THE PALEOPROTEROZOIC MYLONITE OF KINGS CANYON, COLORADO: A CA. 1.6 GA GREENSCHIST FACIES MYLONITE FAR TO THE NORTH OF THE CONCURRENT ACTIVE TECTONIC MARGIN

The main petrological, mineralogical, and geochemical features of the metavolcanites belonging to the Late Proterozoic tectono-sedimentary units of southern Mauritanides (Termessé and Niokolo Koba – Koulountou) and western Hoggar, Algeria (Timesselarsine), permit a reconstruction of the geodynamic environment that controlled the evolution of the Pan-African ranges of western Africa. Flows and pyroclastic beds metamorphosed under greenschist-facies conditions are interbedded within detritic and carbonate platform sediments, suggesting an unstable environment in the vicinity of a continental margin.The association of oceanic tholeiites slightly enriched in LREE and basalts more enriched in TiO2, Zr, Nb, and LREE displaying alkalic affinities suggests intracontinental rifting succeeded by expansion. The lack of ultrabasic – basic complexes (oceanic paleosutures) in southern Mauritanides and their occurrence in the Hoggar suggest that the opening was bigger in the north than in the south.The later calc-alkalic sequences of the Niokolo Koba – Koulountou units are tectonically associated with the Termessé unit. In the Timesselarsine unit, calc-alkalic acid rocks coexist with the basic lavas previously mentioned. Thus, the closure of the rift related to a continental collision appears to be later in southern Mauritanides than in the Hoggar.

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A transition from diagenesis to greenschist facies within a major Variscan fold/thrust complex in SW England

Mineralogical Magazine ◽

10.1180/minmag.1985.049.352.07 ◽

1985 ◽

Vol 49 (352) ◽

pp. 365-374 ◽

Cited By ~ 16

Author(s):

T. J. Primmer

Keyword(s):

Greenschist Facies ◽

North Coast ◽

Illite Crystallinity ◽

Continuous Section ◽

The North ◽

Mineral Assemblages ◽

Facies Metamorphism ◽

O Isotope ◽

Greenschist Facies Metamorphism ◽

Pelitic Rocks

AbstractThe north coast of Cornwall, from Bude to Newquay, provides a continuous section through a major Variscan fold/thrust complex. Illite crystallinity studies have revealed a transition from diagenesis in the north to greenschist facies metamorphism in the south in the Upper Palaeozoic succession. More detailed studies of mineral assemblages in both metabasites and pelitic rocks support the regional pattern of metamorphism indicated by illite crystallinity, and show that locally in the Tintagel district, the grade of metamorphism may have reached middle to upper greenschist facies. An attempt to correlate the above data with temperatures (108–985°C) derived from O-isotope geothermometers is made. Interpretation of the metamorphic data presented helps to emphasize the tectonic importance of the major structures seen in the fold/thrust complex.

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Recent measurements of subsidence in the Ganges-Brahmaputra Delta, Bangladesh

10.5194/egusphere-egu21-6562 ◽

2021 ◽

Author(s):

Michael S. Steckler ◽

Bar Oryan ◽

Md. Hasnat Jaman ◽

Dhiman R. Mondal ◽

Céline Grall ◽

...

Keyword(s):

Sea Level Rise ◽

Sea Level ◽

Land Subsidence ◽

Anthropogenic Activity ◽

Relative Sea Level ◽

The North ◽

Active Tectonic ◽

Relative Sea Level Rise ◽

Tectonic Boundaries ◽

The Ganges

Deltas, the low-lying land at rivers mouths, are sensitive to the delicate balance between sea level rise, land subsidence and sedimentation. Bangladesh and the Ganges-Brahmaputra Delta (GBD) have been highlighted as a region at risk from sea level rise, but reliable estimates of land subsidence have been limited. While early studies in the GBD suggested high rates of relative sea level rise, recent papers estimate more modest rates. Our objective is to better quantify the magnitude, spatial variability, and depth variation of compaction and subsidence in the GBD in order to better evaluate the processes controlling it and the pattern of relative sea level rise in this vulnerable region.With support from the Bangladesh Water Development Board, we have rehabilitated previously installed GNSS and installed new GNSS co-located with Rod Surface Elevation Tables (RSET) to better understand the balance of subsidence and sedimentation in the coastal zone in SW Bangladesh, which is less affected by the active tectonic boundaries to the north and the east. The continuous GNSSs installed in 2003 and 2012 were mounted on reinforced concrete building roofs. GPS stations in the area yield subsidence rate estimates of 3-7 mm/y.&#160; To densify the subsidence data, in early 2020 we resurveyed 48 concrete Survey of Bangladesh geodetic monuments in SW Bangladesh that were installed in 2002. Although only measured at the start and end of the period, the time span between the two measurements is ~18 years enabling us to estimate subsidence over this timespan.Preliminary results show that about &#189; the sites yielded very high subsidence rates; repeat measurements confirm the suspicion that the monuments at these sites are unstable and have undergone localized subsidence from settling or anthropogenic activity. The remaining sites show an increase in subsidence from the NW to the SE, consistent with estimates of average Holocene subsidence (Grall et al., 2018). However, rates from the campaign stations are much higher than those from continuous GNSS sites, but only slightly higher than an RSET site. We interpret that the continuous building GNSS omit very shallow compaction-related subsidence, while RSETs neglect deep subsidence. This is further reinforced by results from a compaction meter consisting of 6 wells from 20 to 300 m depth with vertical optical fiber strainmeters in each well. They show a decrease in compaction with depth. While initial results require further investigation, we highlight the importance of multiple methodologies for interpreting subsidence rates--deep, shallow, natural, anthropogenic--in vulnerable delta regions.

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How reliable are maximum depositional age estimates based on detrital zircon? An example from Early Palaeozoic successions of the Trondheim Nappe Complex, Scandinavian Caledonides

10.5194/egusphere-egu21-3394 ◽

2021 ◽

Author(s):

Deta Gasser ◽

Tor Grenne ◽

Bjørgunn Dalslåen ◽

Trond Slagstad ◽

David Roberts ◽

...

Keyword(s):

Detrital Zircon ◽

Tectonic Setting ◽

Maximum Error ◽

Detrital Zircons ◽

Greenschist Facies ◽

Scandinavian Caledonides ◽

Nappe Complex ◽

Active Tectonic ◽

Depositional Age ◽

Age Estimates

U-Pb age spectra of detrital zircons are widely used to estimate maximum depositional ages (MDA) for sedimentary successions of various age. Different methods have been proposed for calculating an MDA. The most common are based on calculated ages of either the youngest single grain (YSG), the youngest grain cluster composed of three or more grains that overlap at 2&#963; (YGC 2&#963;), or the youngest graphical peak (YPP). Many of these methods produce MDAs consistent with biostratigraphic age or the radiometric age of volcanic horizons within the same unit; however, several studies have shown that MDA estimates based on detrital zircon can be younger than the true depositional age, particularly in active tectonic settings, indicating that the methods should be applied with care for successions where independent depositional age control is lacking.In this contribution we present a compilation of 27 detrital zircon samples from Ordovician to Silurian strata from a part of the Trondheim Nappe Complex of the central Scandinavian Caledonides. The samples belong to six stratigraphically distinct units with independent age control from fossils, dated volcanic horizons or bracketing units of known age. These successions represent various marginal basins filled during the closing stages of the Iapetus Ocean in an overall active tectonic setting with detritus from both continental landmasses and Cambro-Ordovician island arcs. Shortly after deposition, the successions were folded and metamorphosed at up to greenschist facies during Taconian accretionary events and/or the Scandian continent-continent collision.We calculated MDAs by the three methods YSG, YGC 2&#963; and YPP for all samples based on 206Pb/ 238U ages, applying a rigorous discordance filter of 5% (most studies use 10%), in order to use the most reliable analyses possible. Our analysis shows that the YSG MDA is up to 36 m.y. younger than the known depositional age for 17 of the 27 samples, with up to six individual grains giving too young age estimates in some samples. Hence, YSG MDA obviously does not provide a reliable MDA estimate. Of the YGC 2&#963; (weighted mean age) estimates, six are still significantly younger than known depositional age; and an additional seven are younger but overlap with the known depositional age when considering the maximum error on the YGC 2&#963; estimate. The only method which provides an MDA estimate within the age of known deposition or older for all samples is the YPP method.Our results indicate that statistically robust estimates of MDA from detrital zircon data in such an active orogenic setting are provided only by the YPP method; both the YSG and the YGC 2&#963; methods provided unreliably young estimates even with a discordance filter of 5% (using a filter of only 10% makes the problem considerably worse). The spuriously young ages of up to six near-concordant grains in some samples is probably due to concealed lead loss, possibly caused by (fluid-assisted?) recrystallisation of zircon domains during regional greenschist-facies metamorphism shortly after deposition.

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Corona formation around monazite and xenotime during greenschist-facies metamorphism and deformation

European Journal of Mineralogy ◽

10.5194/ejm-32-521-2020 ◽

2020 ◽

Vol 32 (5) ◽

pp. 521-544 ◽

Cited By ~ 1

Author(s):

Felix Hentschel ◽

Emilie Janots ◽

Claudia A. Trepmann ◽

Valerie Magnin ◽

Pierre Lanari

Keyword(s):

Fluid Phase ◽

Eastern Alps ◽

Greenschist Facies ◽

Main Stage ◽

Inductively Coupled ◽

The North ◽

Corona Formation ◽

The Matrix ◽

Heavy Rare Earth Elements ◽

Growth Zones

Abstract. Epidote/allanite–fluorapatite coronae around monazite and xenotime are investigated in Permian pegmatites deformed under greenschist-facies conditions during Alpine tectonometamorphism in the Austroalpine basement, Eastern Alps. The aim was to evaluate the replacement reactions involved in the formation of a corona microstructure, its age and relation to deformation. In the corona core, monazite and xenotime single crystals show domains with different composition and age. Monazite (Mnz1) and xenotime (Xen1) dating by electron microprobe (EPM) reveals an age of 250–287 Ma, consistent with the Permian magmatic age of the pegmatites. These are partly replaced by secondary monazite (Mnz2) and xenotime (Xen2) compositions yielding younger Mesozoic (170–210 Ma) and Alpine (30–120 Ma) ages. The same crystallographic orientation of the primary and secondary monazite and xenotime indicates interface-coupled dissolution–precipitation reactions. Allanite U–Th–Pb dating by laser ablation inductively coupled mass spectrometry in the corona revealed an age of 60±6 Ma, interpreted as the age of corona formation. The coronae around monazite consist of an inner zone of equant fluorapatite grains surrounded by prismatic allanite, which are surrounded by epidote enriched in heavy rare earth elements (HREEs) and REE-poor epidote grains. Compared to coronae around monazite, fluorapatite has higher REE contents and no allanite occurs in the coronae surrounding the xenotime. General reactions for monazite and xenotime breakdown can be written as follows: Mnz1+(Si,Ca,Al,Fe,F)fluid→Mnz2+LREE-Ap+Aln+HREE-Ep+Ep+(Th,U)O2+(Th,U)SiO4,Xen1+(Si,Ca,Al,Fe,F)fluid→Xen2+HREE-Ap+HREE-Ep+Ep+(Th,U)O2. The amount of replacement (judged by the relative proportions of monazite and fluorapatite) is low for monazite included in tourmaline but high within the mylonitic foliation. This dependence on the degree of replacement on the local surrounding microfabric indicates that fluid availability along grain boundaries in the matrix and cracks controlled reaction advancement, allowing the elementary mass transfer required for corona formation (e.g. input of Ca, Al, Si, Fe, F). The oblate shape of the coronae aligned within the foliation of the pegmatites and the deflected foliation around the coronae, without an outer rim of prismatic epidote showing signs of deformation, indicate that the main stage of corona formation took place during deformation and reactions were still ongoing after the main stage of deformation. The corona microstructure documents replacement reactions of a single reactant into multiple distinct mineral growth zones by dissolution and precipitation processes at non-isostatic, greenschist-facies conditions, which prevailed in the area to the north of the Defereggen–Antholz–Vals shear zone between the middle Cretaceous and the Oligocene. These reactions ceased before being completed, and REE gradients within single grains within the corona and on the thin-section scale are preserved, which suggests restricted and/or episodic transport of REE in the fluid phase and/or availability of fluid.

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3-D Inversion of Gravity Data Using Iterative Calculation for Subsurface Modeling of Mt. Pandan, East Java, Indonesia

Jurnal Geofisika ◽

10.36435/jgf.v16i3.388 ◽

2018 ◽

Vol 16 (3) ◽

pp. 28

Author(s):

Waskito Aji ◽

E. J. Wahyudi ◽

D. Santoso ◽

W. G.A. Kadir

Keyword(s):

Gravity Data ◽

Gravity Inversion ◽

Geological Condition ◽

Linear Inversion ◽

Data Inversion ◽

The North ◽

Iterative Calculation ◽

Model Response ◽

Active Tectonic ◽

Surface Geology

Mount Pandan is one of the volcanoes located in East Java.There was an earthquake with magnitude 4.2 onThursday, June 25, 2015, at 10.35 pm located in the area of Mount Pandan. The earthquake indicates the active tectonic of Mount Pandan. In addition, Mount Pandan is one of eleven geothermal prospects in East Java. Thus, it is interesting to do a study related to the geological condition of Mount Pandan. So far, most studies have discussed the related surface geology of Mount Pandan. This study aims to provide information related to the subsurface condition of Mount Pandan based on gravity data. In this research, gravity data inversion modeling technique is used to image the subsurface condition of Mount Pandan. The inversion technique used is non linear inversion with iterative calculation method. In this modeling scheme, the model to be determinedis a layer boundary depth model of two layers of rock based on gravity anomaly data. In each layer the rock is assumed to be composed of a set of rectangular prisms with a certain dimension. A collection of prisms representing the rock layers is then computed by the model response and evaluated by comparing it to the observed data. From this evaluation, a modication of the length of the prism is made so thatthe boundary layer model will change. The process of evalu ation and modication of the model is done iteratively until it reaches the number of predened iterations. The result obtained from this gravity inversion modeling is a model of the subsurface boundary of Mount Pandan. The subsurface model obtained is the existence of a formation resembling a rock intrusion, in this case interpreted as an andesitic rockthat intrudes overlying rock layers. This intrusive formation lies at the peak of Mount Pandan and is continuously to the north, with the undisturbed intrusion located around the summit of Mount Pandan.

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Opening of the north-eastern Atlantic and onshore mountain rise controlled by Fennoscandian deep structure

10.5194/egusphere-egu21-15825 ◽

2021 ◽

Author(s):

Anna Makushkina ◽

Benoit Tauzin ◽

Meghan Miller ◽

Hrvoje Tkalcic ◽

Hans Thybo

Keyword(s):

Continental Margin ◽

Large Scale ◽

Deep Structure ◽

Tectonic Setting ◽

Structural Heterogeneity ◽

Receiver Functions ◽

Mountain Range ◽

North East ◽

The North ◽

Active Tectonic

Large-scale topography is thought to be mainly controlled by active tectonic processes. Fennoscandia is located far from any active tectonic setting and yet includes a mountain range along its passive North Atlantic margin. Models proposed to explain the origin of these enigmatic mountains are based on glacial isostatic adjustments, delamination, long-term isostatic equilibration, and dynamic support from the mantle, yet no consensus has been reached. We show that topography along the continental margin of Fennoscandia may be influenced by its deep structure. Fennoscandia formed by amalgamation of Proterozoic and Archean continental blocks; using both S- and P-receiver functions, we discovered that the Fennoscandian lithosphere still retains the original structural heterogeneity and its western margin is composed of three distinct blocks. The southern and northern blocks have relatively thin crust (~40-45 km), while the central block has thick crust (~60 km) that most likely was formed by crustal stacking during the Proterozoic amalgamation. The boundaries of the blocks continue into the oceanic crust as two major structural zones of the North-East Atlantic, suggesting that the Fennoscandian amalgamation structures determined the geometry of the ocean opening.&#160; We found no evidence for mountain root support or delamination in the areas of high topography that could be related with mountain formation. Instead, our results suggest that both crustal and lithospheric heterogeneity of Fennoscandia along the continental margin might have a control on geodynamic forces that support the rise of Scandinavian mountains.&#160;

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GNSS Constraints to Active Tectonic Deformations of the South American Continental Margin in Ecuador

Sensors ◽

10.3390/s21124003 ◽

2021 ◽

Vol 21 (12) ◽

pp. 4003

Author(s):

José Tamay ◽

Jesús Galindo-Zaldivar ◽

John Soto ◽

Antonio J. Gil

Keyword(s):

Subduction Zone ◽

Main Tool ◽

South American ◽

The South ◽

Nazca Plate ◽

South American Plate ◽

The North ◽

The Pacific ◽

Active Tectonic ◽

Plate Subduction

GNSS observations constitute the main tool to reveal Earth’s crustal deformations in order to improve the identification of geological hazards. The Ecuadorian Andes were formed by Nazca Plate subduction below the Pacific margin of the South American Plate. Active tectonic-related deformation continues to present, and it is constrained by 135 GPS stations of the RENAGE and REGME deployed by the IGM in Ecuador (1995.4–2011.0). They show a regional ENE displacement, increasing towards the N, of the deformed North Andean Sliver in respect to the South American Plate and Inca Sliver relatively stable areas. The heterogeneous displacements towards the NNE of the North Andean Sliver are interpreted as consequences of the coupling of the Carnegie Ridge in the subduction zone. The Dolores–Guayaquil megashear constitutes its southeastern boundary and includes the dextral to normal transfer Pallatanga fault, that develops the Guayaquil Gulf. This fault extends northeastward along the central part of the Cordillera Real, in relay with the reverse dextral Cosanga–Chingual fault and finally followed by the reverse dextral Sub-Andean fault zone. While the Ecuadorian margin and Andes is affected by ENE–WSW shortening, the easternmost Manabí Basin located in between the Cordillera Costanera and the Cordillera Occidental of the Andes, underwent moderate ENE–WSW extension and constitutes an active fore-arc basin of the Nazca plate subduction. The integration of the GPS and seismic data evidences that highest rates of deformation and the highest tectonic hazards in Ecuador are linked: to the subduction zone located in the coastal area; to the Pallatanga transfer fault; and to the Eastern Andes Sub-Andean faults.

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Analysis of tectonic activity of the North Anatolian Fault based on SBInSAR method

E3S Web of Conferences ◽

10.1051/e3sconf/20185500005 ◽

2018 ◽

Vol 55 ◽

pp. 00005 ◽

Cited By ~ 1

Author(s):

Karolina Owczarz

Keyword(s):

North Anatolian Fault ◽

Synthetic Aperture ◽

Tectonic Activity ◽

Sar Images ◽

Synthetic Aperture Radar Interferometry ◽

The North ◽

Tectonic Faults ◽

Surface Displacements ◽

Active Tectonic ◽

Small Baseline

The North Anatolian Fault situated in Turkey is one of the longest and most active tectonic faults in the world. The paper presents an analysis of tectonic activity in its area based on the method of Small Baseline Synthetic Aperture Radar Interferometry. For this purpose 73 satellite SAR images and specialized software GMT5SAR were used with implement the SBAS algorithm. In addition, the most important aspects of data processing and their final products were presented, which determined the surface displacements occurring in the surveyed area from 1 January 2014 to 1 March 2017. The displacements of the SBAS surface area ranged from -10 cm to +10 cm. Based on the obtained results and their analysis, the author also assessed the suitability of SBInSAR technology for areas of land displacement.

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