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首页> 外文期刊>International Geology Review >Combined Paleomagnetic and Petromagnetic Study of the Upper Cretaceous Volcanic Sequence in Western Mexico: Implications for Tectonics and Magnetostratigraphy of the Jalisco Block
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Combined Paleomagnetic and Petromagnetic Study of the Upper Cretaceous Volcanic Sequence in Western Mexico: Implications for Tectonics and Magnetostratigraphy of the Jalisco Block

机译：墨西哥西部上白垩纪火山岩层的古地磁和岩磁组合研究：哈利斯科州地块的构造学和磁地层学意义

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摘要

Results of detailed paleomagnetic studies of the Upper Cretaceous Autlan volcanic sequence from the Sierra Cacoma area (Jalisco block, western Mexico) are reported. The 67.4 +- 1.2 Ma whole-rock K-Ar date for these lavas and the magnetic polarity stratigraphy indicate that flows were emplaced after the Cretaceous Normal Superchron and just prior to the Cretaceous-Tertiary boundary. Fifteen sites (approx 150 oriented samples) were collected along two vertical profiles. Rock-magnetic experiments permit identification of magnetic carriers and assessment of the paleomagnetic stability. Continuous susceptibility measurements versus temperature in most cases yield reasonably reversible curves with Curie points close to magnetite. Judging from the ratios of hysteresis parameters, all samples fall in the pseudo-single domain grain-size region, probably indicating a mixture of multi-domain and a significant amount of single-domain grains. Reliable paleomagnetic directions were obtained for 14 sites, corresponding to 2 normal and 12 reverse polarity sites. Lowermost flows in the sequence yield reverse polarity magnetization, and probably formed during Chron 31r of the reference geomagnetic polarity time scale. The remaining flows, including those belonging to radiometrically dated sites, correspond to Chron 30n. These tentative magnetic correlations suggest that the entire volcanic sequence was emplaced during a time span of about 2 Ma. The mean paleodirection obtained from 14 sites is Inc = 44.2 deg, Dec = 320.6 deg, k = 45, alpha _(95) = 6.0 deg. The mean inclination is in reasonably good agreement with the expected value for 65 and 70 Ma, as derived from reference poles for the North American craton. Declination is significantly different from expected (D = 345.8 deg for 65 Ma and D = 346.2 deg for 70 Ma), which suggests a counterclockwise tectonic rotation of about 25 deg for this region within the Jalisco block.
机译：据报道，对来自山脉塞科马地区（墨西哥西部哈利斯科州）的上白垩纪奥特兰火山序列进行了详细的古磁研究。这些熔岩的67.4±1.2 Ma全岩石K-Ar年代和磁极地层学表明，流动是在白垩纪正时界之后和白垩纪-第三纪边界之前发生的。沿两个垂直剖面收集了15个位置（约150个定向样品）。岩石磁实验可以识别磁性载体并评估古磁稳定性。在大多数情况下，连续磁化率与温度的关系会产生可逆的曲线，居里点接近磁铁矿。从磁滞参数的比率来看，所有样品都落在伪单畴晶粒尺寸区域，这可能表示多畴晶粒和大量单畴晶粒的混合。获得了14个位点的可靠古磁方向，对应于2个法向和12个反极性位点。序列中最低的流会产生反极性磁化，并且可能在参考地磁极性时标的Chron 31r期间形成。其余流量（包括属于放射性测年地点的流量）对应于Chron 30n。这些初步的磁相关性表明，整个火山序列是在大约2 Ma的时间跨度内发生的。从14个位置获得的平均古方向为Inc = 44.2度，Dec = 320.6度，k = 45，α_（95）= 6.0度。从北美克拉通的参考极推导出，平均倾角与65和​​70 Ma的期望值相当合理。倾斜度与预期的明显不同（65 Ma时D = 345.8度，70 Ma时D = 346.2度），这表明哈利斯科州区块内该区域的逆时针构造旋转约为25度。

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