Tectonic reconstruction of the northern Andean blocks: Oblique convergence and rotations derived from the kinematics of the Piedras–Girardot area, Colombia
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Strain partitioning
Echelon formation
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Apparent polar wander
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Felsic
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Southwest Japan rotated clockwise during the late stage of the opening of the Japan Sea, although the exact timing of the rotation is controversial. A recent biostratigraphic study has revealed that the Miocene Oidawara Formation in eastern Southwest Japan was deposited just before 15 Ma; consequently, its paleomagnetic direction may help constrain the timing of rotation. For this purpose, we collected fine felsic tuffs and siltstones at 71 stratigraphic sites (horizons) in the Oidawara Formation. An analysis of alternating field and thermal demagnetization results yielded characteristic remanent magnetization (ChRM) directions for 177 samples. Approximately 80 % (142) of the samples exhibit reverse polarity ChRM directions that are thought to be paleofield directions of reverse polarity Chron C5Br. Normal polarity ChRM directions in 35 samples include primary paleofield records as well as records of secondary magnetization. The data suggest that a short normal polarity interval (microchron or cryptochron) at ~15.8 Ma is present within the dominantly reverse polarity interval of Chron C5Br. Reliable site-mean directions for 19 sites yield a tilt-corrected formation-mean direction of D = 10.5°, I = 41.1°, α 95 = 7.0°, and k = 23.9, indicating virtually no rotation with respect to a reference paleomagnetic direction for the Asian continent. A rotation versus age plot for Southwest Japan indicates that the clockwise rotation started after 17.5 Ma and ceased largely before 15.8 Ma, yielding a rotation rate of ~23°/Myr.
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Magnetostratigraphy
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Polar wander
Felsic
Neogene
Apparent polar wander
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Peninsula
Magnetic declination
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We have studied the paleomagnetism of various Neogene age rocks in southern California as a means of determining the amounts of Oligocene and younger tectonic rotation and translation which has occurred in this region. Our results suggest that fully 25% of this area, in particular the Transverse Ranges, has undergone extreme clockwise rotation. Work in southeastern California implies that 40° of clockwise rotation has occurred here, although paleomagnetic declinations adjacent to a major right lateral fault are apparently rotated over 200°. The crustal block bounded by the San Gabriel and San Andreas fault has undergone a net clockwise rotation of 35° although the data here can be interpreted to show that an original early Miocene rotation of about 50° was followed by a late Miocene or Pliocene counterclockwise rotation of 15°. Paleomagnetic results from the offshore islands suggests that San Clemente, Santa Barbara, and San Nicolas islands have not rotated but Santa Catalina has undergone about 90° of clockwise rotation. All of the northern Channel Islands, including Anacapa, Santa Cruz, Santa Rosa, and San Miguel, are implied to have rotated 70° or 80° clockwise. This result is also found for the Santa Monica Mountains east of Anacapa Island. In the Santa Ynez Range north of the Channel Islands, paleomagnetic study of the Monterey Formation also indicates large clockwise rotations of near 90°. These data also suggest that the Santa Maria Basin is not rotated and that the north boundary of the rotated region is the Santa Ynez River fault. Stratigraphic control on the paleomagnetic data from the Monterey Formation implies that the rotation began about 16 m.y. ago and may be continuing today in the western region. Paleomagnetic inclination data from our study show that the northern Channel Islands, in particular, may have translated 15° northward since middle Miocene time. However, equally valid interpretations of these same data are that the low inclinations are due to the combined effects of erroneous structural corrections, non dipole magnetic field behavior and right offset on the San Andreas fault system. Palinspastic reconstruction of southern California regions for the early Miocene implies that parts of the Transverse Ranges structures were once aligned with north trending extensional structures in the southwestern United States. We propose that Pacific‐American plate interactions both rifted the continental crust to create this pattern and rotated the western‐most structures within a dextral simple shear zone which had a half width of about 400 km.
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New paleomagnetic results from 182 cores sampled at 15 sites in Pliocene and Pleistocene clayey units in the Sant'Arcangelo basin (southern Italy) and its surroundings are presented. They indicate that this area underwent a counterclockwise rotation of 22° after the deposition of the Lower Pleistocene clays.
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Early Pleistocene
Magnetostratigraphy
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Paleomagnetic and geologic studies were made in the Uetsu area, NE Japan, to clarify the Miocene intra-arc deformation associated with the Japan Sea opening. We obtained paleomagnetic directions from the Early to Middle Miocene pyroclastic rocks and lavas in the area. The average paleomagnetic direction is D = 28.0°, I = 48.7° and α95 = 9.6°. It suggests that the clockwise rotation occurred in this area, which is opposite to the counter-clockwise rotation of NE Japan. A fault was discovered which could be the boundary of this rotating domain. The dextral movement is recognized for this fault, concordant with the clockwise rotation in the area. These paleomagnetic and structural data suggest that the clockwise block rotations occurred in the Uetsu area, accompanied by the dextral deformation.
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Magnetic declination
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The Jurassic Scisti Silicei Formation forms part of the Lagonegro superimposed tectonic units I and II that are thought to represent the axial and internal margins of the Mesozoic Lagonegro Basin, prior to nappe formation. Sampling was carried out in the lower (Lagonegro) and upper (Pignola) nappes in two differently oriented anticlines. Single and multi-component magnetizations are present. Isothermal remanence acquisition rates show that magnetite and haematite are present which, in most Lagonegro specimens, show the same direction of magnetization. Comparison of the palaeomagnetic directions with those from Jurassic rocks on the stable African craton indicates a 147° anticlockwise rotation of the lower nappe which is similar to 139° previously reported for the upper nappe at Vietri di Potenza. The same comparisons show a 44° clockwise rotation of the upper nappe at Pignola. These results suggest that the doubled nappe structures, sampled some 50 km apart, resulted from their emplacement by translation with little rotation prior to the opening of the Tyrrhenian Sea and that it was the opening of this Sea that caused the predominantly anticlockwise rotation. This work therefore indicates the way in which palaeomagnetic analyses can be used, even within complex allochthonous areas, as an aid to deciphering their tectonic evolution.
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