1116 ABSTRACT Greater Himalayan sequence rocks ex- posed in the Manaslu–Himal Chuli Hima- laya can be separated into distinct upper and lower parts. Deformation recorded in both parts occurred at temperatures ranging be- tween ~450 °C and ~640 °C and is character- ized by almost equal coaxial and noncoaxial components. Across the upper Greater Hima- layan sequence, peak metamorphic tempera- tures are essentially isothermal, whereas corresponding metamorphic pressure esti- mates across the same section decrease downward with an apparent gradient of 620 bars/km. In the lower Greater Himalayan sequence, however, both metamorphic pres- sure and temperature decrease with struc- tural depth. The abnormal pressure gradient in the upper Greater Himalayan sequence is attributed to ~50% vertical thinning during southward displacement, while the inverted gradient in the lower portion is interpreted to be the result of coeval exhumation and down- ward expansion of the Main Central thrust shear zone and the progressive incorporation of more rock into the Greater Himalayan se- quence. Deformation in the upper portion of the Greater Himalayan sequence was char- acterized by extending flow, i.e., extension in the direction of flow, whereas deforma- tion in its lower portion was characterized by compressing flow, i.e., compression in the direction of flow. Extending flow is a distinc- tive feature of displacement and distortion in deep orogenic hinterlands, while compress- ing flow is emblematic of displacement and distortion in orogenic foreland regions. The transition between the upper and lower parts of the Greater Himalayan sequence there- fore represents a fundamental transition be- tween hinterland-style deformation, involv- ing processes such as lateral midcrustal flow, and foreland-style deformation, involving critical-taper thrust-fold wedge development. INTRODUCTION Kinematic Compatibility in Orogens The stark contrast in style of deformation between the deep interior of the hinterland of an orogen and the shallower regions of its fore- land belies the fact that these disparate geo- logic domains generally form at the same time in response to the same regional plate-tectonic framework. The foreland fold-and-thrust belt is an expanding critical-taper wedge within which detached supracrustal rocks, deformed by listric thrust faulting and thrust-related fold- ing, are compressed horizontally and thickened vertically as they are displaced over the under- riding craton and foreland sediments. Lateral growth of the wedge, induced by plate conver- gence, a high topographic gradient, and associ- ated gravitational spreading, is accommodated within it by horizontal shortening in the direc- tion of displacement. This relationship between displacement and distortion is characteristic of compressing flow (Price, 1972). In contrast, the combination of recumbent folding, shallowly dipping transposition foliation, pronounced stretching, and foreland-directed shear in the deep (mid- to lower crustal) metamorphic core of an orogen involves a relationship between displacement and distortion—extension in the direction of flow—that is characteristic of ex- tending flow (Price, 1972). Compressing flow and extending flow are simple conceptual models of the kinematics of flow that were used by Nye (1952) to ana- lyze the deformation in ice sheets and glaciers. As with ice sheets and glaciers, gravitation- ally driven lateral spreading in orogenic belts involves a continuum between extending flow in the metamorphic core zone beneath the topographically highest parts of an orogen and compressing flow in the topographically lower foreland region. This distinction between displacement and distortion during lateral gravitational spreading, which reconciles the dis- parities in style of deformation between the metamorphic core zone and foreland fold-and- thrust belt, also implies that during orogenesis, diachronous tectonic overprinting occurs as rock moves from a geographic domain of ex- tending flow into an adjacent domain of com- pressing flow (or vice versa). Dynamic relationships between displace- ment and distortion within the hinterland and foreland of the Himalaya-Tibet orogen are evi- dent in both thermo-mechanical mathematical models of lateral midcrustal extrusive flow (e.g., Beaumont et al., 2001, 2004, 2006) and mod- els of critical-wedge taper (e.g., Bollinger et al., 2006; Kohn, 2008) that have been proposed to explain the evolution of the continuing collision between India and Asia. The kinematic transi- tion between the hinterland and foreland is not the focus of these models, however, and as such, both the details and the conceptual framework of this transition remain enigmatic. An under- standing of the commonly cryptic kinematic and temporal transition between the hinterland and foreland regions of the Himalaya, the proto- typical continent-continent collision, is a critical step in elucidating orogenesis in general and, in particular, the development of various orogens that have been modeled after the Himalaya. The Himalaya The geology of the Himalayan arc is com- monly described in terms of three discrete, fault-bounded tectonostratigraphic rock as- semblages, the Lesser and Greater Himalayan sequences and the Tethyan sedimentary se- quence, which were identified by Heim and Gansser (1939) and have subsequently been mapped along the entire length of the Hima- laya (see Hodges, 2000; Yin and Harrison, 2000, and references therein). These three as- semblages have been interpreted as consisting For permission to copy, contact editing@geosociety.org © 2010 Geological Society of America GSA Bulletin; July/August 2010; v. 122; no. 7/8; p. 1116–1134; doi: 10.1130/B30073.1; 11 figures; 1 table; Data Repository item 2010047. † Current address: Department of Geological Sci- ences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan S7N 5E2, Canada; e-mail: kyle.larson@usask.ca. Relationships between displacement and distortion in orogens: Linking the Himalayan foreland and hinterland in central Nepal Kyle P. Larson † , Laurent Godin, and Raymond A. Price Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ontario K7L 3N6, Canada Published online March 29, 2010; doi:10.1130/B30073.1 on March 30, 2010 gsabulletin.gsapubs.org Downloaded from