Correcting for inclination shallowing of early Carboniferous sedimentary rocks from Kyrgyzstan—indication of stable subtropical position of the North Tianshan Zone in the mid-late Palaeozoic
Correcting for inclination shallowing of early Carboniferous sedimentary rocks from Kyrgyzstan—indication of stable subtropical position of the North Tianshan Zone in the mid-late Palaeozoic
复制标题
DOI：
10.1093/gji/ggu177
复制
发表时间：
2014-08
影响因子：
2.8
通讯作者：
U. Kirscher;D. Bilardello;A. Mikolaichuk;V. Bachtadse
中科院分区：
文献类型：
--
作者：
U. Kirscher;D. Bilardello;A. Mikolaichuk;V. Bachtadse
中亚早石炭世的高质量古地磁数据很少，乌拉尔山以东地区最终合并之前该地区的古地理演化仍然相当模糊。在这里，我们提出了古地磁数据早石炭世沉积从两个地区在吉尔吉斯斯坦北天山（NTS）。详细的岩石磁性分析表明，磁铁矿和赤铁矿的存在下，作为磁性载体，在这些红色沉积物。在哈萨克盆地剖面（KEL）中，我们在温度高达680 ° C的逐步热退磁过程中确定了一个高温磁化分量（HTC），倾斜校正后得到的站点平均方向为D = 176.2 ° C，I =− 36.4 ° C，k = 57.4，α95 = 8.9 ° C。在Sonkul盆地（DUN）的样品中确定了两种磁化的HTC，最大阻塞温度为10600 ℃（磁铁矿）和10680 ℃（赤铁矿）。磁铁矿成分也被确定与交变磁场退磁。分别在16个和14个地点确定的这两种成分的地点平均方向分别为：磁铁矿D = 149.3 <$，I =− 50.3 <$，k = 73.6和α95 = 4.3 <$;赤铁矿D = 139.6 <$，I =− 35.1 <$，k = 71.6和α95 = 4.7 <$。所有三个平均方向显示了倾斜校正后的精度参数k的显着增加，表明采集的高温磁化在侏罗纪的主要褶皱事件之前。我们解释的差异，DUN的两个组成部分的倾斜偏置的过程中，由于压实板状赤铁矿颗粒更容易受到影响。对DUN地点100多个单独样品的方向数据应用伸长-倾角（E/I）方法，结果磁铁矿成分的校正值可忽略不计（<5度），而赤铁矿成分的倾角校正值为-35.0度至-50.3度（f = 0.6，误差区间为−41.4至−57.9），该值等于未校正的磁铁矿倾角。由于KEL剖面的样本数量较少，无法应用E/I技术，因此应用了基于等温剩磁的高场各向异性的倾斜校正，得到的校正倾斜为−75.2 ± 4。然而，假设两个研究区域的压实程度相当，并将DUN中获得的平整系数应用于KEL的样品，将导致类似的倾斜度。两个剖面的倾斜变浅表明磁化年龄接近沉积年龄。假设两个地区的方向极性相反，DUN剖面的古纬度为30 ° N，KEL剖面基于各向异性校正的古纬度为60 ° N。然而，这种巨大的差异在地质学上是不太可能的。DUN的磁铁矿成分的倾斜度（不受倾斜变浅的影响）支持北纬30度的古位置。这是由DUN的倾斜变浅校正的赤铁矿组成部分得到的支持，产生了可比的倾斜度。因此，我们的研究结果表明，NTS域在早石炭世位于3000 Ni。此外，在晚古生代之前，NTS带可能并不与波罗的海或西伯利亚相连。
SUMMARY High-quality palaeomagnetic data for the early Carboniferous of Central Asia are scarce and the palaeogeographic evolution of this area prior to final amalgamation of the region east of the Ural mountains is still rather obscure. Here, we present palaeomagnetic data for early Carboniferous deposits from two areas in the Kyrgyz North Tianshan (NTS). Detailed rockmagnetic analysis indicates the presence of magnetite and haematite as magnetic carriers in these red sediments. In the Kazakh basin section (KEL), we identify a high-temperature component (HTC) of magnetization during stepwise thermal demagnetization at temperatures of up to ∼680 ◦ C yielding a site mean direction of D = 176.2 ◦ , I =− 36.4 ◦ , k = 57.4 and α95 = 8.9 ◦ after tilt correction. Two HTCs of magnetization were identified in samples from the Sonkul Basin (DUN) with maximum blocking temperatures of ∼600 ◦ C (magnetite) and ∼680 ◦ C (haematite). The magnetite component was also identified with alternating field demagnetization. The resulting site mean directions for these two components identified in 16 and 14 sites, respectively, are D = 149.3 ◦ , I =− 50.3 ◦ , k = 73.6 and α95 = 4.3 ◦ for the magnetite and D = 139.6 ◦ , I =− 35.1 ◦ , k = 71.6 and α95 = 4.7 ◦ for the haematite component. All three mean directions show a significant increase of the precision parameter k after tilt correction indicating acquisition of the high-temperature magnetization prior to the main folding event in the Jurassic. We explain the difference of the two components of DUN by a process of inclination bias due to compaction to which the platy haematite particles are more susceptible. Applying the elongation-inclination (E/I) method to directional data from over 100 individual samples from location DUN results in a negligible correction for the magnetite component (<5 ◦ ), whereas the inclination of the haematite component corrects from −35.0 ◦ to −50.3 ◦ (f = 0.6, error interval −41.4 ◦ to −57.9 ◦ ), which is then equal to the uncorrected magnetite inclination. The small number of samples from section KEL does not allow application of the E/I technique and inclination correction based on high field anisotropy of isothermal remanent magnetization was applied, yielding a corrected inclination of −75.2 ◦ ± 4 ◦ . Assuming comparable degrees of compaction for both study areas and applying the flattening factor obtained in DUN on samples from KEL, however, would result in comparable inclinations. The identification of inclination shallowing at both sections indicates that the age of magnetization is close to the deposition age. Assuming a reversed polarity of the directions from both areas results in palaeolatitudes of ∼30 ◦ N for section DUN and ∼60 ◦ N for the anisotropy-based correction of section KEL. The large difference, however, is geologically very unlikely. The inclination of the magnetite component of DUN (unaffected by inclination shallowing) favours a palaeoposition of ∼30 ◦ N. This is supported by the inclination shallowing corrected haematite component of DUN yielding a comparable inclination. Therefore, our results indicate that the NTS domain was situated at ∼30 ◦ Ni n the early Carboniferous. Furthermore, the NTS zone was probably not connected to Baltica or Siberia prior to the late Palaeozoic.