Paleomagnetism of the Dazhuqu Terrane, Yarlung Zangbo Suture Zone, Southern Tibet (abstract)

The Dazhuqu terrane comprises several massifs of mid-Cretaceous ophiolitic rocks with the signature of a supra-subduction zone setting. It is associated with the Zedong (island arc) and Bainang (accretionary wedge) terranes, and with these terranes is interpreted to represent remnants of a Jurassic-Cretaceous south-facing intra-oceanic subduction system. Paleomagnetic investigations have been carried out in order to re-assess paleolatitudes at which the ophiolite was generated. For this purpose, oriented samples were collected from several sections of supra-ophiolite deposits, those ages of which were determined using radiolarian fossils. Sampled lithologies include cherts, siliceous mudstones and volcaniclastics. Conventional alternating field and thermal stepwise demagnetization methods were used to analyze specimens. In three sites, Dazhuqu, Donglhe, and Qunrang, characteristic remanent magnetization have been isolated. Occurrence of anti-parallel magnetization components (Dazhuqu) and a positive conglomerate test (Donglhe) suggest a primary origin for remanence. The results of this study indicate generation of the Dazhuqu terrane ophiolite at equatorial latitudes. Some fragments of the ophiolite have experienced a counter-clockwise rotation

A new paleoprecipitation proxy based on soil magnetic properties: implications for expanding paleoclimate reconstructions

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/148590/1/Hyland_et_al_2015_GSA_Bulletin-paleprecipitation_reconstruction.pd

New insights into the evolution of the Yarlung Tsangpo suture zone, Xizang (Tibet), China

Detailed investigations along the Yarlung Tsangpo suture zone, Tibet result in the following conclusions. (1) Arc, forearc, and subduction complex elements of at least one intra-oceanic island arc which once lay within Tethys and accreted to India prior to its collision with Asia are preserved within the suture. (2) Collision-related tectonic mélange is widespread. (3) Temporally distinct conglomeratic molasse units were developed; they are related to different collision events. (4) Improved radiolarian fossil data place constraints on timing of the development of intra-oceanic terranes and mélange formation. (5) A simple unconformable contact occurs between Upper Oligocene-Lower Miocene conglomerates and the southern Lhasa terrane; the so-called south-directed "Gangdese thrust" does not exist. These new results suggest the former existence of an intra-oceanic subduction system within Tethys and indicate that considerable revision of existing models for the evolution of the Yarlung Tsangpo suture zone is required.link_to_subscribed_fulltex

Unraveling the early-middle Paleozoic paleogeography of Kazakhstan on the basis of Ordovician and Devonian paleomagnetic results

It is a common concept that different tectonic units in the western part of the Central Asian Orogenic Belt were united into the landmass of the Kazakhstania continent in the Paleozoic but many important details of its history remain enigmatic and controversial. Recently published paleomagnetic data from this region demonstrate that the ~. 2000. km long horseshoe-shaped Devonian Volcanic Belt was created by oroclinal bending of an originally rectilinear active margin of Kazakhstania. Still, the Silurian and Devonian paleomagnetic results which this interpretation is based upon are limited and unevenly spread along the belt, and additional middle Paleozoic data are highly desirable. Accordingly, we studied three mid-Paleozoic objects from different segments of this volcanic belt. Two of the three new objects yielded paleomagnetic directions that fit perfectly into the oroclinal scenario, whereas the third one provided no interpretable data. The earlier history of Kazakhstania, however, remains misty. We obtained three new Ordovician results in north-central Kazakhstan and found similar inclinations but widely dissimilar declinations. Previously published data show a large scatter of Ordovician declinations in South Kazakhstan and Kyrgyzstan as well. We analyzed all seven Middle-Late Ordovician paleolatitudes and came to the conclusion that a nearly E-W trending active margin of the Kazakhstania landmass had existed at low (~. 10°S) latitudes at that time. We hypothesize that this margin of the Kazakhstania landmass collided with another island arc, called Baydaulet-Akbastau, and with the Aktau-Junggar microcontinent by the Ordovician-Silurian boundary. As a result of this collision, subduction ceased, and regional deformation, magmatism, and rotations of crustal fragments took place in most of Kazakhstania. In Silurian time, Kazakhstania moved northward crossing the equator and rotating clockwise by ~ 45°. This changed the orientation of the Kazakhstania to NW-SE, and thereby established the (rectilinear) predecessor of the modern curved Devonian Volcanic Belt

The similar to 270 Ma palaeolatitude of Baltica and its significance for Pangea models

To better constrain Baltica's position within Pangea, we conducted a palaeomagnetic study of Permo-Triassic dykes from the Oslo Graben, as a follow-up to an initial, but rather limited, study by Torsvik and colleagues in 1998. The age of these so-called Lunner dykes had previously been determined as similar to 240 Ma in that study, but details in their analyses and new Ar-40/Ar-39 ages reveal that there may have been some argon loss in the initially dated dyke minerals and that a combined (weighted mean) age of 271 +/- 2.7 (2 sigma) Myr for the dykes is preferable. We find two major components of magnetization in our samples: one carried by an Fe-sulphide (likely pyrrhotite) and the other carried by low-Ti magnetite; these magnetization components may be found together (superposed) in a given sample or they may occur apart. Micronmetre-sized crystals of Ti-Fe oxides, observed with a scanning electron microscope (SEM) show exsolution lamellae, formed upon cooling from intrusion temperatures. Assuming that the submicronmetre-sized (Ti)-magnetite grains that carry a stable remanence are of the same generation as the observed larger grains, we interpret the magnetite remanence in the dykes as of primary, thermoremanent origin. The sulphide remanence appears to be slightly younger, as seen by the SEM observations of pyrite framboids and a Fe-sulphide grain invading a Ti-magnetite grain. Moreover, the sulphide mineralization is likely of region-wide hydrothermal origin. The magnetizations carried by the pyrrhotite and magnetite have nearly identical directions and so, must be nearly of the same age. For this study, we sampled 56 sites including 39 dykes, 10 baked-contact rocks and 7 host rocks removed from the immediate dyke contacts. The dykes and the contact rocks have the same SW and up directions of magnetization, and contain the Fe-sulphide or the magnetite magnetization or both, as diagnosed by their relative unblocking temperatures. However, all the sampled carbonate and igneous host rocks far away from the dykes also have the same directions. Thus, all of the 10 originally planned contact tests are inconclusive. The new palaeopoles of this study are a few degrees apart; the magnetite pole (from dykes only, N = 25) is located at 51 degrees N, 164 degrees E, K = 69, A(95) = 3.5 degrees, whereas the pole calculated from iron sulphide magnetic directions (all rock types, N = 20) is at 54 degrees N, 166 degrees E, K = 112, A(95) = 3.1 degrees. All directions are of reversed polarity, suggesting that the magnetization was acquired during the Kiaman Reversed Superchron. The palaeomagnetic mean result from the magnetite-bearing sites implies a palaeolatitude of Oslo of 23 degrees N, whereas the palaeolatitude calculated from the pyrrhotite magnetizations is 25-27 degrees N, depending on choice of host lithologies. As noted in many previous publications, the palaeomagnetic poles for the late Palaeozoic and Early-Middle Triassic are in conflict with classical Pangea reconstructions. The poles with ages of 250 +/- 10 Ma, in particular, previously showed a discrepancy of some 25 degrees or more, when the Gondwana and Laurussia continents are restored to their juxtapositions in the Pangea-A fit, before the opening of the Atlantic Ocean. Proposed solutions to this conundrum have been controversial, involving doubts about (1) the geocentric coaxial dipole field model, (2) the reliability of the palaeomagnetic results or their ages, or (3) the validity of the Pangea-A reconstruction, leading to proposals of a Pangea B reconstruction in which Gondwana is displaced some 3500 km to the east with respect to Laurussia. The significance of our new result for this Pangea controversy resides in its improved age within an early Guadelupian (mid to late Permian) time interval where few results exist from well-dated igneous rocks in either Baltica or Laurentia. There are quite a few results from sedimentary rocks, but these may be suspected to suffer inclination shallowing, and are therefore less suitable to settle a palaeolatitudinal argument