Collisional Orogeny in the Scandinavian Caledonides (COSC): Some preliminary results from drilling of the 2.276 km deep COSC-2 borehole, central Sweden

2021 ◽  
Author(s):  
Christopher Juhlin ◽  
Bjarne Almqvist ◽  
Mark Anderson ◽  
Mark Dopson ◽  
Iwona Klonowska ◽  
...  
Keyword(s):  
High Amplitude ◽  
Granitic Rocks ◽  
Magnetic Data ◽  
Precambrian Basement ◽  
Tectonic Model ◽  
Core Recovery ◽  
Alum Shale ◽  
Borehole Seismic ◽  
Seismic Reflectors ◽  
Seismic Measurements

<p>COSC investigations and drilling activities are focused in the Åre-Mörsil area (Sweden) of central Scandinavia. COSC-2 was drilled with nearly 100% core recovery in 2020 to 2.276 km depth with drilling ongoing from mid-April to early August. Drilling targets for COSC-2 included (1) the highly conductive Alum shale, (2) the Caledonian décollement, the major detachment that separates the Caledonian allochthons from the autochthonous basement of the Fennoscandian Shield, and (3) the strong seismic reflectors in the Precambrian basement.</p><p>Combined seismic, magnetotelluric (MT) and magnetic data were used to site the COSC-2 borehole about 20 km east-southeast of COSC-1. Based on these data it was predicted that the uppermost, tectonic occurrence of Cambrian Alum shale would be penetrated at about 800 m, the main décollement in Alum shale at its stratigraphic level at about 1200 m and the uppermost high amplitude basement reflector at about 1600 m. Paleozoic turbidites and greywackes were expected to be drilled down to 800 m depth. Below this depth, Ordovician limestone and shale with imbricates of Alum shale were interpreted to be present. Directly below the main décollement, magnetite rich Precambrian basement was expected to be encountered with a composition similar to that of magnetic granitic rocks found east of the Caledonian Front. The actual depths of the main contacts turned out to agree very well with the predictions based on the geophysical data. However, the geology below the uppermost occurrence of Alum shale is quite different from the expected model. Alum shale was only clearly encountered as a highly deformed, about 30 m thick unit, starting at about 790 m. Between about 820 and 1200 m, preliminary interpretations are that the rocks mainly consist of Neo-Proterozoic to Early Cambrian tuffs. Further below, Precambrian porphyries are present. The high amplitude reflections within the Precambrian sequence appear to be generated by dolerite sheets with the uppermost top penetrated at about 1600 m. Several deformed sheets of dolerite may be present down to about 1930 m. Below this depth the rocks are again porphyries.</p><p>A preliminary conclusion concerning the tectonic model is that the main décollement is at about 800 m and not at 1200 m. Also the thickness of the lowermost Cambrian/uppermost Neoproterozoic sediments on top of the basement is much greater than expected (hundreds of meters instead of tens of meters) and likely to have been thickened tectonically. Detailed studies are required to assess the actual importance of the “main décollement” and the degree, type and age of deformation in its footwall. We can also conclude that the Precambrian basement is very similar to the Dala porphyries succession that are typically present farther south.</p><p>An extensive set of downhole logging data was acquired directly after drilling. Borehole seismic measurements in 2021 will help to define and correlate seismic boundaries with lithology and structures in the core. Unfortunately, work for describing the geology of the drill core in detail is still on hold due to Covid-19.</p>

Isotopic data bearing on the origin of Mesozoic and Tertiary granitic rocks in the western United States

1984 ◽  
Vol 310 (1514) ◽  
pp. 743-753 ◽  
Keyword(s):  
Granitic Rocks ◽  
Precambrian Basement ◽  
Regional Survey ◽  
Regional Patterns ◽  
Peraluminous Granite ◽  
Isotopic Compositions ◽  
Geographic Coverage ◽  
Magma Flux ◽  
Archean Basement ◽  
Mantle Magma

A regional survey of initial Nd and Sr isotopic compositions has been done on Mesozoic and Tertiary granitic rocks from a 500 000 km 2 area in California, Nevada, Utah, Arizona, and Colorado. The plutons, which range in composition from quartz diorite to monzogranite, are intruded into accreted oceanic geosynclmal terrains in the west and north and into Precambrian basement in the east. Broad geographic coverage allows the data to be interpreted in the context of the regional pre-Mesozoic crustal structure. Initial Nd isotopic compositions exhibit a huge range, encompassing values typical of oceanic magmatic arcs and Archean basement. The sources of the magmas can be inferred from the systematic geographic variability of Nd isotopic compositions. The plutons in the accreted terrains represent mantle-derived magma that assimilated crust while differentiating at deep levels. Those emplaced into Precambrian basement are mainly derived from the crust. The regional patterns can be understood in terms of: (1) the flux of mantle magma entering the crust; (2) crustal thickness; and (3) crustal age. The mantle magma flux apparently decreased inland; in the main batholith belts purely crustal granitic rocks are not observed because the flux was too large. Inland, crustal granite is common because mantle magma was scarce and the crust was thick, and hot enough to melt. The values of peraluminous granite formed by melting of the Precambrian basement depend on the age of the local basement source.

The Precambrian problem in younger orogenic zones: An example from Japan

1968 ◽  
Vol 5 (3) ◽  
pp. 643-648 ◽  
Author(s):  
Tatsuro Matsumoto ◽  
Masaru Yamaguchi ◽  
Takeru Yanagi ◽  
Susumu Matsushita ◽  
Ichikazu Hayase ◽  
...  
Keyword(s):  
Radiometric Dating ◽  
Granitic Rocks ◽  
Northwestern Part ◽  
Precambrian Basement ◽  
Early Paleozoic ◽  
Central Japan ◽  
Field Evidence ◽  
Orogenic Zone

We have examined some of the presumed Precambrian basement metamorphic and granitic rocks in Japan, through radiometric dating as well as on field evidence, and have found that mineral ages of about 175 to 250 m.y. are abundant in the Hida area, northwestern part of central Japan, that a number of thrust rocks in southwestern Japan show ages of 400 to 450 m.y., and that the oldest of the measured samples is about or somewhat over 500 m.y. Little evidence is available to support a view that the Pre-Sinian rocks, if ever existent, have remained unaltered under such a polycyclic orogenic zone as that represented by Japan, although remnants of the youngest Precambrian to Early Paleozoic cycle can be detected.

An improved tectonic model for the Eocene opening of the Norwegian–Greenland Sea: Use of modern magnetic data

2007 ◽  
Vol 24 (1) ◽  
pp. 53-66 ◽  
Author(s):  
O. Olesen ◽  
J. Ebbing ◽  
E. Lundin ◽  
E. Mauring ◽  
J.R. Skilbrei ◽  
...  
Keyword(s):  
Magnetic Data ◽  
Greenland Sea ◽  
Tectonic Model ◽  
Sea Use

scholarly journals Basement Classification through Enhanced Magnetic Data Reductions in parts of Ekiti State, Southwestern Nigeria

2017 ◽  
Vol 6 (1) ◽  
pp. 1
Author(s):  
Adedibu Akingboye ◽  
Abimbola Ogunyele
Keyword(s):  
Automatic Gain Control ◽  
Gain Control ◽  
High Amplitude ◽  
Analytic Signal ◽  
Magnetic Data ◽  
Horizontal Gradient ◽  
Southwestern Nigeria ◽  
Basement Rocks ◽  
Low Amplitude ◽  
Data Reductions

Enhanced magnetic data reductions via the use of various forms of filters were employed for basement classification in parts of Ekiti State. The data reductions and enhancement involve: reduction to equator (RTE), regional and residual, automatic gain control (AGC), downward continuation, upward continuations (1, 2, and 3 km), analytic signal (AS) and horizontal gradient (HG) to map and delineate basement rocks and structures, while surface relief and spectral plot were used to determine depth to top of magnetic sources. The images revealed that the study area is characterized by different lithologies. The rocks evinced lineaments and faults trending NE-SW (G–G’, H–H’, J–J’, K–K’), NNE-SSW, E-W (minor) and approximately N-S, while the dykes are in NW-SE, NNW-SSW directions. The analytic signal (AS) and horizontal gradient (HG) revealed high amplitude reversed Z-like shape as migmatite rocks, differentiating them from the two flanks with low amplitude signals as schist and quartzite schist of Ijero and Aramoko and the granitic intrusive within these migmatised rocks around Ijan, Gbonyin and Ise/Otun. The shaded relief and the spectral plot showed that the total depth to top of magnetic sources ranged from 20m to 1.8km for shallower and deeper sources respectively.

3D slowness time coherence for sonic imaging

Geophysics ◽  
2019 ◽  
Vol 84 (5) ◽  
pp. D179-D189 ◽  
Author(s):  
Nicholas N. Bennett
Keyword(s):  
Feature Extraction ◽  
Modeling And Simulation ◽  
Ray Tracing ◽  
Propagation Direction ◽  
Central Issue ◽  
Imaging Results ◽  
Critical Interpretation ◽  
The Many ◽  
Borehole Seismic ◽  
Seismic Measurements

Although sonic imaging can provide higher resolution images of the near-wellbore region than lower frequency seismic and borehole seismic measurements, many challenges confront its more widespread use. The traditional sonic imaging workflow of first filtering the borehole modes and then migrating the underlying reflected arrival events ignores a critical interpretation step, namely, characterizing these reflected arrivals in terms of their azimuths, raypath types, and other attributes. Furthermore, using sonic imaging results in subsequent modeling and simulation workflows requires determining the 3D coordinates or at least the true dip and azimuth of these near-wellbore reflectors, and feature extraction from noisy 2D sonic imaging migration images cannot provide either of these. To address these interpretation challenges and develop a means of mapping these reflectors without requiring a migration, a central issue that arises is whether we can determine the slowness and propagation direction of a reflected wavefield using a standard array of receiver sensors mounted around the circumference of a tool sonde. To accomplish this task, we have developed 3D slowness time coherence (STC). We combine an automated time pick with a ray-tracing procedure and our 3D STC processing to evaluate the many candidate arrival events that may be present in the filtered waveform measurements, which leads to a 3D map of the reflectors that can be readily integrated into digital models of the surrounding subsurface as well as logs of reflector true dip and azimuth that can be compared with similar logs produced from borehole images.

Geology and Ages of Buried Precambrian Basement Rocks, Manitoulin Island, Ontario

1975 ◽  
Vol 12 (7) ◽  
pp. 1175-1189 ◽  
Author(s):  
W. R. Van Schmus ◽  
K. D. Card ◽  
K. L. Harrower
Keyword(s):  
Granitic Rocks ◽  
Precambrian Basement ◽  
Aeromagnetic Data ◽  
Grenville Province ◽  
Metasedimentary Rocks ◽  
Basement Rocks ◽  
Metavolcanic Rocks ◽  
Northern Half ◽  
East End ◽  
Archean Basement

The geology of the buried Precambrian basement under Manitoulin Island in northern Lake Huron, Ontario, has been re-evaluated on the basis of aeromagnetic data, well cuttings, core samples, and rubidium–strontium and uranium–lead geochronologic data on some of the subsurface samples. We conclude that the northern half of the island is underlain in part by Huronian metasedimentary rocks, but that these are absent from the southern part of the island, which is underlain by granitic, gneissic, and metavolcanic rocks. Granitic and gneissic rocks are also present under the northern half of the island.Geochronologic data show that rocks underlying major positive aeromagnetic anomalies are quartz-monzonitic composite plutons which are about 1500 ± 20 m.y. old. Surrounding metasedimentary. gneissic, and granitic rocks are at least 1700 m.y. old. No evidence was found for extrapolation of the pre-Huroman Archean basement beneath Manitoulin Island; if it is present it has been affected by younger metamorphic overprinting.The south west ward extension of the boundary zone between the Grenville Province and rocks to the west can he traced along the east end of Manitoulin Island on the basis of aeromagnetic data.

GEOLOGY OF THE ZECHSTEIN BASEMENT OF THE LEGNICA-GŁOGÓW COPPER DISTRICT (LGOM) AND ITS SURROUNDINGS: A CRITICAL OVERVIEW

2017 ◽  
pp. 0-0 ◽  
Author(s):  
Hubert KIERSNOWSKI ◽  
Zdzisław Petecki
Keyword(s):  
Fracture Zone ◽  
Sedimentary Cover ◽  
Middle Part ◽  
Magnetic Data ◽  
Lower Permian ◽  
Areal Extent ◽  
Tectonic Model ◽  
Critical Overview ◽  
Deep Fracture ◽  
Northern Segment

On the background of a critical review of earlier studies and concepts concerning the structure of the pre-Permian basement of LGOM, the authors’s own, modified version of its tectonic model is presented, accompanied by that explaining the sedimentation and areal extent of the Lower Permian sedimentary cover. Integrated sedimentological, structural and tectonic analysis was applied in conjunction with geophysical analysis based on gravimetric and magnetic data. As a result of the analysis, the pre-Permian basement of the LGOM area is interpreted to be part of the northern Variscan internides and, at the same time, to constitutea northerly extension of the Fore-Sudetic block (Sieroszowice block) . Within the LGOM area, in its southern part, there are middle Odra fault zone, comprising the southern middle Odra fault (Lubin trough) and northern middle Odra fault, separated with the Sieroszowice block. In the middle part of the LGOM area located is the Głogów trough, cut by the Silesia-Lubusz fault in its northern segment and by the southern part of the vast middle Odra deep fracture zone.

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