| 1. |
- Elming, Sten-åke, et al.
(författare)
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A palaeomagnetic study and age determinations of Tertiary rocks in Nicaragua, Central America
- 2001
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Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 147:2, s. 294-309
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Tidskriftsartikel (refereegranskat)abstract
- Palaeomagnetic, K-Ar and Ar40/Ar39 measurements have been made to elucidate Nicaragua's plate tectonic history. These show that the Nicaraguan Highland rotated 90° counterclockwise between 30 and 18 Myr ago, but the Pacific Coastal Plain has not rotated.A lack of data has prevented a direct comparison with the tectonic history of adjacent areas, in particular, the Chortis Block, which is the part of the Caribbean Plate that is geologically most closely related to the Nicaraguan Highland. However, our data suggest that the Caribbean Plate in this part of Central America was not consolidated until c. 15 Ma.
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| 2. |
- Artyushkov, E V, et al.
(författare)
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The cause of loss of lithospheric rigidity in areas far from plate tectonic activity
- 2000
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Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 143:3, s. 752-776
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Forskningsöversikt (refereegranskat)abstract
- Significant losses of lithospheric strength are generally considered to be almost entirely associated with abnormal heating or steep lithospheric bending and/or stretching near to active plate boundaries. Several areas-the western Greater Caucasus, the North Crimean basin, the Carpathian foredeep, the Peri-Caspian basin and the Trans-Caspian areas-are shown to have steep basement slopes, usually comprising a difference in height of several kilometres over lateral distances of only 20-30 km, corresponding to very low, similar to 3-5 km, effective elastic thicknesses of the lithosphere. Each of these areas is shown to have undergone rapid steepening of the basement slope, usually within 1-2 Myr but in up to 10 Myr in some areas. At such times, these localities were far from active plate boundaries and in positions where bending forces could not have been transmitted to them from far-distant plate activities. Surface and/or subsurface loading can similarly be excluded as mechanisms for such steepening, and there is no apparent outflow of crustal materials into adjacent regions. It is suggested that such rapid subsidence far from plate tectonic activity is caused by rapid increases in the local density of the lithosphere. This could occur as a result of, for example, a gabbro-eclogite transformation in the lower crust, catalysed by the infiltration of volatiles from the asthenosphere. The resultant contraction of the mafic rocks would be non-uniform in space and produce high deviatory stresses, reducing the viscosity in the lower crust to similar to 10(23) Pa s. This would result in the rapid subsidence of the top of this layer, accompanied by steep ductile bending of the overlying upper crust. Such steer downwarping of the basement would be accompanied by a similar steepening of the underlying weakened mantle. The formation of such steep slopes thus indicates a weakening of the entire lithospheric layer, most probably due to the infiltration of volatiles from the asthenosphere, and unrelated to coeval plate tectonic activity.
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| 3. |
- Eneroth, Erik
(författare)
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Preservation of a magnetization carried by magnetite-inclusions during amphibolite grade metamorphic conditions? An example from the Sarek Dykeswarm, Northern Swedish Caledonides
- 2002
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Ingår i: Geophysical Journal International. - 0956-540X. ; 150:1, s. 65-78
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Tidskriftsartikel (refereegranskat)abstract
- An outcrop consisting of a 608 Myr old sheeted dyke complex and dyke-intruded metasediments has been investigated palaeomagnetically. It is located in the interior of a nappe the basal and upper parts of which suffered penetrative deformation and amphibolite grade metamorphism during the Caledonian orogeny. The only trace from the orogeny at the outcrop is a widely spaced network of brittle and ductile shear zones crosscutting dyke contacts. Foliated amphibolite consisting of hornblende, quartz, plagioclase, garnet and sphene occur locally within them. The opaque mineralogy of the dykes differs from that commonly found in continental dykes, with ilmenite and pyrrhotite being the major visible opaque phases, and there are indications that magnetite inclusions exsolved from silicates carry a primary remanence even though Ar-39-(40) Ar mica ages were reset at ca. 430 Myr during the orogeny. A secondary component carried by pyrrhotite is less prominent but falls on the Baltican APW path at 430 Myr. The inferred primary component has a large directional uncertainty, but indicates low palaeolatitudes for the Baltscandian margin during rifting.
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| 4. |
- Juhlin, C., et al.
(författare)
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Crustal reflectivity near the Archean-Proterozoic boundary in northern Sweden and implications for the tectonic evolution of the area
- 2002
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Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 150:1, s. 180-197
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Tidskriftsartikel (refereegranskat)abstract
- Sm–Nd isotope ratios of 1.9–1.8 Ga granitoids delineate the Archaean–Proterozoic boundary in northern Sweden, an important feature in the Fennoscandian Shield. The boundary strikes approximately WNW–ESE and is defined as a c. 20 km wide zone with juvenile Palaeoproterozoic rocks to the SSW and Archaean and Proterozoic rocks, derived to a large extent from Archaean sources, to the NNE. It therefore constitutes the strongly reworked margin of the old Archaean craton. Extrapolation of the boundary offshore into the Bothnian Bay and correlation with the marine reflection seismic BABEL Lines 2 and 3/4 indicates that the boundary dips to the south-southwest, consistent with interpretation of the Sm–Nd data. In order to tie the BABEL results with onshore surface geology and obtain detailed images of the uppermost crust a short (30 km of subsurface coverage) pilot profile was acquired in the Luleå area of northern Sweden during August 1999. The profile consisted of a high-resolution shallow component (1 kg shots) and a lower-resolution deep component (12 kg shots). Both components image most of the reflective crust, with the deep component providing a better image below 10 s. Comparison of signal penetration curves with data acquired over the Trans-Scandinavian Igneous Belt (a large batholith) indicate the transparent nature of the crust there to be caused by geological factors, not acquisition parameters. Lower crustal reflectivity patterns on the Luleå test profile are similar to those observed on the BABEL lines, suggesting the same lower crust onshore as offshore. Interpreted Archaean reflective upper crust in the NE extends below more transparent Proterozoic crust in the SW. This transparent crust contains a number of high-amplitude reflectors that may represent shear zones and/or mafic rock within granite intrusions. A marked boundary in the magnetic field in the SW has been interpreted as being the result of a gently west-dipping contact zone between meta-sediments and felsic volcanic rocks, however, the seismic data indicate a near-vertical structure in this area. By correlating the onshore and offshore seismic data we have better defined the location of the Archaean–Proterozoic boundary on the BABEL profiles. Our new interpretation of the crustal structure along the northern part of the BABEL Line 2 shows a more bi-vergent geometry than previous interpretations. Comparison of the re-interpreted crustal structure in northern Sweden with that found in the Middle Urals shows several similarities, in particular the accretion of a series of arcs to a stable craton. Based on this similarity and geological data, we deduce that a continental arc accreted to the southwestern margin of the Archaean craton at c. 1.87 Ga. Shortly thereafter, the Skellefte island arc underthrust the continental arc owing to a collision further to the southwest resulting in the bi-vergent crustal structure observed today.
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| 5. |
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| 6. |
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| 7. |
- Shomali, ZH
(författare)
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Empirical Green's functions calculated from the inversion of earthquake radiation patterns
- 2001
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Ingår i: GEOPHYSICAL JOURNAL INTERNATIONAL. - : BLACKWELL SCIENCE LTD. - 0956-540X. ; 144:3, s. 647-655
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, the empirical Green's function is calculated based on a method that inverts the radiation patterns of earthquakes. The method utilizes several close-lying earthquakes with known focal mechanisms that are received by a common seismic station
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| 8. |
- Sjöberg, Lars Erik, et al.
(författare)
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Land uplift near Vatnajokull, Iceland, as observed by GPS in 1992, 1996 and 1999
- 2004
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Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 159:3, s. 943-948
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Tidskriftsartikel (refereegranskat)abstract
- Warming of the climate in the 20th century has been manifested by an ablation of Europe's largest ice cap, Vatnajokull in Iceland. The thin elastic lithosphere and the low-viscosity asthenosphere are responding to the reduction in mass by current land uplift in the vicinity of the ice cap suggested to be of the order of 5-10 mm yr(-1): lithosphere thickness and asthenosphere viscosities compatible with these values have been inferred. From our repeated GPS epoch campaigns in 1992, 1996 and 1999 uplift rates are estimated to be of the order of 5-19 mm yr(-1), and the uplift rate is decreasing by -0.11 +/- 0.01 mm yr(-1) km(-1) with radial distance from the centre of the ice cap. These results deviate from previous Earth rheology models estimated for the region. Our data indicate that the lithosphere thickness might be of the order of 10-20 km and the asthenosphere viscosity may be as low as 5 x 10(17) Pa s, but these parameters need a careful fitting to the estimated uplift rates.
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| 9. |
- Sjöberg, Lars Erik, et al.
(författare)
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The atmospheric geoid effects in Stokes' formula
- 2000
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Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 140:1, s. 95-100
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Tidskriftsartikel (refereegranskat)abstract
- The application of Stokes' formula requires that the atmospheric effect on the gravity anomaly is removed. We show that this direct effect reaches about -40 cm over the Himalayas and Antarctica. The restoring of the atmospheric masses yields the indirect atmospheric effect, reaching about -20 cm for the same regions. Consequently, the total atmospheric effect on the geoid is of the order of - 60 cm. However, for most areas close to sea level, the correction is within a few centimetres. Furthermore, the total atmospheric geoid effect is derived for the truncated as well as the modified Stokes formula. It is emphasized that the traditional (IAG) approach to adding a direct atmospheric effect to gravity may lead to a serious geoid bias in the truncated Stokes formula. However, as all the parameters of the bias are known, it can easily be corrected. In contrast, we suggest that the total atmospheric effect on the geoid be determined separately. In this approach the bias is avoided.
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| 10. |
- Stich, Daniel, et al.
(författare)
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Source parameters of the M w = 6.1 1910 Adra earthquake (southern Spain)
- 2003
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Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 155:2, s. 539-546
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Tidskriftsartikel (refereegranskat)abstract
- We examine and model analogue recordings from 6 early mechanical seismographs for the 1910 June 16 earthquake at Adra, Southern Spain. Modern standard, time-domain analysis techniques were applied to the historical data to estimate the source parameters of the event: The regional sparse network data were inverted for the deviatoric seismic moment tensor. The best moment tensor solution corresponds to a M o = 1.50 . 10 18 Nm, M w 6.1 oblique strike-slip event at 16 km depth. Our preferred faulting solution is: strike 122°, dip 80° rake - 137°, in very good agreement with available neo- and seismotectonic data. The source time function of this earthquake was estimated by deconvolving recordings of a M w 5.5 aftershock that occurred the same day. The time function indicates a total rupture time of 4.5 s, corresponding to estimates for mainshock rupture length of 12 km, and stress drop of 29 bar.
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