Abstract
SUMMARY In this paper, we test the role of vertical axis rotations during transpressional mountain building. To this end, we carried out a palaeomagnetic study in the NE Gobi Altai of southern Mongolia, sampling widely exposed lower Cretaceous lavas allowing comparison of rotation histories of the Ih Bogd, Baga Bogd and Artz Bogd restraining bends at the eastern termination of the Bogd strike-slip zone. We provide new 40 Ar/ 39 Ar ages to show that the stratigraphy of mafic lavas and fluvio-lacustrine sediments on the southern flanks of Mt Ih Bogd and Mt Baga Bogd have ages between ∼125 and ∼122 Ma, and a mafic sill that intrudes the sequence has an age of 118.2 ± 0.8 Ma. The lavas are older than previously dated lavas south of Artz Bogd, with ages of 119‐115 Ma. Palaeomagnetic results from the 119‐115 Ma lavas south of Artz Bogd show a significant steeper inclination than both results from 125 to 122 Ma lavas of Baga Bogd and Ih Bogd, as well as from newly sampled and previously published younger lavas and necks of the 107‐92 Ma Tsost Magmatic Field and Shovon and Khurmen Uul basalts. We explain this result by insufficient averaging of secular variation and small errors induced by overcorrection of bedding tilt. We show that individual lavas in the SE Artz Bogd locality represent individual spot readings of the Earth’s magnetic field and integrate all results obtained from lower Cretaceous lavas in the Gobi Altai. We present a pole, or rather, an apparent polar wander path without significant plate motion from the reference positions of Eurasia, from ∼125 to 95 Ma, with n = 126, λ = 80.8 ◦ , φ = 158.4 ◦ , κ = 25.3, A95 = 2.5, palaeolatitude = 48.2 with a scatter Sλ = 16.7 (Sl = 15.3, Su = 17.8) and a regionally consistent direction for the Gobi Altai of D/I = 11.1 ◦ /65.9 ◦ , � D/� I = 3.8 ◦ /1.9 ◦ . This is one of the best-determined palaeopoles/APWP’s for Asia. Formation of the Ih Bogd, Baga Bogd and Artz Bogd restraining bends was thus not associated with vertical axis rotations larger than our error margin of ∼10 ◦ .
Highlights
Transpressional mountain belts form a distinct and unique class of orogen, in which high elevations can be established with relatively small amounts of thrust displacement compared to collisional and subduction related orogens in active margin settings (Scotese 2001; Cunningham 2005; van Hinsbergen et al 2005; Hafkenscheid et al 2006)
In western and Central Mongolia, two transpressional orogenic belts formed around a passive indentor formed by the Hangay dome (Cunningham 1998, 2005; Fig. 1): the right-lateral Mongolian Altai in the west (Cunningham et al 1996b, 2003a), which formed since the Oligocene (Howard et al 2003, 2006) and the left-lateral Gobi Altai in the south (Cunningham et al 1996a, 1997), which formed since the late Miocene (∼8 Ma: Vassallo et al 2007)
The average direction obtained from the lower Cretaceous lavas at Unegt Khairkhan suggest very large rotations which can by no means be confirmed through structural geological observations, and we interpret these results as bearing of no regional importance
Summary
Transpressional mountain belts form a distinct and unique class of orogen, in which high elevations can be established with relatively small amounts of thrust displacement compared to collisional and subduction related orogens in active margin settings (Scotese 2001; Cunningham 2005; van Hinsbergen et al 2005; Hafkenscheid et al 2006). In western and Central Mongolia, two transpressional orogenic belts formed around a passive indentor formed by the Hangay dome (Cunningham 1998, 2005; Fig. 1): the right-lateral Mongolian Altai in the west (Cunningham et al 1996b, 2003a), which formed since the Oligocene (Howard et al 2003, 2006) and the left-lateral Gobi Altai in the south (Cunningham et al 1996a, 1997), which formed since the late Miocene (∼8 Ma: Vassallo et al 2007) In both ranges, deformation continues today (Baljinnyam et al 1993; Ritz et al 1995; Bayarsayhan et al 1996; Carretier et al 1998, 2002; Philip & Ritz 1999; Ritz et al 2003; Vassallo et al 2005). We tested for regional vertical axis rotations by comparing our palaeomagnetic results with apparent pole positions of stable Eurasia in the early Cretaceous published by Besse & Courtillot (2002), Schettino & Scotese (2005) and Torsvik et al (in press)
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