Italian tectonics, corrected

With the earthquake in Italy last night (our time) come some of the most ham-handed tectonic explanations ever (this is stuff that makes the Grumpy Geophysicist officially grumpy).  The official USGS explanation is:

The August 24, 2016 normal faulting earthquake is an expression of the east­west extensional tectonics that now dominate along the Apennine belt, primarily a response to the Tyrrhenian basin opening faster than the compression between the Eurasia and Africa plates.

This is totally off the wall and so of course has been repackaged by the BBC:

The Tyrrhenian Basin, or Sea, which lies to the west of Italy, between the mainland and Sardinia/Corsica, is slowly opening up.

Scientists say this is contributing to extension, or “pull-apart”, along the Apennines. This stress is compounded by movement in the east, in the Adriatic.

The result is a major fault system that runs the length of the mountain range with a series of smaller faults that fan off to the sides. The foundations of cities like Perugia and L’Aquila stand on top of it all.

OK, here’s the problem with this explanation.  If the Tyrrhenian Sea is opening up, something else must be closing (Earth is not expanding, sorry Carey fans).  The general picture, as has been put forward by Malinverno and Ryan or Wiki Royden, goes like this: the ocean floor to the east of Italy (the Adriatic) is sinking into the mantle.  The subduction zone (where the Adriatic goes under Italy) is basically moving to the east, dragging Italy along for the ride.  To compensate for this, new ocean floor is opening up on the west side of Italy (the Tyrrhenian Sea).  This was drawn by Malinerno and Ryan like this:


East-west section from the Tyrrhenian Sea (at left, pattern 2) across Italy, with the Apennine accretionary prism (pattern 5) and on into the Adriatic (Ad, at right). Oldest section at top, youngest at the bottom. Apennines are mainly stuff scraped off the Adriatic plate. From Malinverno and Ryan, Tectonics, 1986.

If the Tyrrhenian Sea was opening faster than the convergence on the east side of Italy, then you should see compression (thrust faulting) all the way across, but you do not.  Instead, this earthquake and others in the area are extensional, north-south trending normal faults. However, these are paralleling thrust faults farther east, so some additional thought is needed.

Now given the thrust faults on the east edge of Italy, the normal faults of the Apennines seem strange, but in fact this happens in many places.  Here, the buildup of the mountains  has produced gravitational stresses in the Apennines that favor normal faulting.  The fact the sub-Adriatic lithosphere is driving the subduction zone to the east is what prevents the lithosphere from being in compression, so the relatively low potential energy in the Apennines can be expressed as normal faulting (in contrast, the very high compressional stresses from the Indian-Asian collision require very high mountains before you see any extensional faulting, and there the extensional faults are perpendicular to the thrust belt, not parallel as in Italy).

Look, it is probably a bit more complex that “it is caused by the Tyrrhenian Sea opening” but not so much that it excuses such a misleading explanation.

This is, by the ways, an example of a style of tectonics that probably produced the late Paleozoic Antler orogeny in the western U.S.  Similar stuff goes on behind “retreating” subduction zones in parts of the western Pacific, but lacking the continental material to reproduce Italy in quite the same way.


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