24 December 2010
Today’s Friday Fold comes to us via Pete Berquist of Thomas Nelson Community College in Hampton, Virginia. Check it out:
Pete explains what’s going on here:
I cannot provide an exact location but this is within the Mars Hill Terrane (MHT), which is an distinctive swath of Mesoproterzoic basement extending ~50 km x 100 km within the North Carolina Blue Ridge. It may be analogous with the Stage Road Gneiss and Fries Fault in southwestern Virginia, but that’s pretty speculative. MHT rocks are anomalous in many regards to the surrounding Blue Ridge basement and, as such, are interpreted as an accreted terrain within the Grenvillian Blue Ridge. The MHT, compared to the generalized view of Proterozoic Blue Ridge basement [within the Eastern Blue Ridge (EBR) & Western Blue Ridge (WBR)] includes:
- a greater diversity of rock types when compared to the surrounding Eastern and Western Blue Ridge provinces.
- granulite grade rocks, that appear to be absent from the EBR and WBR and restricted to the MHT.
- very, very old zircon ages (1.8Ga magmatic ages and >2.0Ga detrital ages). It should be noted that some (John Aleinikoff and Scott Southworth) interpret the 1.8Ga ages as detrital grains based on zircon morphology. However, isotope geochemisty can permit a magmatic origin.
- isotopic signatures (Sm-Nd, primitive Nd-model ages, and Pb-isotopes) demonstrating provenance to some suspect terrane (Kalahari? Amazonia?) accreted during Grenvillian orogenesis.
The MHT includes pretty structures, too.
I suspect that the folds displayed in this picture are Paleozoic rather than Proterozoic but I have no age data to support that assertion, only a regional sense of field relations.
Anyway, here are some additional papers if you might be interested in learning more about the glorious MHT:
This is my second week in a row with a guest Friday fold — and both were contributed spontaneously, without me asking! So… let it be known that if you know of a cool fold with a cool story attached, I’m happy to feature it here and make it world famous.