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Structural Setting, Paleomagnetism, and Magnetic Fabric of Miocene Plutons in a Transpressional Sinistral Shear Zone, Tonalá, Chiapas, Mexico: Evidence of Shortening During Magma Emplacement
Tectonics ( IF 4.2 ) Pub Date : 2021-01-28 , DOI: 10.1029/2020tc006559
Roberto S. Molina Garza 1 , John W. Geissman 2, 3 , Tomás Peña Alonso 4 , Jorge Aranda Gómez 1 , Timothy Wawrzyniec 5
Affiliation  

We studied the structure, paleomagnetism, and magnetic fabric of Miocene granitoids intruded along the left‐lateral, transpressional, Tonalá shear zone in Chiapas, southern Mexico. Magmatic fabrics delineate patterns of regional folding striking parallel to the shear zone, suggesting syn‐magmatic shortening. Magnetic susceptibility fabrics are well developed, and mean magnetic susceptibility averages 18.6 × 10−3 SI. The magnetic fabric of the plutons is controlled primarily by MD and finer PSD magnetite, which also carries the magnetic remanence. The orientations of macroscopic mineral fabrics and magnetic susceptibility fabrics are consistently parallel in protomylonites, but their coincidence is good to poor in granitoids that preserve magmatic fabrics. Most site mean directions are NW‐NE directed of moderate positive inclination (or antipodal), but on a local scale site, mean directions are very discordant. The overall mean direction, omitting highly discordant sites, is of Decl. = 359.6° and Incl. = +41.9° (k = 14.4, α95 = 8.1°, N = 24 sites); it is nearly concordant with the North America reference direction, and the slight difference may indicate a gentle, north‐side down regional tilt. High directional dispersion is interpreted to reflect relative rotation about local vertical to sub‐vertical axes of plutons, or parts of plutons, along the shear zone, with some rotations interpreted to be caused by progressive deformation within the shear zone. Paleomagnetic and AMS data from the protomylonite rocks are interpreted to have been affected by the distortional strain, which accommodated shape and volume change in the rock. We suggest that the introduction of granitic magma into middle crustal levels critically enhanced the crustal response to shortening within the shear zone, possibly due to melt compressibility.

中文翻译:

墨西哥恰帕斯州Tonalá的超压正中剪切带中新世Plutons的结构环境,古磁性和磁性结构:岩浆侵袭期间缩短的证据

我们研究了中新世花岗岩沿墨西哥南部恰帕斯州左侧,压变,Tonalá剪切带侵入的结构,古磁性和磁性构造。岩浆织物描绘出平行于剪切带的区域褶皱样式,表明岩浆缩短。磁化率织物发达,平均磁化率平均值为18.6×10 -3SI。Plutons的磁性结构主要由MD和更细的PSD磁铁矿控制,该磁铁矿还具有剩磁。原生矿物中宏观矿物织物和磁化率织物的取向始终是平行的,但在保存岩浆织物的花岗岩类中,它们的重合性好或差。大多数站点的平均方向是中等偏正(或对向)的西北偏北方向,但是在局部尺度上,平均方向非常不一致。省略高度不一致的站点的总体平均方向为Decl。= 359.6°,包括 = + 41.9°(ķ  = 14.4,α 95  = 8.1°,Ñ = 24个网站);它几乎与北美参考方向一致,略有差异可能表明区域向北倾斜平缓。高方向性色散被解释为反映沿剪切区围绕胶体或部分胶体的局部垂直轴到亚垂直轴的相对旋转,其中一些旋转被解释为是由剪切区内的渐进变形引起的。来自原次隆隆岩的古磁和AMS数据被认为受变形应变的影响,变形应变适应了岩石的形状和体积变化。我们认为,将花岗岩岩浆引入中地壳层可极大地增强地壳对剪切带内缩短作用的响应,这可能是由于熔体的可压缩性所致。
更新日期:2021-02-18
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