Teach the Earth > Structural Geology > Deformation Rate Enigmas

More thoughts from Rick on the topic of deformation rates

Basically, we measure deformation over some 15 orders of magnitude of temporal sampling window, from the 30 seconds or so during an earthquake to the cycle of super continent formation (~600 Ma). Geodesists, who work at time scales of 101 years) would like to know how their measurements relate to mountain building whereas geologists (who work at time scales of 106 years and greater, in general) would like to know, say, how their measurements relate to earthquakes. A problem which varies in severity from area to area is that some significant proportion of the geodetic interseismic strain field is elastic and is never recorded in any straightforward way in the geologic record. For example, the the Chilean coastal cordillera where I've been working the last few years, the interseismic GPS signal is one of significant horizontal shortening (some of the the highest GPS shortening rates in the central Andes) parallel to the plate convergence. Yet there are *no* thrust faults parallel to the plate boundary in the forearc at all – instead, we only find geological evidence of extension parallel to the plate boundary. Of course this is probably part of interseismic loading and elastic rebound but is representative of cases where geologic and geodetic are completely at odds with one another – GPS only samples part of an ongoing process whereas geology probably samples multiple unrelated processes.

I would pose the general question as: "how does instantaneous deformation integrate over 105 to 107 years to produce the mountain belts and basins we see today?" This 4D problem is hampered by the fact that so far reliable deformation rates are relatively rare at "paleoseismic" time intervals (102 - 105 years) especially for thrust and normal faults. The other reason why this is a very difficult problem is that the true uncertainties associated with both paleoseismic and geologic sampling of deformation (seldom treated explicitly) are vastly larger than the uncertainties associated with geodetic measurements.

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