Wednesday, March 16, 2011

Where it All Began, Part II

Continued from Where It All Began, which ended:
That, anyway, is one way to make a geologist, although I’m sure there are also other ways. One thing that geologists are sure of is the existence and viability of other ways — this idea is technically referred to as “multiple working hypotheses,” and it’s sometimes stated as, “if ten geologists mapped the same area, you’d end up with at least ten different maps."

Half Dome

The western U.S. — from the subduction-bounded and transform-faulted Pacific coast to the uplifted and locally hog-backed eastern border of the Rocky Mountains — is a tectonically active region. This activity is revealed in the Mesozoic intrusion, cooling, and crystallization of the great Sierra Nevada batholith (65 to 210 million years ago), in the Holocene to Miocene faulting of the San Andreas fault system (0 to 35 million years ago), and in the Pleistocene to Pliocene uplift of the Sierra Nevada Mountains as a block (1 to 3 million years ago especially, with some uplift still continuing). The west coast has been an active continental margin for a long time — back, way back into the early Paleozoic and perhaps beyond into the Precambrian. I was born and grew up in this active geologic area.

When I was almost eleven, my family and I left the west coast semi-permanently and moved to the mostly inactive, even passive, continental margin of the east coast. The east coast — as geologists learned when the theories of continental drift and plate tectonics came together in the 60s and 70s — has been, for the most part, tectonically inactive since the Paleozoic era, with a smattering of activity in the Mesozoic, and even less activity since then (though not entirely zero). It took geologists years to figure out how plate tectonic theory could apply to such a long dead area (geologic history here and here). It’s now known to be a passive margin.

I never felt truly at home there: the mountains were not mountains, the coastlines were not coastlines — unless we drove all the way to Maine — and no huge masses of granite stuck out anywhere to provide a feeling of solidity or to remind me of home. Eventually, I found pieces of home in the granite quarry of Mt. Airy, North Carolina, and in unakite outcrops hidden somewhere amongst the creeping, overgrown underbrush of the Blue Ridge Mountains (unakite = altered granite). I was also rather partial to the kyanite of Willis Mountain, and enjoyed looking for twinned staurolite crystals. These, and other rocks and minerals, are reminders of the the Appalachians' earlier, more tectonically active heydey, but I was not introduced to them until relatively late in my east coast stage of life. I was a geologic orphan.

When I returned to the west in 1975, I bypassed all known modes of geologic transportation — continental drift, wind and river transport, landslide, fault creep, thrust faulting, and valley rifting — and, instead, traveled across the continent in my ‘72 Opel, with everything I owned fitting inside except my full length mirror. I deposited myself like a graded bed on the east side of the Sierra Nevada and once again drew renewed strength from its high prominences and great length.

I was now in the province of the intermountain west, an area of complex and still unfolding geology, an area of tension and extension that formed, and still form, the quite obvious elevation extremes of basins and ranges that reach from the Sierra Nevada to the Wasatch Range. (Reno, Nevada, and Salt Lake City, Utah, are moving away from each other at a rate of about ½ inch per year.) The topography is simple and seemingly endless: up and down, up and down, up and down: corrugated or not corrugated, depending on how you like to characterize things. Dutton (1886) described our western ranges as looking like "an army of caterpillars crawling northward" — ranges of the Basin and Range province crawl northwestward across Arizona toward Nevada, and crawl northeastward across Nevada toward the Snake River plain.

I was also in a region shaped by tectonic processes more subtle than those that created the definitive horst-graben topography of the Basin and Range, processes that were active long before the land now known as California even existed — before magma arose from the melting zone deep beneath the west coast’s subducting plate to form a batholith that is intermittently present from at least Mexico to the northern reaches of Canada, and even into Alaska. First, sedimentary layers and volcanic flows had accumulated in former oceans and basins; then, as part of repeated accretionary events beginning in the Paleozoic, western siliceous cherts, siltstones, and pillows had been pushed over eastern carbonates along a thrust fault hundreds of miles long: the Roberts Mountains Thrust.
Carlin Unconformity

I was in a land of slammed-together accreted terranes, of compression and obduction, of province-wide, low-angle reverse faults. I was also in a land of thin and hot crust, a land whose huge-caldera volcanism and volcanic pyrotechnics in the Tertiary could easily make the eruptions of Mt. St. Helens, Mt. Mazama, Mt. Pelé, and Krakatoa look puny. In short, I was in the Great Basin — a region that overlaps with the Basin and Range province but is by no means identical to it — where all rivers, creeks, and dry washes flow inward into inland lakes or dry playas, and not outward to the sea.

I was, at long last, in Nevada.

Nevada, the silver state, has long been known for its bonanza gold and silver, and has more recently been known for its no-seeum, low-grade, disseminated gold (not so low-grade anymore, but that's another story). After moving here, I expanded my horizons: Carlin-type gold deposits in 1976, eastern Nevada in 1980, and the Mojave desert in 1981. Because of my own expansion, I gradually came to know that I was in the land of detachment faulting: a land where extreme extension and tectonic denudation have exposed the tortoise-shell–shaped cores of metamorphic core complexes that cut through the North American Cordillera in a band running from north-central Mexico into British Columbia. Detachment faults — large, low-angle normal faults of regional extent — separate the ductilely deformed cores from the broken and faulted rock formations lying tilted and detached above.

I moved here in 1975 and have been extended, disseminated, and detached ever since.

Some References:
Cathro, R.J., 2010, Nevada-type gold deposits (Part 4): CIM Magazine, v. 5, no. 7, p. 88-90.

Dutton, C.E., 1886, Mount Taylor and the Zuni Plateau, p. 105-198 in Volume III of Report of the Secretary of the Interior; being part of the Message and Documents Communicated to the Two Houses of Congress at the Beginning of the First Session of the Forty Ninth Congress in Five Volumes: Washington, Government Printing Office.

Gilbert, G.K., 1890, Lake Bonneville: U.S. Geological Survey Monograph 1, 438 pp.

McPhee, John, 1981, Basin and Range: Farrar; Straus & Giroux, New York.

Troxel, B.W., and Wright, L.A., 1987, Tertiary extensional features, Death Valley region, eastern California: Geological Society of America Centennial Field Guide-Cordilleran Section, 1987, p. 123-132.


Gaelyn said...

You've been through an interesting progression.

Kea Giles said...

More girls need to read your blog. Wonder how to do that...

Silver Fox said...

Gaelyn - a geologic progression. ;)

Kea, I'm not sure how to do that in particular...

Geology Happens said...

An excellent post, a great read!

Silver Fox said...

Thanks, GH. :)