Strength of the Lithosphere Or, on how the geophysicist stole the Christmas tree

A Gourmet Perspective…

Strength Envelopes Q. - It seems like any permutation of pore pressure, hydrostatic fluid pressure and mantle strength could give a model. Why do the authors emphasize three? Are there other models? In what way do these three models serve as end member cases? Also, why do they only need a limited set of minerals for these diagrams? (Fig 2 and 7). What about these minerals make them characteristic?

Strength Envelopes Parameter Creeping SAF Indiana Hispaniola Surface heat flow (mW/m2) 75 – 85 (used 80) 37-71, used average ~54 20-30 (used both as end-members) Upper crust Heat production (µW/m3) 1.1-1.7, used 1.5 1.1 0.7 Lower crust heat production (µW/m3) 0.4 0.1 Upper crust conductivity (W/m C) 2.4 3.7-5.0 Lower crust conductivity (W/m C) 2.5 3.1 1.9 Upper crustal thickness (km) 14 25 15 Moho depth (km) 40 Upper crustal pore fluid factor (λ) 0.357 Inferred stress state S/R S Log strain rate -12 -18

Strength Envelopes

Strength Envelopes Brittle failure dominant mechanism Ductile deformation mechanisms dominant

Rheology Models

Two Regions of Interest Continental interiors Plate boundary zones Strong faults Higher viscosity Weak faults Lower viscosity

Strength Envelopes

Differences in Post-seismic Models

Questions Question 1 An interesting argument of the paper is that the viscosity of the lower crust or asthenosphere may vary with time and/or applied stress. What are the relevant observations that support this idea? Are these related to a specific deformation process? Are there processes that might mimic the deformation signal of time- dependent viscosity and if so, can the problem be resolved? What rheological model(s) discussed in the paper would allow for a time-varying viscosity? Why does a stress-dependent rheology lead to apparent variation in viscosity in time?

Questions Question 2 The paper discusses various processes that are sensitive to viscosity and can be measured using geodetic techniques (postseismic transients, lake drainage, and glacial retreat). Take a look at the table of viscosities estimated using each of these methods and answer the following questions: What are the spatial scales associated with each type of process, and what depth extent are they likely to be most sensitive to, if any? Are there systematic differences in the viscosity estimates from: The different processes? Continental/plate boundary areas? What other processes could conceptually show similar effects to viscoelastic processes? When/on what scales might those processes be safely neglected? What differences in lithospheric structure or characteristics (globally) did the authors of the paper suggest as one of the reasons for the different viscosities found in the lower crust and upper mantle? (Hint: think about plate boundaries vs. cratons and relevant differences)