海原地区早白垩世古地磁结果及其构造意义

NEW EARLY CRETACEOUS PALEOAGNETIC RESULTS FROM THE HAIYUAN AREA AND ITS TECTONIC IMPLICATIONS

通过海原地区早白垩世 1 3个采点的古地磁研究 ,揭示了一组高温特征剩磁分量 .在 95 %置信度下通过倒转检验 ,采样剖面获得的下白垩统李洼峡组和和尚铺组的磁性地层结果 ,显示多个正、反极性带 ,与早白垩世早期的极性特征相似 ,说明这组高温分量很可能代表岩石形成时的原生剩磁 ,其特征剩磁方向为 :偏角D =1 2 .7° ,倾角I=5 0 .2° ,α95=6 .3° ;相应的极位置为 :经度 φ =2 1 8.0°E ,纬度λ =78.2°N ,dp=5 .7°,dm=8.4° ,古纬度 ρ =3 1 .0° .通过对比华北地块鄂尔多斯盆地的早白垩世古地磁结果 ,表明采样地区自早白垩世以来相对于华北鄂尔多斯盆地未发生明显的构造旋转和纬度方向上的位移 .这说明海原断裂东南段并未发生大规模的左旋走滑运动 ,印度—欧亚板块碰撞挤压作用对青藏高原东北部海原地区的影响已经很小 .

WTHTThe paleomagnetic results are presented for 13 sites from lower Cretaceous red beds in the Haiyuan area,northwest of Ninxia province. Thermal demagnetization of the samples allows us to isolate a high-temperature component which passed postive reversal test. The preliminary magnetosratigraphic result from Liwaxia and Heshangpu sections seems to correlate with the globe lower Cretaceous magnetic polarity. It is suggested that the high-temperature component possibly represents the primary magnetization. The corresponding paleople lies at 78.2°N,218.0°E (d p=5.7°,d m=8.4). The tectonic-corrected data are D=12.7°,I=50.2°α 95 =6.3°.Comparing with the lower cretaceous poles from the Ordos basin in the west of North China Block,we conclude that Haiyuan area could suffered a non-significant northward motion and rotation with respect to the stable Ordos basin since the lower Cretaceous. The result indicates that Haiyuan fault zone does not yield a large-scale left-slip eastward movement in the southeast end,and that the effect of collision between India and Asia on Haiyuan area diminished.