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Treatise on Geophysics - Avouac- draft#1
California Institute of Technology
Mountain Building: From earthquakes to geological deformation 1
1. Introduction 2
2. Geodynamical Setting of the Himalaya. 3
2.1 The Himalaya as a result of the India-Asia collision. 3
2.2 Variation of crustal thickness across the Himalaya 4
2.3 Geological architecture of the Himalayan range and southern Tibet 5
2.6 Crustal-scale structural models of the Himalaya. 7
2.7 Geophysical constraints on the structure of the crust 8
3.1 Active thrusting and folding in the sub-Himalaya 9
3.1.1 Structural evolution of the sub-Himalaya 9
3.1.2 River incision across the sub-Himalaya 10
3.1.3 Converting incision rates to uplift rates in the sub-Himalaya 10
3.1.4 Converting uplift rates to horizontal shortening from area balance 12
3.1.5 Converting uplift rates to horizontal shortening from the fault-bend fold model 12
3.2 River incision, erosion and uplift across the range. 13
3.2.1 Fluvial incision across the whole range 13
3.2.2 Denudation across the whole range 14
3.2.3 Holocene kinematics of overthrusting along the MFT-MHT 14
4. Longer term geological deformation and exhumation. 15
4.1 Foreland deposition: a record of underthrusting 15
4.2 Structural evolution of the thrust package 17
4.3. Exhumation of the Lesser and High Himalaya: a record of overthrusting. 17
5. Kinematic and mechanical models of crustal deformation 19
5.1 Thermo-kinematic model of the evolution of the range since 15 Ma. 20
5.2 Modeling deformation and surface processes based on continuum mechanics 21
6. Geodetic deformation and the seismic cycle 23
6.1 Large earthquakes in the Himalaya. 24
6.2 Geodetic deformation in the Nepal Himalaya. 25
6.3 Microseismic activity in the Nepal Himalaya. 26
6.4 What controls the downdip end of the locked portion of the MHT? 27
6.5 A model of the seismic cycle in the central Nepal Himalaya 28
6.6 Geodetic deformation, Seismic coupling, and recurrence of large earthquakes in the Himalaya 28
6.7 Is interseismic strain stationary? 29
7. Discussion 31
7.1 The critical wedge theory: does it apply to the Himalaya? 31
7.2 Evidence for low friction on the Main Himalayan Thrust. 34
7.3 Importance of the brittle-ductile transition 35
7.4 Metamorphism during underthrusting. 35
7.5 How does the steep front of he High Himalaya relate to tectonics, erosion and climate? 36
7.6 The elevation and support of mountain ranges: effect of climate and lower crustal flow 37
7.6 The fate of the Indian crust and mantle lithosphere. 39
8. Conclusions 39
Tables and Figure Caption 57
Mountain ranges are the most spectacular manifestation of continental dynamics. The fact that some mountain ranges were able to maintain their topography over tens of millions years, while their erosion was feeding large sedimentary basins, is unambiguous evidence that tectonic forces can cause sustained uplift of subsidence of the continental crust. Geologists noticed quite early on that most mountain ranges are contractional orogens, the result of horizontal contraction of the continental crust, and that they tend to form long belts separating domains with often quite different geological history (e.g.,[Willis, 1891], [Argand, 1924]). A rapid tour of active mountain ranges on earth todays show that contractional mountain ranges can arise in a variety of contexts. Some have formed along converging plate boundaries as the result of collisions which can involve two continents (along the Himalaya for example, as detailed in this review), a continent and an island arc (in Taiwan for example [Malavieille, et al., 2002a]) or a continent and an oceanic plateau (the Southern Alps of New Zealand for example [Walcott, 1998]). Contractional mountain ranges can also form along subduction zones without being necessarily collisional features. In the Andes for example, the stresses transmitted across a subduction zone appear to be sufficient to cause trench-perpendicular shortening (e.g., [Lamb, 2006]). Probably because high heat flow in the backarc zone weakens the the continental lithosphere [Hyndman, et al., 2005]. Mountain ranges are thus often closely associated with converging plate boundaries. However, active mountain building can also occur far away from plate boundaries, the Tien Shan, in central Asia, being an outstanding example (e.g.,[Hendrix, 1992],[Avouac, et al., 1993]).
Mountains are major players in the interactions between solid Earth and its climate. Surface processes play a key role in the mechanics of mountain building not only because of the isostatic response to erosion [Molnar and England, 1990a], but because they contributes to focalizing crustal deformation leading to a positive feedback between erosion at the surface and shortening of the crust[Avouac and Burov, 1996] . Climate may also influence the mechanics of mountain building along subduction zones, through its influence on the amount of sediments delivered to the trench and hence on the mechanical coupling across the plate boundary [Lamb and Davis, 2003]. Mountain ranges affect atmospheric circulation and the distribution of precipitation, and consequently drainage patterns [Ruddiman and Kutzbach, 1991; Raymo and Ruddiman, 1992; Ramstein, et al., 1997]. Their erosion influences eventually their elevation through isostasy, and influences the chemistry of the atmosphere through a variety of chemical reactions and through burial of organic matter [Kerrick and Caldeira, 1993; Derry and France-Lanord, 1997; France-Lanord and Derry, 1997; Kerrick and Caldeira, 1999]. Finally, mountain ranges and their piedmontsare also a primary locus of geo-hazards, earthquakes, landslides and floods in particular.
For all these reasons, understanding better orogenic processes is a fundamental issue in geology. The anatomy of mountain ranges, and the tectonic processes at work deep in the crust, might be best studied from the investigation of the exhumed core of ancient orogens. However a lot of insight on orogenic processes can be gained by ausculting orogens that are actively deforming. The main intent of this book chapter is to illustrate that point and show how the study of active processes can shed light on how mountain ranges form and evolve over a geological period of time.
This chapter focuses on the Himalaya is undoubtfully the the world’s most impressive example of an active collisional orogen. This incomparably long and high mountain arc is the setting of rapid, on-going crustal shortening and thickening, intense denudation driven by the monsoon climate, and frequent very large earthquakes. The relation of this range to plate tectonics has long been recognized [Dewey and Bird, 1970]. As reviewed in this chapter, we now have a reasonably solid understanding of the structure of the range, of its petro-metamorphic history, and of the kinematics of its active deformation. The long-term geological history of the range – from several millions to a few tens of millions of years – has been documented by structural, thermobarometric and thermochronological studies. Morphotectonic investigations have revealed its evolution over the past several thousands or tens of thousands of years. And, finally, geodetic measurements and seismological monitoring have revealed the pattern of strain and stress build-up over several years. This chapter shows that the results of these investigations can be assembled into a simple, coherent picture of the structure and evolution of the range. Some emphasis is put on surface processes, which play a key role in interpreting orogenesis. These processes carved the morphologic features that are used to deduce vertical displacements. They generated the molasse deposits that filled the subsiding foreland basin, providing a stratigraphic record of mountain building. And they influenced the evolution of the range by changing its thermal structure and stress field via redistribution of surface mass. Surface processes therefore contribute to recording the geological history of an orogen, and they also participate in the mountain-building process itself.
The Himalaya is consequently one of the best places on Earth where the geological history of a mountain belt can be compared with its current tectonic processes. Although some aspects might be specific to the setting of this mountain range, the tectonic processes at work there are presumably the same as those at work in any other contractional mountain range.
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