Continental collision causes deformation in the crust along shear zones. However, the physical and chemical conditions at which these zones operate and the deformation processes that enable up to ...hundreds of km of tectonic transport are still unclear because of the depth at which they occur and the challenges in imaging them. Ancient exhumed collision zones allow us to investigate these processes much better, for example at the COSC‐1 borehole in the central Scandinavian Caledonides. This study combines data from the COSC‐1 borehole with different seismic measurements to provide constraints on the spatial lithological and textural configuration of the Seve Nappe Complex. This is one of the few studies that shows that core‐log‐seismic integration in metamorphic rocks allows to identify the spatial distribution of major lithological units. Especially gamma ray logs in combination with density data are powerful tools to distinguish between mafic and felsic lithologies in log‐core correlation. Our results indicate that reflections along the borehole are primarily caused by compositional rather than textural changes. Reflections in the Seve Nappe Complex are not as distinct as in greater depths but continuous and several of them can be linked to magmatic intrusions, which have been metamorphically overprinted. Their setting indicates that the Seve Nappe Complex consists of the remnants of a volcanic continental margin. Our results suggest that ductile‐deformed middle crustal reflectivity is primarily a function of pre‐orogenic lithological variations which has to be considered when deciphering mountain building processes.
Plain Language Summary
Areas where continents collide experience different kind of deformation. However, these processes and the conditions at which they take place are difficult to study because of the great depth at which they occur. Former collision zones that are closer to the surface these days allow the investigation of these processes much better, for example at the COSC‐1 borehole in the central Scandinavian Caledonides. The challenge remains to image the remnant of these processes in high detail but at the same time over a large area. This study combines data from the COSC‐1 borehole with different geophysical measurements to better understand the lithology and structure of the Seve Nappe Complex. We show that the combination of these data allows us to distinguish between rocks from mafic and sedimentary origin. Our results indicate that the geophysical data along the borehole image the change of the composition of the rocks which probably originates from magmatic intrusions and have been overprinted by geological processes, rather than from fracture zones.
Key Points
Core‐log‐seismic integration works well with metamorphic rocks in the middle crust
Log analysis can distinguish between former mafic and sedimentary lithologies
Reflections at COSC‐1 are caused primarily by compositional variation
Knowledge of the in situ stress state of the Earth's crust plays a key role in understanding geological processes including plate tectonics, earthquakes, slope failure, and igneous emplacement. In ...this paper, we determine the in situ stress orientation from the PTA2 borehole on the island of Hawai'i, drilled into a lava flow dominated sequence between Mauna Kea and Mauna Loa. High‐resolution acoustic images were collected from the open hole interval 886 m to 1,567 m. Based on identification of 371 borehole breakouts for a total length of 310 m, the mean orientation of the minimum horizontal principal stress is N106° and remains constant across different volcanic rock fabrics. Changes in borehole breakout shape are linked to the different strength of volcanic facies and intra‐facies. The orientation of the present‐day stress field at Mauna Kea deviates from the plate forces and regional tectonic stress field. We interpret the compressive stress regime at the PTA2 site as resulting from the competing gravitational fields of the large topographic highs of Mauna Kea and Mauna Loa. Our study reveals that the mass accumulation associated with shield volcano growth imparts significant local variations to the subsurface stress state on volcanic islands consisting of overlapping shield volcanoes. The results have significant implications for stress accumulation leading to brittle failure and flank collapse, along with potentially influencing magma accumulation and ascent pathways during volcanic island evolution. This study provides the first insights into the orientation of the present‐day stress field between the major island forming shield volcanoes of Hawai'i.
Key Points
First documentation of the subsurface in situ stress state between the major island forming shield volcanoes of Mauna Loa and Mauna Kea
Borehole breakouts analysis constrains the present‐day stress field between large shield volcanoes on the island of Hawai'i
Stress state evolution is linked to the progressive build up and interaction of gravitational loads between large shield volcanoes
To help understand volcanic facies in the subsurface, data sets
that enable detailed comparisons between down-hole geophysical data and cored
volcanic intervals are critical. However, in many cases, ...the collection of
extended core intervals within volcanic sequences is rare and often
incomplete due to challenging coring conditions. In this contribution we
outline and provide initial results from borehole logging operations within
two fully cored lava-dominated borehole sequences, PTA2 and KMA1, on the Big
Island of Hawai`i. Data for spectral gamma, magnetic susceptibility, dipmeter
resistivity, sonic, total magnetic field, temperature and televiewer wireline
logs were successfully acquired for the open hole interval ca. 889 m to 1567 m within the PTA2 borehole. Spectral gamma was also collected from inside the
casing of both wells, extending the coverage for PTA2 to the surface and
covering the interval from ca. 300 to 1200 m for KMA1. High-quality core
material was available for both boreholes with almost complete recovery which
enabled high-resolution core-to-log integration. Gamma data are generally low
commonly in the range ca. 7–20 gAPI but are shown to increase up to API of
ca. 60 with some intrusions and with increases in hawaiite compositions in
the upper part of PTA2. Velocity data are more variable due to alteration
within porous volcanic facies than with burial depth, with a general degrease
down-hole. The high-resolution televiewer data have been compared directly to
the core, enabling a comprehensive analysis of the variations in the
televiewer responses. This has enabled the identification of key features
including individual vesicles, vesicle segregations, strained vesicles,
chilled margins, rubble zones, intrusive contacts and pāhoehoe lobe
morphologies, which can be confidently matched between the televiewer data
and the full diameter core. The data set and results of this study include
findings which should enable improved borehole facies analysis through
volcanic sequences in the future, especially where down-borehole data and images
but no core are available.
Heat flow (Q) determined from bottom-hole temperatures measured in oil and gas wells in Alberta show a large scatter with values ranging from 40 to 90 mW m−2. Only two precise measurements of heat ...flow were previously reported in Alberta, and were made more than half a century ago. These were made in wells located near Edmonton, Alberta, and penetrated the upper kilometre of clastic sedimentary rocks yielding heat flows values of 61 and 67 mW m−2 (Garland & Lennox). Here, we report a new precise heat flow determination from a 2363-m deep well drilled into basement granite rocks just west of Fort McMurray, Alberta (the Hunt Well). Temperature logs acquired in 2010-2011 show a significant increase in the thermal gradient in the granite due to palaeoclimatic effects. In the case of the Hunt Well, heat flow at depths >2200 m is beyond the influence of the glacial-interglacial surface temperatures. Thermal conductivity and temperature measurements in the Hunt Well have shown that the heat flow below 2.2 km is 51 mW m−2 (±3 mW m−2), thermal conductivity measured by the divided bar method under bottom of the well in situ like condition is 2.5 W m−1 K−1, and 2.7 W m−1 K−1 in ambient conditions), and the geothermal gradient was measured as 20.4 mK m−1. The palaeoclimatic effect causes an underestimate of heat flow derived from measurements collected at depths shallower than 2200 m, meaning other heat flow estimates calculated from basin measurements have likely been underestimated. Heat production (A) was calculated from spectral gamma recorded in the Hunt Well granites to a depth of 1880 m and give an average A of 3.4 and 2.9 μW m−3 for the whole depth range of granites down to 2263 m, based on both gamma and spectral logs. This high A explains the relatively high heat flow measured within the Precambrian basement intersected by the Hunt Well; the Taltson Magmatic Zone. Heat flow and related heat generation from the Hunt Well fits the heat flow-heat generation relationship determined for other provinces of the Canadian Shield. However, this relationship could not be established for Q estimates from industrial temperatures data for the study area that includes the Taltson Magmatic Zone and neighbouring Buffalo High and Buffalo Utikuma domains to the west. It appears that the spatial wavelength of heat generation change is much smaller than that of heat flow. Thermal modelling of heat flow and heat generation data from the Hunt Well, using mantle heat flow contributions of 15 ± 5 mW m−2 results in lithosphere-asthenosphere boundary depth estimates of near 200 km. This mantle heat flow value is consistent with the range for the stable continental areas, 15 (±3) mW m−2.
The Collisional Orogeny in the Scandinavian Caledonides (COSC) scientific
drilling project aims to characterise the structure and orogenic processes
involved in a major collisional mountain belt by ...multidisciplinary geoscientific research. Located in western central Sweden, the project has
drilled two fully cored deep boreholes into the bedrock of the deeply eroded Early Paleozoic Caledonide Orogen. COSC-1 (2014) drilled a
subduction-related allochthon and the associated thrust zone. COSC-2 (2020,
this paper) extends this section deeper through the underlying nappes (Lower Allochthon), the main Caledonian décollement, and the upper kilometre of basement rocks. COSC-2 targets include the characterisation of orogen-scale detachments, the impact of orogenesis on the basement below the detachment, and the Early Paleozoic palaeoenvironment on the outer margin of palaeocontinent Baltica. This is complemented by research on heat flow, groundwater flow, and the characterisation of the microbial community in the present hard rock environment of the relict mountain belt. COSC-2
successfully, and within budget, recovered a continuous drill core to 2276 m depth. The retrieved geological section is partially different from the expected geological section with respect to the depth to the main
décollement and the expected rock types. Although the intensity of synsedimentary deformation in the rocks in the upper part of the drill core might impede
the analysis of the Early Paleozoic palaeoenvironment, the superb quality of
the drill core and the borehole will facilitate research on the remaining
targets and beyond. Protocols for sampling in the hard rock environment and
subsequent sample preservation were established for geomicrobiological
research and rock mechanical testing. For the former, a sparse sample series along the entire drill core was taken, while the target of the latter was the décollement. COSC-2 was surveyed by a comprehensive post-drilling downhole logging campaign and a combined borehole/land seismic survey in autumn 2021. This paper provides an overview of the COSC-2 (International Continental Scientific Drilling Project – ICDP 5054_2_A and 5054_2_B boreholes) operations and preliminary results. It will be complemented by a detailed operational report and data publication.
The possible benefits of measuring the magnetic flux density in three components continuously along a borehole have been recognized a long time ago by researchers who developed models and ...interpretation schemes for 3-component magnetic borehole data (Parker and Daniell,1979; Gallet and Courtillot, 1989).Common borehole methods provide data not allowing for an orientation with respect to a global reference, since this requires a highly accurate orienta tion system independent of the magnetic measurements. A first attempt to obtain the orientation of the sonde was made by Bosum et al. (1988) using a mechanical gyro and accelerometers. However, at that time the data quality of the gyro did not allow for a continuous 3-component measurement. Steveling et al. (2003) provide an example from the Hawaii Scientific Drilling Project (HSDP) drill hole, where directional information of magnetization was used to separate massive lavas from hyaloclastites. However, their directional analysis was limited to the inclination because information on the tool rotation around the vertical axis was not available.Here, we describe the successful development of an orientation procedure with very high resolution independent of magnetic data. Test data were acquired in the 2.5-km-deep ICDP Outokumpu Research Hole in eastern Finland (Kukkonen, 2007) with the so-called Göttinger Borehole Magnetometer (GBM). The sonde uses three fiber optic gyros (FOGs) exhibiting a small drift of 1.5°h-1 and a high resolution of 9x10-5 degrees. In combination with a built-in Förster magnetometer triplet, the GBM can record the magnetic field in three components as well as the tool orientation continuously. In the Outokumpu drill hole, errors (root mean square) were 0.14° for the inclination and 1.4° for the declination of the magnetic flux density.
Deeply rooted thrust zones are key features of tectonic processes and the evolution of mountain belts. Exhumed and deeply eroded orogens like the Scandinavian Caledonides allow us to study such ...systems from the surface. Previous seismic investigations of the Seve Nappe Complex have shown indications of a strong but discontinuous reflectivity of this thrust zone, which is only poorly understood. The correlation of seismic properties measured on borehole cores with surface seismic data can constrain the origin of this reflectivity. To this end, we compare seismic velocities measured on cores to in situ velocities measured in the borehole. For some intervals of the COSC-1 borehole, the core and downhole velocities deviate by up to 2 km s−1. These differences in the core and downhole velocities are
most likely the result of microcracks mainly due to depressurization.
However, the core and downhole velocities of the intervals with mafic rocks
are generally in close agreement. Seismic anisotropy measured in laboratory
samples increases from about 5 % to 26 % at depth, correlating with a transition from gneissic to schistose foliation. Thus, metamorphic foliation has a clear expression in seismic anisotropy. These results will aid in the evaluation of core-derived seismic properties of high-grade metamorphic rocks at the COSC-1 borehole and elsewhere.
We established a cable-free memory-logging system for drill-string-deployed geophysical borehole measurements. For more than 20 years,
various so-called “logging while tripping” (LWT) techniques have ...been available in
the logging service industry. However, this method has rarely been used in
scientific drilling, although it enables logging in deviated and unstable
boreholes, such as in lacustrine sediment drilling projects. LWT operations
have a far lower risk of damage or loss of downhole logging equipment compared with
the common wireline logging. For this
purpose, we developed, tested, and commissioned a modular memory-logging system that does not require drill string
modifications, such as special collars, and can be deployed in standard
wireline core drilling diameters (HQ, bit size of 96 mm, and PQ, bit size of 123 mm). The battery-powered, autonomous
sondes register the profiles of the natural GR (gamma radiation) spectrum, sonic
velocity, magnetic susceptibility, electric resistivity, temperature, and
borehole inclination in high quality while they are pulled out along with the drill
string. As a precise depth measurement carried out in the drill rig is
just as important as the actual petrophysical downhole measurements, we
developed depth-measuring devices providing a high accuracy of less than 0.1 m deviation from the wireline-determined depth. Moreover, the modular structure of
the system facilitates sonde deployment in online mode for wireline
measurements.
In connection with the Collisional Orogeny in the Scandinavian Caledonides (COSC) project, broad-band magnetotelluric (MT) data were acquired at 78 stations along a recent ca. 55km- long NW-SE ...directed reflection seismic profile (referred to as the COSC Seismic Profile; CSP), with the eastern end located similar to 30 km to the west of the orogenic Caledonian front. The MT component of the project aims at (i) delineating the highly conductive (similar to 0.1 Omega . m) alum shales that are associated with an underlying main decollement and (ii) calibrating the MT model to borehole logs. Strike and distortion analyses of the MT data show a 3-D structure in the western 10 km of the profile around the 2.5 km deep COSC-1 borehole (IGSN: ICDP5054EHW1001) and a preferred strike angle of N34 degrees E in the central and eastern parts of the profile. 2-D modelling of MT impedances was tested using different inversion schemes and parameters. To adjust the resistivity structure locally around the borehole, resistivity logging data from COSC-1 were successfully employed as prior constraints in the 2-D MT inversions. Compared with the CSP, the model inverted from the determinant impedances shows the highest level of structural similarity. A shallow resistor (> 1000 Omega . m) in the top 2-3 km depth underneath the western most 10 km of the profile around COSC-1 corresponds to a zone of high seismic reflectivity, and a boundary at less than 1 km depth where the resistivity decreases rapidly from > 100 to < 1 Omega . m in the central and eastern parts of the profile coincides with the first seismic reflections. The depth to this boundary is well constrained as shown by 1-D inversions of the MT data from five selected sites and it decreases towards the Caledonian front in the east. Underneath the easternmost part of the profile, the MT data show evidence of a second deeper conductor (resistivity < 1 Omega . m) at > 3 km depth. Based upon the COSC-1 borehole logs, the CSP reflection seismic image, and the surface geologic map, the MT resistivity models were interpreted geologically. In the vicinity of COSC-1, the resistor down to 2-3 km depth pertains to the metamorphic Middle Allochthon. The up to 1000-m-thick shallow resistor in the central and eastern parts of the profile is interpreted to overly an imbricated unit at the bottom of the Lower Allochthon that includes the alum shales. In the MT resistivity model, the 300-500 m thick imbricated unit masks the main Caledonian decollement at its bottom. A second possible interpretation, though not favoured here, is that the decollement occurs along a much deeper seismic reflection shallowing from 4.5 km depth in the west to similar to 600 m depth in the east. An additional borehole (COSC-2) is planned to penetrate the Lower Allochthon and the main decollement surface in the central part of the profile and can provide information to overcome this interpretational ambiguity. Using a synthetic study, we evaluate how resistivity logs from COSC-2 can improve the 2-D inversion model.
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