MOUNTAIN BUILDING, EROSION,AND THE SEISMIC CYCLE INTHE NEPAL HIMALAYAJEAN-PHILIPPE AVOUACCalifornian Institute of Technology, Pasadena, USA1. INTRODUCTIONThe Himalaya is the most impressive example on earth of an activecollisional orogen. It combines rapid crustal shortening and thickening,intense denudation driven by the monsoon climate, and frequent very largeearthquakes along an incomparably long and high mountain arc. It hastherefore been the focus of a variety of investigations that have addressedvarious aspects of mountain building on various timescales. Geological andgeophysical studies give some idea of the structure of the range and physicalproperties at depth. The long-term geological history of the range, over sayseveral millions to a few tens of millions of years, has been documented bystructural, thermobarometric, and thermochronological studies. Morpho-tectonic investigations have revealed its evolution over several thousands ortens of thousands of years; and geodetic measurements and seismologicalmonitoring have revealed the pattern of strain and stress built-up overseveral years. This chapter is an attempt to show that the results of theseinvestigations can be assembled into a simple and coherent picture of thestructure and evolution of the range. The author also intends to illustratethe interplay between these various processes operating at different time-scales. One important example of processes that interact via feedbackmechanisms is particularly clear in the Himalaya: the thermal structure ofthe range, which is a result of the long-term crustal deformation and patternof ex humation, governs, through its influence on rheology, the pattern ofdeformation as well as the seismic behavior of the range-bounding thrustfault.In this chapter the key role played by surface processes is emphasized.These processes have carved morphologic features that can be used to deducevertical displacements. They also have generated the molasse deposits thathave filled the subsiding foreland basin, providing a record of mountainbuilding. In addition, they have participated actively in the evolution of theADVANCES IN GEOPHYSICS, VOL. 461ß 2003 Elsevier Inc. All rights reservedISSN: 1474-8177DOI: 10.1016/S0065-2687(03)46001-9range by influencing the thermal structure and the stress field throughredistribution of mass at the earth surface. Sur face process es must thereforebe taken into account in any analysis of the mechanics of mountain build-ing. They are also probably the major factors that differentiate intra-continental megathrust from subduction zones.This chapter is not a comprehensive review of the Himalaya. Forpedagogic reasons the author mainly descri bes studies carried out acrossthe Himalaya of central Nepal because he is most familar with this area thathas attracted particular attention over the last decade.The author first briefly introduces in Section 1 the geodynamic settingand presents in Section 2 a model of the development of the Himalayanorogen and foreland basin. In Section 3 the structure and kinematicevolution of the Himalaya as constrained from surface geology, geo-chronology, and geophysical investigations is reviewed in more detail.Section 4 describes how the kinematics of thin-skinned deformationalong the Himalayan foothills can be determined from the deformation ofabandoned river terraces. In Section 5 the pattern of river incision acrossthe whole rang e is described and it is shown that it basically reflects thekinematics of overthrusting. Section 6 discusses geodetic measurements ofcrustal deformation, historical seismicity, and the seismic cycle along theHimalaya. Section 7 discusses some general questions about continentaldeformation and seismicity:– How is deformation distributed throughout the range, and where are thefaults capable of producing very large recurrent earthquakes?– What can we learn about future large earthquakes from seismicity anddeformation monitored over a limited period of time?– What proportion of crustal deformation is expressed in the seismicity?– Starting with recent deformation, measured over a decade with geodetictechniques, can we extrapolate backwards to explain the long-termhistory of the range as expressed in its structural geology?– How does the erosion rate compare with tectonic uplift?2. AN ACTIVE C OLLISIONAL OROGEN2.1. Geodynamical Setting and Key Structural FeaturesThe Himalayan arc is one of the major zones of deformation thathave absorbed the indentation of India into Eurasia (e.g., Powell and2AVOUACConaghan, 1973). The collision started about 50 My r ago and produceda combination of lateral escape and crustal thickening that has givenrise to the highest topographic features on earth (e.g., Molnar andTapponnier, 1975; Harrison et al., 1992; Tapponnier et al., 2001) (Fig. 1).Subsequently, India and stable Eurasia continued converging at a rateof about 5 cm/year (Patriat and Achache, 1984). At present, the 4–5 cm/yearof northward displacement of India relative to stable Eurasia is stillbeing absorbed by a combination of horizontal shear and crustalshortening. This is demonstrated both by the pattern and kinematics ofactive faults in Asia (Molnar and Tapponnier, 1975; Avouac andTapponnier, 1993) and by GPS measurements (Larson et al., 1999)(Fig. 2a), although the respective contribution of these two mechanismsto the overall deformation remains a matter of debate (Tapponnier et al.,2001; Wang et al., 2002). Across the Himalaya of central Nepal a fractionof this convergence (estimated at about 2 cm/year) is absorbed bycrustal shortening, as shown from GPS geodetic campaigns carried out overthe last decade (Fig. 2b). Ongoing crustal shortening across the Himalaya isalso manifested by recurring large earthquakes with magnitude MwFIG. 1. Sketch showing how indentation of India into Eurasia, since the onset of thecollision about 50 Myr ago, has been absorbed by a combination of crustal thickening andlateral escape.MOUNTAIN BUILDING, EROSION, AND THE SEISMIC CYCLE 3above 8, such as the Bihar–Nepal earthquake of 1934 or the Kangraearthquake of 1905 (Fig. 3, Table 1).Relics of the Tethys ocean that used to separate the northern margin ofIndia from the active southern margin of Eurasia can now be tracedalong the Indus-Tsangpo suture zone (ITSZ) (e.g., Burg, 1983; Searle et al.,1987) well north of the Himalayan summits (Figs. 1 and 4). To thesouth, Cambrian to Eocene Tethyan sediments deposited on the