Growth, but no seismicity

To the east of the Zagros Mountains, a 1.5-km-elevation section of the Eurasian plate lies wedged between the Indian and Arabian plates. Known as the Turkish–Iranian Plateau, the section extends from the Persian Gulf north to the Caspian Sea and east to Afghanistan. Understanding how such plateaus grow in elevation and breadth requires collective examination of several forms of evidence.

Situated in both Iran and Iraq, the Zagros fold-and-thrust belt comprises the southwestern part of the plateau and a mountainous foothill zone of rocks deformed by the collision of the Arabian and Eurasian plates. The belt is young and actively deforming. Studying it can help determine how plateau growth actually occurs: Whether such plateaus achieve critical elevation in one place and then enlarge laterally or whether the entire plateau rises together is a topic of much speculation.

‘There’s a lot of interest in the idea that if you rapidly lose bits of the lower part of the lithosphere [over a couple of million years], you would get a rapid surface uplift across the whole area,’ says Mark Allen of Durham University. Allen is the lead author of a Tectonics paper that investigates plateau growth using a combination of evidence derived from seismology, geomorphology, and bedrock geology.

But in their study, Allen and his coworkers propose that the high plateau is built gradually and incrementally, with active faults switching off in higher parts of the fold-and-thrust belt, which then becomes incorporated into the plateau.

‘There are no sharp cut-offs to be seen in the geology or geomorphology,’ explains Allen. Rather, it is gradational, and major faults do not coincide with sharp observable changes. There is a topographic inflection near the regional limit of seismic activity. But similar, albeit inactive, geological faults also occur much farther north of this limit, and there is minimal crustal shortening in that region at upper crustal levels.

The crucial correlation is a cut-off in seismicity at an elevation of about 1250 m, which occurs regardless of the local geology. ‘If you’ve already thickened and raised the crust, it’s like a lid pressing down and suppressing further thrust faulting,’ adds Allen.

The study provocatively indicates that the mountains and plateau could keep rising after earthquakes stop. Beyond the 1250-m elevation contour, the thrust earthquakes that lead to crustal thickening cease, yet elevation continues to increase away from the Persian Gulf. Allen and his coworkers suggest that the land rises because the layer of lower crust is still shortening and thickening, thus pushing the surface up; The thick and elevated crust has prevented further seismicity. This theory is yet untested.

‘This does not match the view of [fold-and-thrust] belts that there is a huge, low-angle thrust underlying the entire mountain range,’ says Allen. ‘We’re suggesting that there is no evidence of such a thrust underneath the Zagros to generate earthquakes, so rather than a single fault, you have a zone that is squishy and can be deformed in a ductile manner.’

Had the plateau grown vertically in one rapid event, as would be caused by mantle uplift, there would be strange swells and rises in the mean topography that do not correlate with any crustal features. There would likely be dramatic changes in river profiles, and widespread, synchronous magmatism. Instead, magmatism on the plateau has occurred in different centers over at least the last 10 million years.

The next step is a similar analysis of comparable plateaus—Tibet and the South American Andean Plateau—that will aim to analyze seismicity and geomorphologic evidence for drastic change versus incremental adjustment.