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首页> 外文期刊>Journal of Geophysical Research. Biogeosciences >Vertical axis rotation in the Bolivian orocline, South America 1. Paleomagnetic analysis of Cretaceous and Cenozoic rocks
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Vertical axis rotation in the Bolivian orocline, South America 1. Paleomagnetic analysis of Cretaceous and Cenozoic rocks

机译：南美玻利维亚斜盘的垂直轴旋转1.白垩纪和新生代岩石的古磁分析

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The results of a paleomagnetic study are reported for 45 new sites or localities in the central Andes of Bolivia and northern Chile (17 degreesS-23 degreesS), South America, in intrusive, volcanic and sedimentary rocks, ranging in age between similar to7 and similar to 80 Ma. These outcrop across the width of the northern and southern limbs of the Bolivian orocline, from the sub-Andean zone in the east to the volcanic arc in the west. For most localities a mean primary magnetization has been identified which is assumed to reflect a geocentric axial dipole field. A comparison of directions of magnetizations with those predicted by the relevant paleomagnetic reference poles for stable South America reveals declination anomalies which are interpreted in terms of the rotation about a local vertical axis of rigid crustal blocks (hundreds of meters to tens of kilometers across), relative to stable South America, These data, when combined with previous paleomagnetic studies of Upper Cretaceous and Cenozoic rocks, show a pattern of vertical axis rotations which defines three regional domains, extending across the width of the Bolivian Andes. The differences between the various domains are best seen in rocks in the age range similar to 20 to 80 Ma; younger rocks acquired a primary magnetization at later stages in the rotation history. For rocks older than 20 Ma, crustal blocks in domain 1, on the northern flank of the Bolivian orocline, have undergone an anticlockwise rotation about a vertical axis in the range similar to8 to similar to 33 degrees, with a geometric mean of similar to 22; blocks from domain 2 immediately south of the main oroclinal bend have rotated between similar to 19 degrees anticlockwise and similar to 40 degrees clockwise, with a geometric mean of similar to8 degrees clockwise. Even farther south, in domain 3b, blocks have rotated between similar to3 degrees and similar to 57 degrees clockwise, with a geometric mean of 37 degrees. A localised area (domain 3b), in which blocks have undergone similar to 32 degrees clockwise rotation in the last similar to 65 Myr, ties within the southeastern part of domain 1. The northern boundary of domain I and the southern boundary of domain 3a are as yet not well defined. The amount and timing of Cenozoic vertical axis rotation vary across the width of the Andes, and are interpreted to record both regional bending of both limbs of the Bolivian orocline and small block rotation in zones of distributed sinistral or dextral shear during three stages in the Cenozoic: (1) similar to 45 to similar to 25 Ma, (2) similar to 25 to similar to 10 Ma, (3) similar to 10 Ma to 0. Bending of the Bolivian Andes during the last similar to 25 Myr was accommodated, relative to stable South America, by gradients of shortening along the eastern margin of the Andes, particularly in the sub-Andean zone. [References: 69]
机译：据报道，在玻利维亚中部安第斯山脉和南美洲智利北部（17度至23度）内的侵入，火山和沉积岩中有45个新的地点或地点的古地磁研究结果，年龄介于7岁至7岁之间。到80 Ma从东部的亚安第斯山脉带到西部的火山弧，它们从玻利维亚海床北，南四肢的宽度露出。对于大多数地区，已经确定了平均一次磁化强度，假定该平均初级磁化强度反映了地心轴向偶极子场。将磁化方向与有关南美洲稳定的相关古磁参考极所预测的磁化方向进行比较，可以发现磁偏角异常，这些磁偏角可以通过围绕刚性地壳块的局部垂直轴（数百米至数十公里）的旋转来解释，相对于稳定的南美地区，这些数据与先前的上白垩纪和新生代岩石的古磁研究相结合，显示出垂直轴旋转的模式，该模式定义了三个区域性区域，横跨玻利维亚安第斯山脉的宽度。在年龄范围为20至80 Ma的岩石中，最好能看到不同区域之间的差异。年轻的岩石在旋转历史的后期阶段获得了一次磁化强度。对于年龄大于20 Ma的岩石，玻利维亚奥克拉底北侧区域1中的地壳已绕垂直轴逆时针旋转，旋转范围类似于8到33度，其几何平均值为22 ;位于主口折弯以南的区域2的块在逆时针之间旋转了大约19度，在顺时针方向旋转了40度之间，其几何平均值在顺时针方向旋转了8度。在区域3b的更南端，块顺时针旋转了大约3度到大约57度之间，几何平均值为37度。一个局部区域（区域3b），其中区域在区域1的东南部分内受相似于32顺时针方向旋转的块的最后一个类似于65 Myr的约束。区域I的北部边界和区域3a的南部边界为尚未明确。新生代垂直轴旋转的数量和时间在安第斯山脉的整个宽度上变化，并被解释为记录了在新生代的三个阶段中玻利维亚Orocline的两个肢体的局部弯曲和分布的左旋或右旋剪切带中的小块旋转：（1）近似于45近似于25 Ma，（2）近似于25近似于10 Ma，（3）近似于10 Ma等于0。在最后一个近似25 Myr的过程中，玻利维亚安第斯山脉发生了弯曲，相对于稳定的南美而言，沿安第斯山脉东缘（尤其是在安第斯南部地区）沿缩短的坡度减小。 [参考：69]

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