Dear Richard, David and Jean-Paul
Thanks for the very helpful comments and suggestions
Actually, we are working in the western Chihuahua, Mexico where we identified a block surrounded by mobile belts (at three sides) characterized by contrasting negative gravity anomalies (~ -200 m gal), high initial Sr ratios and perhaps low heat flow than the surrounding regions(mobile belts). The surrounding mobile belts have Precambrian rocks while there are almost no Precambrian rocks are present within the block, and there is a general consensus that most of the northern Mexico is predominantly underlain by Jurassic basement and there is no Precambrian crust, recently, I acquired U-Pb zircons (260 analyses) and apatites( 150 analyses) dates from the granodiorites from the same region and interestingly, none of the zircons yielded inherited ages that is no age older than Early Cretaceous are found. Crustal thickness estimates in northern Mexico are around 35-43 km and there are arguments about the lithosphere thickness but it would be somewhere between 80-100km. Seismic data is scarce, I think what we have identified is a high elevation sialic block, that is more felsic having with high initial Sr ratio, cold (low heat flow) and less dense (gravity data) and this require a more buoyant lithosphere and perhaps, more dense mobile belts have underthrusted this block.
I personally do not think that it is a chipped off block from north American craton in north, as Richard pointed, in that case it would be more dense and over that long period of time most of the topography would have eroded. I consider it as a more younger, locally generated block by the repeated action of mobile belts and underlain by more buoyant lithosphere and less dense crust than the normal. And perhaps underthrusting of surrounding mobile belts have facilitated the block to attain its present elevation.
Please suggest what else could be a likely scenario? Comments and suggestions will be greatly appreciated.
Thanks
Munazzam Ali
University of Texas at El Paso
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