Paleornagnetic and geochronological constraints on the post-collisional northward convergence of the southwest Tian Shan, NW China
Paleornagnetic and geochronological constraints on the post-collisional northward convergence of the southwest Tian Shan, NW China
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DOI：
10.1016/j.tecto.2005.08.018
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发表时间：
2005-11-16
期刊：
影响因子：
2.9
通讯作者：
Zhu, RX
中科院分区：
文献类型：
--
作者：
Huang, BC;Piper, JDA;Zhu, RX
本文报道了中国天山山脉西南部沱云盆地古近系玄武岩熔岩和一个由至少14个玄武岩流层和10个红层层组成的夹层红层序列的古地磁和年代学相结合的研究。两种玄武岩基质样品的40Ar/39Ar等时线年龄分别为58.5 +/- 1.3 Ma （2 sigma, MSWD = 0.9）和60.4 +/- 1.3 Ma （2 sigma, MSWD = 1.7）。这些结果与之前公布的类似古近系玄武岩的K-Ar稀释年龄（61.7 +/- 2.3 Ma）进行了比较，证实了该盆地较年轻的岩浆活动以侵入和挤压活动为代表。退磁和成分分析发现，在250-350℃或5mt以下的峰值退磁步骤去除次生剩余物后，存在一种稳定的特征剩余物（ChRM），其极性主要是相反的。岩石磁性分析发现，伪单畴磁铁矿或钛磁铁矿是载体。稳定ChRM通过折叠测试；它可能是在熔岩就位时形成的。整体或收集的结果表明，沱云盆地上、下玄武岩系列（类似于115 Ma）的古地磁数据在95%显著性水平上无法区分，表明该构造域在始新世早期印度/亚洲碰撞开始前的大约50 Ma时间内相对于地球自转轴基本保持静止。因此，碰撞前天山西南部可能没有发生古地磁探测到的地壳缩短。沱云盆地的古地磁资料也表明，在60 Ma左右，天山与欧亚大陆参考纬度之间的古纬度差异很小或没有差异。这支持了先前的证据，即自印度/亚洲碰撞开始和喜马拉雅形成以来，天山附近的中亚地块不太可能相对于欧亚大陆经历明显的北辐合。(c) 2005 Elsevier b.v.。版权所有。
A combined paleomagnetic and geochronological Study is reported of Paleogene basalt lavas and an intercalated red bed succession, comprising a minimum of 14 basalt flows and 10 red bed horizons in the Tuoyun Basin of the southwest Tian Shan Range, China. Two basalt matrix samples yield 40Ar/39Ar isochron ages of 58.5 +/- 1.3 Ma (2 sigma, MSWD = 0.9) and 60.4 +/- 1.3 Ma (2 sigma, MSWD = 1.7). These compare well with a previously published K-Ar dilution age of 61.7 +/- 2.3 Ma for comparable Paleogene basalts and confirm that the younger Pulse of magmatism in this basin is represented by both intrusive and extrusive activity. Demagnetization and component analysis identify a stable characteristic remanence (ChRM) with predominantly reversed polarity following removal of secondary remanence by peak demagnetization steps below 250-350 degrees C or 5 mT. Rock magnetic analysis identifies pseudo-single domain magnetite or titanomagnetite as carriers. The stable ChRM passes a fold test; it was probably acquired at the time of lava emplacement. Results from the bulk or the collection imply that paleomagnetic data from the upper and lower (similar to 115 Ma) basalt series in the Tuoyun Basin are not distinguishable at the 95% significance level and indicate that this tectonic domain remained essentially stationary with respect to the Earth's spin axis for similar to 50 Ma prior to onset of the India/Asia collision in early Eocene times. It is therefore probable that no paleomagnetically detectable crustal shortening Occurred in the southwest Tian Shan prior to collision. Paleomagnetic data sets from the Tuoyun Basin also show that little or no paleolatitude difference is present between the Tian Shan and the reference latitude of Eurasia at similar to 60 Ma. This supports previous evidence suggesting that central Asian blocks in the vicinity of the Tian Shan are unlikely to have experienced appreciable northward convergence relative to Eurasia since onset of the India/Asia collision and initiation of the Himalaya. (c) 2005 Elsevier B.V.. All rights reserved.