Continental Faults at Depth: the roots of the debate

Ge277-2011

• Summary: How continents deform is a much debated question. Whether surface strain is localized on a few active faults is one difficult issue, but it is tractable (from the comparison of geodetic and geologic slip rates). What happens below the seismogenic depth is much more difficult to assess.

 List of topics offered for

discussion• Is continental deformation distributed of localized?

– What can we tell from geological observations?– What can we tell from geophysical observations?– What can we tell from geodetic observations?– What can we tell from geochemistry observations?– What can we infer from experimenal rock rheology and modelling?

• Where does the strength of the continental lithosphere reside?– In the crust– In the Lithospheric mantle

•  • Agenda, Readings and presentations are posted at:• ftp://ftp.gps.caltech.edu/pub/avouac/Ge277-2011/

World seismicity (data source: USGS) and velocities relative to ITRF1997 at Geodetic sites (Sella et al, 2000)

The distribution of seismicity, quaternary faulting and geodetic displacement shows, to fisrt order a ‘plate-like’ behavior.

Tectonic map of Eurasia (Emile Argand, 1928).

All continents suffered successive orogenic episodes

Velocities relative to Eurasia, Calais et al (2006), Bettinelli et al (2006), Simons et al (2007), Gan et al (2007), Yu et al (in prep), and Chlieh et al (in press)

A BA

B

Velocities relative to Eurasia, Calais et al (2006), Bettinelli et al (2006), Simons et al (2007), Gan et al (2007), Yu et al (in prep), and Chlieh et al (in press)

10-8 yr-1

)()( 22

212

121 tr

Strain rate computed from the technique of Tape, Muse and Simons (2009).

(Tape et al, 2009)

What happens at depth? (cf Thatcher, 1983)

(Johnson and Segall, 2004)

(Bourne et al, 1998)

go

(Johnson and Segall, 2004)

Geodetic strain is time dependent. It varies troughout the seismic cycle

(Johnson and Segall, 2004)

Geodetic strain is time dependent. It varies troughout the seismic cycle

Inferences made from Experimental Rock studies regarding the rheology of the

lithosphere Wet quartz

Dry Olivine

(See in particular Kohlstedt et al, 1995)

From E. Burov

Phe predicted strength envelope depends a lot on the assumption regarding which minerals govern the rheology of the lower crust. (NB: model 4 is for an oceanic lithosphere)

600

Oceanic earthquake depths

1350

(Jackson et al, 2008)

Insight from seismicityChen and Molnar [1983] discussed the presence of a weak, aseismic middle-

lower crust sandwiched between the seismic upper crust and seismic uppermost mantle [showing that the ‘jelly sandwich’ model may be valid

The Jelly Sandwich

How certain can we be about the focal depths of Chen and Molnar’s earthquakes?

• Mackwell et al [1998] suggests that dry diabase or wet granulite in the lower crust could be quite strong relative to wet upper mantle olivine

• The strength of the continental lithosphere might ly in the brittle portion.

Jackson [2002]

• In a very influential paper Chen and Molnar (1983) proposed that the distribution of seismicity with depth would reflect the strength envelopes as inferred from laboratory laws.

• The paucity of EQ in the upper mantle in continental area suggests that deformation in the continental lithosphere is only viscous and that most of the strength of the continental lithosphere is in the brittle crust (Jackson, 2001)