利用地貌形态估算西秦岭-松潘构造结及邻区的下地壳黏滞系数

doi:10.3969/j.issn.0253-4967.2020.01.011

摘要/Abstract

摘要：

西秦岭-松潘构造结下地壳黏滞系数的定量化研究是理解青藏高原东缘及东北缘动力过程的基础。为进一步认识该区域岩石圈动力学的演化过程, 建立下地壳流与不同时间尺度岩石圈变形特征的相互联系, 文中以下地壳管道流模型为基础, 利用地貌形态估算下地壳的黏滞系数, 探讨深部岩石圈流变学过程如何作用于上地壳形变和构造地貌特征; 同时结合GPS速度场分析现今的地壳形变, 进一步研究区域弥散构造变形过程。结果表明: 1)若尔盖-红原盆地北侧及东北侧下地壳的黏滞系数小于东侧及东南侧; 2)下地壳流具有向NE低黏滞系数区流动的趋势, 较好地解释了该区域的造山运动过程、弧形等高线分布及“V”形展布断裂的发育; 3)GPS数据揭示的现今地表运动方向与黏滞系数反演的下地壳历史演化方向一致, 说明下地壳与上地壳可能具有良好的耦合特征。研究结果最终为解释不同走向和性质的断裂系发育、造山带形成、宏观地貌发育特征以及深入探讨青藏高原东北缘岩石圈的流变学和隆升动力学提供了依据。

关键词: 西秦岭-松潘构造结, 下地壳黏滞系数, GPS速度场, 构造地貌

Abstract:

The western Qinling-Songpan tectonic node is located at the intersection of three major tectonic units of Tibetan plateau, the South China Block and the Ordos Block, and is at the forefront of the northeastern margin of Tibetan plateau. It has unique geological and dynamic characteristics from the surface to the deep underground. Based on the model for ductile flow in the lower crust, the geomorphological form is used to estimate the viscosity of the lower crust, and how the rheological process of the deep lithosphere acts on the upper crust deformation and structural geomorphology. And combined with GPS velocity field data, the current crustal deformation is analyzed to further study the regional dispersive deformation process. The results show that the viscosity of the north and northeast of the Zoige-Hongyuan Basin is smaller than that of the east and southeast. Therefore, the lower crust flow has a tendency of flowing to the northeastern low viscosity zone. We believe that when the lower crust flows from the central plain of the Qinghai-Tibet Plateau to the rigid Sichuan Basin with a higher viscosity of the lower crust, it cannot flow into the basin, and part of the lower crust flow accumulate here, causing the upper crust to rise, and the uplifting led to the formation of the Longmen Mountains and a series of NNE-striking faults as well. When the lower crust flows to the northeast direction with a low viscosity, the brittle upper crust is driven together. Because of the remote effects from the Ordos Basin and the Longxi Basin, the mountains in this region are built slowly and the stepped arc-shaped topography of the current 3 000-meter contour line and the 2 000-meter contour line are developed. At the same time, a series of NWW-trending left-lateral strike-slip faults are developed. This explains the seismogenic tectonic model of the western Qinling-Songpan tectonic node as from NWW-trending left-lateral strike-slip faulting to the NNE-trending right-lateral strike-slip faulting and both having a thrust component. The current crustal movement direction revealed by the GPS velocity field is consistent with the direction of historical crust evolution of the lower crust revealed by the viscosity, implying that there is a good coupling relationship between the lower crust and upper crust. The results provide a basis for studying the development of fault systems with different strikes and properties, the formation of orogenic belts, the macroscopic geomorphological evolution characteristics, and the rheological and uplift dynamics of the lithosphere in the northeastern margin of the Tibetan plateau.
In addition, our research differs from the previous studies in the spatial and temporal scale. Previous studies included either the entire Qinghai-Tibet Plateau or only the eastern margin of the Qinghai-Tibet Plateau. However, our analysis on the contours and topographical differences in the topography of the western Qinling-Songpan tectonic knot reveals that the study area is controlled by the lower crust flow. Our results are confirmed by various observations such as seismology, magnetotellurics and geophysical exploration. Moreover, the previous studies did not point out enough that the elevation contours are elliptical, and the elliptical geomorphology further illustrates that the formation and evolution of the Qinghai-Tibet Plateau has rheological characteristics and also conforms to the continuous deformation mode. Meanwhile, in terms of time scale, the evolution time of the study area is divided into three types of simulation time according to geochronology. And the GPS velocity field is introduced to observe the present-day crustal deformation.

Key words: western Qinling-Songpan tectonic node, lower crustal viscosity, GPS velocity field, tectonogeomorphology

中图分类号:

P315.2

魏聪敏, 葛伟鹏, 张波. 利用地貌形态估算西秦岭-松潘构造结及邻区的下地壳黏滞系数[J]. 地震地质, 2020, 42(1): 163-181.

WEI Cong-min, GE Wei-peng, ZHANG Bo. ESTIMATING THE LOWER CRUSTAL VISCOSITY OF THE WESTERN QINLING-SONGPAN TECTONIC NODE AND ITS ADJACENT AREAS BY USING LANDFORM MORPHOLOGY[J]. SEISMOLOGY AND GEOLOGY, 2020, 42(1): 163-181.

图/表 6

图 1 西秦岭-松潘构造结的活动断裂与历史地震分布图F1西秦岭北缘断裂; F2临潭-宕昌断裂; F3迭山-光盖山断裂; F4迭部-白龙江断裂; F5塔藏断裂; F6岷江断裂;F7龙日坝断裂; F8哈南-稻畦子断裂; F9武都-康县-略阳断裂; F10两当-江洛断裂; F11日月山断裂; F12鄂拉山断裂

Fig. 1 Distribution of active faults and historical earthquakes in the western Qinling-Songpan continental tectonic node.

图 2 区域地貌和条带状剖面位置分布图橘黄色长条为本文所取的地形条带, 褐色实线为4km等高线, 红色实线为3km等高线,黄色实线为2km等高线, 绿色实线为1km等高线

Fig. 2 Regional topography and distribution of location of banded profiles.

图 3 模型结果与地形剖面黑色实线为真实地形剖面, 红色实线为黏滞系数反演获得的地形剖面

Fig. 3 Model results and topographic profiles.

表1 西秦岭-松潘构造结及邻区岩石圈黏滞系数估计

Table1 Approximate lithospheric viscosity estimation of the western Qinling-Songpan continental tectonic node and the adjacent regions

带状剖面		范围/km	长度/km	最大/m	最小/m	黏滞系数范围/Pa·s	黏滞系数估计/Pa·s
A	1	255<d<350	95	4 000	3 100	10¹⁹<Y<10²⁰	4×10¹⁹
	2	120<d<350	230	4 200	3 100	10¹⁸<Y<10¹⁹	4×10¹⁸
B	1	240<d<300	60	4 800	2 200	10²⁰<Y<10²¹	5×10²⁰
C	1	60<d<350	290	4 300	2 000	10¹⁸<Y<10¹⁹	3×10¹⁸
D	1	210<d<250	40	4 200	2 000	10²⁰<Y<10²¹	6×10²⁰
	2	75<d<350	275	4 000	2 000	10¹⁸<Y<10¹⁹	9×10¹⁸
E	1	130<d<205	75	4 500	2 800	10¹⁹<Y<10²⁰	3×10¹⁹
	2	230<d<275	45	4 000	2 000	10²⁰<Y<10²¹	5×10²⁰
	3	130<d<350	220	4 500	2 000	10¹⁹<Y<10²⁰	2×10¹⁹
F	1	100<d<175	75	4 000	2 700	10¹⁹<Y<10²⁰	8×10¹⁹
	2	185<d<220	35	4 000	2 500	10²⁰<Y<10²¹	2×10²⁰
	3	250<d<350	100	3 300	1 500	10¹⁹<Y<10²⁰	5×10¹⁹
	4	185<d<350	165	4 000	1 500	10¹⁹<Y<10²⁰	6×10¹⁹
G	1	130<d<165	35	4 400	3 700	10¹⁸<Y<10¹⁹	8×10¹⁸
	2	185<d<235	50	4 500	2 700	10¹⁹<Y<10²⁰	9×10¹⁹
	3	240<d<350	110	3 000	1 500	10¹⁹<Y<10²⁰	7×10¹⁹
	4	180<d<350	170	4 500	1 500	10¹⁹<Y<10²⁰	7×10¹⁹
H	1	180<d<210	30	4 500	2 700	10²⁰<Y<10²¹	3×10²⁰
	2	180<d<350	170	4 500	1 400	10¹⁹<Y<10²⁰	4×10¹⁹
I	1	190<d<280	90	4 500	1 500	10¹⁹<Y<10²⁰	1×10²⁰
	2	190<d<350	160	4 500	800	10¹⁹<Y<10²⁰	9×10¹⁹

图 4 1999—2007年(a)与2009—2018年(b)期间相对于天水-成县块体的GPS速度场蓝色箭头为连续观测站结果, 红色箭头为流动观测站结果

Fig. 4 GPS velocity field with respect to the Tianshui-Chengxian block between 1999—2007(a) and 2009—2018(b).

图 5 西秦岭松潘构造结岩石圈三维模型绿色条带为3km等高线区域, 黑色箭头为下地壳流流动方向; 地壳结构来自深震反射及远震接收函数等结果(高锐等, 2006; Zhang et al., 2010; Yang et al., 2012; 姚志祥等, 2014; 朱介寿等, 2017)

Fig. 5 A 3D model of the lithosphere in western Qinling-Songpan continental node.

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