Tectonic mode switches and the nature of orogenesis

Gordon Lister and Marnie Forster

Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

The birth and death of many mountain belts occurs in lithosphere that over-rides major subduction zones.  Roll-back creates a gravitational potential well into which the orogen collapses. This motion, coupled with stress guides, can "pull" an orogen apart. A slowing of roll-back (or of hinge retreat) means that the subduction flexure may subsequently begin to be "pushed back" or be "pushed over" by the advancing orogen. The consequence of such changes in relative motion is that orogenic belts are affected by abrupt tectonic mode switches. The change from "push" to "pull" leads to a sudden change from horizontal extension to horizontal shortening, potentially throughout the entire mass of the orogenic lithosphere that over‑rides the subducting slab. The sequencing of these tectonic mode switches affects the thermal evolution of the orogen, and thus fundamentally determines the nature of orogenesis. In consequence high pressure metamorphic rocks are found in orogens characterized by push-pull sequences while high temperature metamorphic rocks are found in orogens characterized by pull-push sequences (Lister and Forster, 2008).

However the real complexity evident in the evolution of any orogen begins to emerge once we begin to consider movement in three dimensions.  Motion orthogonal to an arcuate mountain front is typical of the geometry of collapse, where the orogen has spread over or been pushed over the adjacent foreland.  For example it can reasonably be inferred that the Tibetan crust collapsed southward to create the modern arcuate shape of the southern boundary of the Himalayan mountain chain (see Figure below).  In fact the mountain front defines an almost perfect small circle, with a radius of 1696 ± 55 km (Bendick and Bilham, 2001). GPS measurements suggest this flow is still occurring: present day movement is taking place in directions orthogonal to the modern arc (Jade et al., 2004).  England and Molnar (2005) provide a convincing argument that crustal flow in Tibet is driven by the gravitational potential energy of the collapsing orogen: in their words, the orogen behaves more like a 'fluid' than a 'plate'.  Forward motion of the Indian indentor is accommodated in the west by the left-lateral Chaman fault zone on the boundary between Afghanistan and Pakistan, and in the east (in Myanmar) by the right-lateral Sagaing fault zone.

Geometry of a mountain front reflecting the impact of collapse driven by the gravitational potential energy of the orogen. Arrows show GPS motions relative to India (after Jade et al., 2004). The coloured slip lines stem from epicenters of 575 earthquakes in the period 1976-2006 in the depth range 0-60 km, as recorded in the Harvard Centroid Moment Tensor database
(http:/ www.seismology.harvard.edu/CMTsearch.html). Map produced using Program eQuakes superimposed on images derived from NOAA (http://ibis.grdl.noaa.gov/cgi-bin/bathy bathD.pl). Each slip line shown represents a choice of one of two conjugate fault plane solutions derived from the centroid moment tensor: each choice based on details of the local geology. Thrusts (red), normal faults (blue), left-lateral strike-slip faults (green) and right lateral (yellow). Motion orthogonal to the mountain front is typical of the geometry of collapse where the orogen has spread over, or been pushed over the adjacent descending slab. Left-lateral Chaman fault zone (dashed) in the west (a). Right-lateral Sagaing fault zone (dashed) in the east (b).

The main focus of the India-Asia collision is now in the NW, under the ranges of the Hindu Kush.  The impact of an indentor can be inferred from the paired clusters of strike-slip faults.  In the South, the small-circle geometry of the Himalayan mountain front is diagnostic of the fluid-like behaviour of this collapsing orogen, reflecting the impact of radiating viscous flow driven by the gravitational potential energy of the collapsing Tibetan Plateau.  Thrusts radiate orthogonally from the orogenic welt defined by the Tibetan plateau, southward, northward, and eastward.  The effects of eastward flow of the collapsing Tibetan Plateau is particularly evident in the fold and thrust belt in Sichuan Province, the locus of several catastrophic earthquakes (yellow dots in Figure).

In contrast, in the Myanmar crust, there are two almost orthogonal competing movement patterns.  Shortening occurs in the foreland fold and thrust belt because this zone accommodates WSW directed motion of crust flowing out from the Myanmar hinterland. The Sagaing wrench fault zone marks the locus of accumulating right-lateral offsets, periodically accommodating distortions caused by relative plate motion.  At the same time concentric left-lateral strike-slip faults accommodate flow around the eastern syntaxis, causing distortion of the Sagaing Fault. These concentric left-lateral strike-slip faults are most evident in the green lines that show the trend of fault plane slip vectors associated with left-lateral strike-slip earthquakes.  As we move from north to south in a semicircle around the eastern syntaxis the movement direction associated with these earthquakes changes from towards ~90° to towards ~250°. This is a movement pattern that suggests mass flow in the deeper crust driven by the WSW-directed roll-back of the tearing Myanmar slab that lies beneath this zone. 

Bendick, R., Bilham, R., 2001. How perfect is the Himalayan arc? Geology 29, 791-794, doi:10.1130/0091-7613(2001)029<0791:HPITHA>2.0.CO;2

England, P., Molnar, P., 2005. Late Quaternary to decadal velocity fields in Asia. Journal of Geophysical Research 110, B12401, doi:10.1029/2004jb003541.

Jade, S., Bhatt, B.C., Yang, Z., Bendick, R., Gaur, V.K., Molnar, P., Anand, M.B., Kumar, D., 2004. GPS measurements from the Ladakh Himalaya, India: preliminary tests of plate-like or continuous deformation in Tibet. Geol. Soc. Amer., Bull. 116, 1385–1391, doi:10.1130/B25357.1

Lister, G. and Forster, M. 2008.  Tectonics mode switches and the nature of orogenesis.  Lithos, in press.