler portions of a mantle wedge. At lower crustal conditions, the bulk density of the HA lithologies is greater than the background mantle, indicating the feasibility of lower crustal foundering into a mantle wedge where the HA was incorporated in the subduction channel to reach its peak conditions. Hydration of the HA body while in the subduction channel likely provided the change in density necessary to facilitate its rapid exhumation to the surface. Thus, the HA cumulate likely represents a piece of the subduction system that is rarely preserved, as well as a key component in the compositional evolution of the continental crust.

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