Monday, August 10, 2009

Where on (Google) Earth #170

WoGE #170. Click to Enlarge.

Yay! I just won another Where on (Google) Earth! [Note: the WoGE link is dead, at least until Ron Schott gets back from the field, so there is no location information or winners list currently available.]

I won WoGE #169 by Peter Luffi, which was a bit of the Denali Fault near Cantwell, Alaska. Here's how I found it. First, I thought the area looked familiar, and it in fact reminded me of Tertiary volcanic rock sitting on top of Tertiary sediments. Using this false idea, I immediately started hunting in Idaho, thinking of east-west rivers and the Eocene Challis Volcanics. I soon gave up on that area and piddled around, hovering over the western U.S. in Google Earth.

The scale of the hills and rivers in the image compared to hills and rivers in Idaho and other western states, made me realize that I would need to look elsewhere. The large wash in the northwestern corner of WoGE#169 looked like glacial rivers and washes I've seen in Canada and Alaska. I thought about that and came up with the east-west valley area where the bus goes to Denali from Denali Park headquarters, some few miles north of the actual #169 area. I checked that area out thoroughly to no avail, and realized it was incorrect; nevertheless I kept looking in the region, thinking that I should find the Eocene Cantwell Formation. (Stuck in the Eocene, for some reason.) I was fortunately in the right area for the wrong reason. As soon as I thought about east-west faults, the fate of WoGE#169 was sealed. The portion of the Denali Fault shown is just west of the area of the 2002 7.9 earthquake.

To win this WoGE #170, be the first to put the correct latitude and longitude in the comments, and describe the geology of the area! No Schott Rule this time. If you win, it's up to you to choose the next WoGE!

Them thar's the rules.


Gaelyn said...

No idea where, but looks like there's a trapped valley in between those angular mountains.

Peter L said...

View centered at 36.372N, 120.669W; this is the New Idria serpentinite diapir which forms the core of the Coalinga antiform in the southern Diablo Range, California. It is flanked by folded Mesozoic and Lower Tertiary marine sedimentary units (visible in the southern and northern thirds of the view). All these units together form an asymmetric actively rising anticline. Northwards, the serpentinite diapir thrusts along high-angle faults over Late Cretaceous shales and sandstones of the Upper Great Valley Sequence and metagreywackes of the Jurassic and Cretaceous Franciscan Complex; in the southern part it is in thrust contact chiefly with the Franciscan Complex. The far northeastern corner of the view is dominated by mostly Eocene marine sediments. The New Idria serpentinite has been buoyantly detached from its mantle source region (still undecided whether it represents underplated Farallon abyssal peridotites, the mantle section of the Coast Range Ophiolite, or the mantle wedge itself) and, upon diapiric ascension, it has entrained a series of low- and high-grade crustal blocks (greenschists, blueschists, jadeitites, garnet amphibolites, and eclogites).

Silver Fox said...

That's a great description, Peter, of the New Idria antiform. I'd only mention that New Idria is in the north part of the view, more-or-less the center of the historic mercury mining in the district.

So - it's up to you again - for WoGE #171! :)

Peter L said...

Yes, Silver Fox, you're right, I've overlooked the New Idria mercury mining. And I also have entirely forgotten that here is the locus tipicus for benitoite (BaTiSi3O9, a cyclosilicate formed in hydrothermally altered serpentinites), named after the near-by San Benito River. It's the official state gem of California and, more importantly to me, owing to its intense blue cathodoluminescence, it is the "spot finder" sample used in the adjustment of beam size and shape of electron microprobes :-)

Anyway, I'll post #171 asap.

Silver Fox said...

I forgot about the benitoite - there are some great little rock shops in New Idria - or at least *were* in 1982!

Peter L said...

Folks, WoGE#171 is ready for inspection. Have fun!