New paleomagnetic and geochronologic results from Ethiopian Afar: Block rotations linked to rift overlap and propagation and determination of a ~2 Ma reference pole for stable Africa
New paleomagnetic and geochronologic results from Ethiopian Afar: Block rotations linked to rift overlap and propagation and determination of a ~2 Ma reference pole for stable Africa
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DOI：
10.1029/2001jb000645
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发表时间：
2003-02
影响因子：
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通讯作者：
T. Kidane;V. Courtillot;I. Manighetti;L. Audin;P. Lahitte;X. Quidelleur;P. Gillot;Y. Gallet;J. Carlut;T. Haile
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作者：
T. Kidane;V. Courtillot;I. Manighetti;L. Audin;P. Lahitte;X. Quidelleur;P. Gillot;Y. Gallet;J. Carlut;T. Haile
[1]1995-1996年法国-埃塞俄比亚联合实地考察产生了新的地质年代学和古地磁数据，大大扩展了我们对阿法尔凹陷最近岩浆和构造历史的了解。24个新的K-Ar年龄介于0.6 ~ 3.3Ma之间。地磁极性与地磁极性时标（GPTS）有很好的一致性。不确定度小于50 kyr的8个年龄测定结果可用于GPTS的未来重新评估（奥杜威/松山上、下倒转和留尼旺和猛犸亚时）。通过对133个测点865个岩心的古地磁分析，证实了低钛磁铁矿是特征剩余磁化强度（ChRM）的主要载体。阳性倾斜测试（基于分别具有63个和23个位点的两个亚组）证实该ChRM可能是主要磁化。主要古地磁研究结果如下。根据位于东非大裂谷北方终端两侧的26个地点，确定了一个2百万美元的稳定非洲参考极。它位于l = 87.2°N，f = 217.1°E（A95 = 4°）。在231个层状熔岩流中发现了4.6 ± 1.8°（2s）的倾斜变浅，与轴向偶极子的6 ± 2%的全球轴向四极一致。结合Acton等人的早期数据。[2000]，我们的新数据允许确定五个单独的平均古磁场方向，断层边界块先前确定的构造分析在阿法尔中部。这些都遭受了微不足道的旋转垂直轴，因为侵位的熔岩。这与以前在阿法尔省东部发现的三个主要区块的重大轮换形成鲜明对比。总的来说，阿法尔中部和阿法尔东部相对于参考方向的十度差分别为2 ± 4°和13 ± 4°，与Manighetti等人[2001 a]的模型一致。看来，在过去的$3马阿法尔凹陷被广泛包围的陷阱一样的玄武岩，这是由一个单一的，但复杂的物理（构造）过程，结合扩散延伸，裂谷定位，传播，跳跃和重叠，书架断层变形。
[1] Joint French–Ethiopian field trips in 1995–1996 yield new geochronologic and paleomagnetic data, which significantly expand our knowledge of the recent magmatic and tectonic history of the Afar depression. Twenty-four new K-Ar ages range from 0.6 to 3.3 Ma. There is quite good agreement between magnetic polarities and Geomagnetic Polarity Timescale (GPTS). Eight age determinations with uncertainty less than 50 kyr can be used in future reassessments of the GPTS (upper and lower Olduvai/Matuyama reversals and Reunion and Mammoth subchrons). Paleomagnetic analysis of 865 cores from 133 sites confirms that low-Ti magnetites are the main carrier of the Characteristic Remanent Magnetization (ChRM). A positive tilt test (based on two subgroups with 63 and 23 sites, respectively) confirms that this ChRM is likely the primary magnetization. The main paleomagnetic results can be summarized as follows. A $2 Ma reference pole for stable Africa is determined based on 26 sites located on either side of the northern termination of the East African rift. It is located at l = 87.2°N, f = 217.1°E (A 95 = 4°). A 4.6 ± 1.8° (2s) inclination shallowing is identified within a population of 231 stratoid lava flows, consistent with a global axial quadrupole of 6 ± 2% of the axial dipole. Combined with earlier data of Acton et al. [2000], our new data allow mean paleomagnetic field directions to be determined for five individual, fault-bounded blocks previously identified by tectonic analysis within central Afar. These all have suffered negligible rotations about vertical axes since emplacement of the lava. This contrasts with the significant rotations previously uncovered to the east in Djiboutian Afar for three major individual blocks. Taken altogether, the declination differences with respect to reference directions are 2 ± 4° for central Afar and 13 ± 4° for eastern Afar, consistent with the model of Manighetti et al. [2001a]. It appears that in the last $3 Ma the Afar depression was extensively floored by trap-like basalts, which were deformed by a single but complex physical (tectonic) process, combining diffuse extension, rift localization, propagation, jumps and overlap, and bookshelf faulting.