Steady state erosion of critical Coulomb wedges with applications to ...

B01411 HILLEY AND STRECKER: STEADY STATE EROSION OF CRITICAL COULOMB WEDGES
Figure 8. Geologic map of the Himalaya. Ongoing collision of India with respect to Asia has produced
15.5–19.5 mm/yr of convergence between the two. The orogenic wedge is constituted by, from northeast to
southwest, ophiolites and sutures, metamorphosed sedimentary rocks from the Tethys basin, high-grade
metamorphic rocks of the High Himalayan Crystalline, lower-grade metamorphic rocks of the Lesser
Himalaya, and sedimentary foreland deposits of the Subhimalaya. Various volcanic and crystalline rocks
are included within these units. Geology and structures simplified after Hodges et al. [2000, and references
therein].
apatite, sphene, and zircon fission track thermochronology
[Liu, 1982], respectively. Exhumation rates over these timescales
are 5–6, 5, and 5–9 mm/yr, respectively.
4.1.2. Himalayan Orogenic Wedge
[25] Northward translation of India with respect to Eurasia
and the resulting continental collision has produced one of
the most spectacular active mountain belts on Earth [e.g.,
Molnar and Tapponnier, 1975]. This collision has been
ongoing since 50 Ma [Rowley, 1996; Searle et al., 1997;
Ratschbacher et al., 1994] and has resulted in a total of 1800
and as much as 2750 km of relative displacement between
India and Eurasia in the western and eastern Himalaya,
respectively [Dewey et al., 1989]. Global Positioning System
(GPS) reoccupations of sites across the Himalaya indicate
that the current rate of convergence is 17.5 ± 2 mm/yr [Bilham
et al., 1997], and most of the resulting interseismic contraction
is concentrated in the northern parts of the Himalaya
[Jackson and Bilham, 1994; Bilham et al., 1997]. However,
studies of uplifted and warped Quaternary terraces along the
southern margin of the Nepalese Himalaya record similar
deformation rates [Lave and Avouac, 2000]. Therefore, while
high interseismic strain may accumulate within the core of the
Himalaya, it is apparently released primarily along its southern
margin. This continued contraction within the Himalaya
has produced an orogenic wedge 250 km in width, from
northeast to southwest [DeCelles and DeCelles, 2001].
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B01411
[26] The geology of the Himalayan orogenic wedge is
shown in Figure 8. The regional geology reflects the progressive
incorporation of material into and exhumation of the
wedge from northeast to southwest. The major lithotectonic
units exposed in the wedge are, from northeast to southwest:
[27] 1. Ophiolites and crystalline batholiths represent the
suture between the Indo-Eurasian plates and the eroded
remnants of the volcanic arc that formed behind the
northward directed subduction [Dewey and Bird, 1970;
Tapponnier et al., 1981].
[28] 2. These rocks are thrust onto a thick section of
sedimentary rocks that represent the deposition of material
into the Tethys basin prior to collision [S¸engör et al., 1988;
Gaetani and Garzanti, 1991].
[29] 3. A series of high-grade metamorphic rocks are
juxtaposed with these metasediments by a low-angle normal
fault within the orogen’s interior [e.g., Caby et al., 1983;
Burg et al., 1984; Burchfiel et al., 1992; Searle, 1986;
Herren, 1987; Valdiya, 1989]. These rocks are bounded to
the southwest by a north dipping low-angle thrust system
[e.g., Hodges, 2000] and comprise a fault-bounded wedge
whose structural thickness varies along strike between 3.5
and 11.7 km.
[30] 4. The high-grade metamorphic rocks supersede
weakly to moderately metamorphosed clastic sediments of
the Lesser Himalaya 8 to 10 km thick, and form a series of