用重力资料研究青藏高原东北缘深部岩石圈形变特征
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摘要
青藏高原东北缘位于青藏、鄂尔多斯和阿拉善3个块体的结合部位,地形地貌复杂、构造活动强烈、地震异常活跃。祁连山断裂带是青藏高原的东北边界,现代构造活动和变形比较集中,是研究岩石圈动力学最典型的地区。作者在研究区切了穿过祁连山断层并与其走向相垂直的4条测线。对于每条测线,利用二维弹性板理论,通过调试岩石圈有效弹性厚度Te,试验了横向不均匀弹性刚度的单一弹性板模型和有断点的2个独立弹性板模型,得到的结论是变刚度单一弹性板模型与我们的观测布格重力异常值吻合得很好。说明祁连山断层并没有穿过莫霍面进入岩石圈上地幔,是地壳内大断裂,即青藏高原内部构造单元。Te值由西南向东北呈现出逐渐增大的趋势,这应当与3个块体在板块热构造、岩石圈物质组成、构造活动性等方面的差异有关。
The northeastern edge of the Qinghai-Tibetan Plateau lies in the convergence zone of the Tibetan block,Ordos block and Ala-Shan Block.The topography is quite complicated,and the tectonic movement is active,with abnormally active earthquakes.The Qilian Shan fault is the northeastern boundary of the Tibetan Plateau,with strong modern tectonic activities and deformation.Therefore the Qilian Shan fault is a typical area for lithoshphere dynamics study.We project four profiles across the Qilian Shan Fault,basically vertical to the strike of the fault.For each profile,on the basis of 2D elastic plate theory, by modifying the effective elastic thickness of the lithosphere,we test both one continuous plate and two separate plates with a broken point beneath the study area,the effective elastic thickness being allowed to vary laterally.Our conclusion is that the continuous plate model can well explain the observed Bouguer gravity anomalies,while the broken plate model cannot.The result indicates that the Qilian Shan Fault does not cut through the Moho discontinuity into the upper mantle of the lithosphere,and is an interior tectonic unit of the Tibetan Plateau.The values of the effective elastic thickness gradually increase from southwest to northeast,which should be relevant to the large differences in the thermal structure of the blocks,the lithospheric compositions and the block tectonics etc.
The northeastern edge of the Qinghai-Tibetan Plateau lies in the convergence zone of the Tibetan block,Ordos block and Ala-Shan Block.The topography is quite complicated,and the tectonic movement is active,with abnormally active earthquakes.The Qilian Shan fault is the northeastern boundary of the Tibetan Plateau,with strong modern tectonic activities and deformation.Therefore the Qilian Shan fault is a typical area for lithoshphere dynamics study.We project four profiles across the Qilian Shan Fault,basically vertical to the strike of the fault.For each profile,on the basis of 2D elastic plate theory, by modifying the effective elastic thickness of the lithosphere,we test both one continuous plate and two separate plates with a broken point beneath the study area,the effective elastic thickness being allowed to vary laterally.Our conclusion is that the continuous plate model can well explain the observed Bouguer gravity anomalies,while the broken plate model cannot.The result indicates that the Qilian Shan Fault does not cut through the Moho discontinuity into the upper mantle of the lithosphere,and is an interior tectonic unit of the Tibetan Plateau.The values of the effective elastic thickness gradually increase from southwest to northeast,which should be relevant to the large differences in the thermal structure of the blocks,the lithospheric compositions and the block tectonics etc.
引文
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[20]张永志,王卫东.青藏高原东北缘断层活动变形的模拟研究[J].大地测量与地球动力学,2004,24(1):63-67.
[21]郭飚,刘启元,陈九辉,等.青藏高原东北缘—鄂尔多斯地壳上地幔地震层析成像研究[J].地球物理学报,2004,47(5):790-797.
[22]赖晓玲,张先康,方盛明.青藏高原东北缘壳幔过渡带研究[J].地震学报2004,26(2):132-139.
[23]刘宝峰,李松林,张先康,等.玛沁-靖边剖面S波资料研究与探讨[J].地震学报,2003,25(1):82-88.
[24]许忠淮,汪素云,裴顺平,青藏高原东北缘地区Pn波速度的横向变化[J].地震学报,2003,25(1):24-31.
[25]赵金仁,李松林,张先康,等.青藏高原东北缘莫霍界面的三维空间构造特征[J].地球物理学报,2005,48(1):78-85.
[26]金煜,姜效典.岩石圈动力学[M].北京:科技出版社,2002:75-96.
[27]侯康明,张忻,刘小龙.祁连山构造带的新构造变形机制[J].华南地震,1999,19(2):50-56.
[2]郑斯华.青藏高原地震的震源深度及其构造意义[J].中国地震,1995,11(2):99-106.
[3]傅容珊,李力刚,黄建华,等.青藏高原隆升过程三阶段模式的动力学模拟[C].寸丹集,庆贺刘光鼎院士工作50周年学术论文集,北京:科学出版社,1998.596-610.
[4]傅容珊,黄建华,李力刚,等.青藏高原隆升三阶段模型的数值模拟[J].地学前缘,2000,7(4):588-596.
[5]熊熊.青藏高原隆升的地幔动力学机制研究进展,青藏高原岩石圈现今变动与动力学[M].北京:地质出版社,2001:214-222.
[6]Tapponnier P,Xu zhiqin,Roger F,et al.Oblique stepwise rise andgrowth of the Tibet Plateau[J].Science,2001,294:1671-1677.
[7]Forsyth D W.Subsurface loading and esti mates of the flexure rigidityof continental lithosphere[J].J Geophs Res,1985,90;12623-12632.
[8]McNutt M K.Lethospheric flexure and thermal anomalies[J].JGeophys Res,1984,89;209-222.
[9]Burov E B,Diament M.Flexure of the continental lithosphere withmultilayered rheology[J].Geophys J Int,1992,109;449-468.
[10]丁国瑜.中国岩石圈动力学概论[M].北京:地震出版社,1991,144-153.
[11]邵辉成,苏刚.鄂尔多斯地块周缘近期地震活动趋势分析[J].西北地震学报,1999,21(4):395-406.
[12]Arnaud N O,Vidol P H,Tapponnier P,et al.The high K2O Vol-canismof northwestern Tibet:Geochemistry and tectonic i mplica-tions[J].Earth and Planetary Science Letters,1992,111:351-367.
[13]张先康,李松林,王夫运,等.青藏高原东北缘、鄂尔多斯和华北唐山震区的地壳结构差异———深地震测深的结果[J].地震地质,2003,25(1):52-60.
[14]国家地震局地质研究所,宁夏地震研究所.海原活动断裂带[M].北京:地震出版社,1990.
[15]国家地震局地质研究所,国家地震局兰州地震研究所.祁连山-河西走廊活动断裂系[M].北京:地震出版社,1993:175-210.
[16]李松林,张先康,张成科,等.玛沁-兰州-靖边地震测深剖面地壳速度的初步研究[J].地球物理学报,2002,45(2):210-217.
[17]薛光琦,钱辉,姜枚,等.青藏高原东北部天然地震探测与岩石圈深部特征[J].地球学报,2003,24(1):19-26.
[18]陈九辉,刘启元,李顺成,等.,青藏高原东北缘———鄂尔多斯地块地壳上地幔S波速度结构[J],地球物理学报.2005,48(2):333-342.
[19]汪素云,高阿甲,许忠淮,等.青藏高原东北地区地震重新定位及其活动特征[J].地震学报,2000,22(3):241-248.
[20]张永志,王卫东.青藏高原东北缘断层活动变形的模拟研究[J].大地测量与地球动力学,2004,24(1):63-67.
[21]郭飚,刘启元,陈九辉,等.青藏高原东北缘—鄂尔多斯地壳上地幔地震层析成像研究[J].地球物理学报,2004,47(5):790-797.
[22]赖晓玲,张先康,方盛明.青藏高原东北缘壳幔过渡带研究[J].地震学报2004,26(2):132-139.
[23]刘宝峰,李松林,张先康,等.玛沁-靖边剖面S波资料研究与探讨[J].地震学报,2003,25(1):82-88.
[24]许忠淮,汪素云,裴顺平,青藏高原东北缘地区Pn波速度的横向变化[J].地震学报,2003,25(1):24-31.
[25]赵金仁,李松林,张先康,等.青藏高原东北缘莫霍界面的三维空间构造特征[J].地球物理学报,2005,48(1):78-85.
[26]金煜,姜效典.岩石圈动力学[M].北京:科技出版社,2002:75-96.
[27]侯康明,张忻,刘小龙.祁连山构造带的新构造变形机制[J].华南地震,1999,19(2):50-56.
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