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SOPHIA OF WISDOM III - GEOPHYSICS
LIBRARY OF SOPHIA OF WISDOM III
SOPHIA OF ALL SOPHIA OF WISDOMS
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CAROLINE E. KENNEDY - CAROLINA KENNEDIA_________________
JULY 23, 2007
RE: GEOPHYSICS
Seismology
The decoupling zone along the Hormuz Salt layer results in mismatches between known faults mapped at the surface and the measured earthquakes, most of which take place in the basement depths (>14 km). However, seismic maps show that most seismic activity takes place along the Main Zagros Thrust and southwest in the fold belt; there is a sharp break in seismicity visible just to the northeast of the MZT.
Berberian, 1981
Gravity Models
The most obvious first order observation to be made from Bouguer gravity anomalies is that the gravity field is more or less inverse to the topographic profiles. With respect to isostatic residual anomalies, we most readily note the negative values within the Zagros Range.
Bouguer gravity anomalies
The Main Zagros Thrust, the present day location of the suture zone, is highlighted in orange. Three profiles were analyzed, the southern-most one, highlighted in red, will be the focus of later figures (all from Snyder and Barazangi [1986]). The Persian Gulf is outlined in blue, and the other two profiles are highlighted in black. The anomalies reach ~ -225 mGal in the suture zone and ~ -20 mGal in the Persian Gulf.
Isostatic residual anomalies
As above, the Main Zagros Thrust is highlighted in orange, the Persian Gulf in blue, and the three profiles are highlighted in red (southern) or black. Values are near 0 on the Arabian craton, but decrease to ~ -80 mGal near the Gulf coast and in the folded belt, and increase to ~ -40 mGal in the suture zone.
Figure from Snyder and Barazangi (1986)
Isostatic Compensation
Such negative residual anomalies suggest that there is more to the crustal structure than isostatic compensation can accommodate. Constraints on density contrasts allow the authors to suggest the presence of low-density underthrust sediments within the Zagros Suture Zone as a solution to the isostatic compensation modeling problems. This model will not solve the isostatic problems in the Persian Gulf, however, and flexural deformation of the Moho is called upon to explain the anomalies in that area (Snyder and Barazangi, 1986; see below).
With the observed isostatic residual anomaly highlighted in green, the difference between the observed values and the profile calculated from known density constraints (black) is obvious, and it becomes clear that isostatic compensation cannot fully explain the observed gravity patterns in the Zagros (Snyder and Barazangi, 1986).
Moho (Bouguer anomalies)
Bouguer gravity anomalies can help to better constrain the Moho and thus perhaps aid in resolving the isostatic compensation predicament. Moho depths are modeled at 40 km northeast of the fold belt and dip to 58-65 km beneath the Main Zagros Thrust. As noted earlier, the high topography in the Zagros appears to not to coincide with Moho depth. Bouguer anomalies suggest that topography coincides with deformation in the upper basement.
The observed Bouguer gravity data depicted against topography in the southern profile shows that the high topography coincides better with basement deformation than changes in the Moho.
Subducting Slab?
How can the isostatic compensation quandry be satisfactorily modeled, given the density constraints and gravity observations? Isostatic compensation alone can?t explain the observations, and neither can a crustal root. A single intermediate-depth earthquake at 107 km, as well as the presence of the active volcanic belt behind the MZT (to the northeast) provide a possible answer: the remnance of a piece of oceanic lithosphere, still attached to the Arabian plate as it continues to converge towards Eurasia (Snyder and Barazangi, 1986).
If Bouguer models incorporate the effects of the suggested oceanic slab, the observed gravity data (highlighted in green) can be matched extremely well, provided that the slab present does not extend to depths below about 100 km. A slab larger than this, as depicted in Model C to the right, clearly does not match the observed gravity data.
How About Flexure?
Put quite simply, flexure doesn?t cut it either. Simple flexure models aren?t able to simultaneously match all aspects of the Moho as determined from gravity observations.
No reasonable scenario can model the observed shape of the Moho. ?A? attempts to fit the Moho by topographic loading and an end force, and ?B? attempts to model the Moho in two ways; by topography and basement thickening, or by topography, basement thickening, and subsurface loading, evidence for which is entirely absent in the gravity observations (Snyder and Barazangi, 1986).
Well then, What???
The preferred model suggested by Snyder and Barazangi (1986), and the most comprehensive and thorough model I was able to find, incorporates a little of everything!
The preferred model incorporates density constraints (provided by seismic data) and model characteristics based on gravity observations (Figure from Snyder and Barazangi [1986]).
In addition to the decoupling zone along the Hormuz salt layer (see Geology section), a second decoupling layer along the Moho is incorporated, allowing the rigid mantle to slide beneath the shortening Arabian crust as well as the lithosphere of Central Iran. The rigid mantle allows the lower crust to deform plastically, trapped between the brittle upper crust and the rigid mantle. In this scenario, the lower crust behaves like a compressed hydraulic fluid. The Arabian craton area and the region surrounding the Main Zagros thrust are both dominated by isostatic forces, as the plastically deforming lower crust can simultaneously raise the surface and lower the Moho while retaining local isostatic balance. Meanwhile, the Persian Gulf region is dominated by flexural forces resulting from the loading of the Zagros, which produce a more gently dipping Moho as observed with gravity data. Though the potential remnant slab is not included in the final model, the possibility of its existence is not ignored either, as evidenced by the Zagros Volcanic Belt and the intermediate-depth earthquake location.
Some Quick Comparisons?
A Cornell group involved with the Comprehensive Test Ban Treaty is putting together a database for the Middle East, and has come out with some Moho depth data for the region. The regional scale of their data really restrains their results to a first-order comparison with that of Snyder and Barazangi (1986), but it helps to put the entire region in perspective, as well as corroborate earlier results.
This Depth to Moho map shows the Zagros having crustal thicknesses on the order of 50 km, thinning to 40 km toward the SW in the fold belt, similar to thicknesses presented above. In the region of the Middle East, the Zagros display the thickest crust, not too surprising as they are actively deforming (Moho map from Seber et al., 1997)
Seber et al. (1997)
Though the finer details of the crustal structure are impossible to resolve at such a regional level, the shape of the Moho presented here resembles that presented by Snyder and Barazangi (1986), a testimony to the ability and usefulness of GIS in such applications as well as reassurance that neither new data nor a larger quantity of data result in any glaring differences in the projected geometry of the Moho.
Just For FUN!!!
Cornell also has a really fantastically fun website (http://atlas.geo.cornell.edu/webmap/?Submit=Start+Application+) that allows you, the browser, to go ahead and set your own parameters pertaining to several different geological and geophysical aspects. So just for fun, why not have the GIS create a Bouguer anomaly map, depth to Moho map, a couple of Bouguer profiles, and a couple of crustal cross sections of a somewhat more local scale? Here?s what I came up with:
The Bouguer anomaly map shows no surprise; negative anomalies of around ?250 mGal near the suture zone, and increasing on either side. The depth to Moho map shows the suture zone at about 54 km depth, which is only slightly shallower than suggested by the geophysical evidence presented above. The crustal profiles closely match the Bouguer profiles (also not a surprise). It would have been really great to try to get a more detailed topography cross-section of the first few kilometers only, but as yet I have not figured out how to make the cross-section on such a small scale. None of the GIS material really does much beyond prove that the first order observations made in the 80?s are still valid, and that added information, whether it is in the form of new data or merely more of it, does not drastically alter the results that are the basis for Snyder and Barazangi?s model (1986). I also think that the application of GIS to such large scale tectonic and structural questions is really exciting and holds many prospects for future work in both GIS and in regional tectonics.
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