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.
Another view of the dike, from Zoback and Thompson (1978), used here as per GSA fair use policy. (Thanks, Andrew Alden!)
The pinnacles are two shadowy dots on the ridgeline near the center of this MSRMaps airphoto, west of an in-canyon set of builings and trees.
The [semi] coressponding MSRMaps topographic map indicates that the pinnacles are less than 40 feet high. I would guess that they aren't even 20 feet high, based on my mid-July, driving-through-canyon perspective. 
Just a quick post showing a photo I took about two weeks ago at 5:50 am, on my early morning ride to work. We're looking easterly at a broad, generally flat-topped sloping part of the southern Snowstorm Mountains. The entire mesa-like, gently north-dipping hill may be called Jake Creek Mountain, or perhaps only its highest peak, barely visible from this angle, is called Jake Creek Mountain. Part of the abrupt slope to the south is called the Owyhee Bluffs.
You'll have to see the second post in this mini-series on the Titus Canyon megabreccia to get a sense of scale (or the first, or see the next to last photo of this post). Very roughly, the horizontal fracture or shadow above the wash (most of the center of the photo, a quarter of the way up) is 3 to 4 feet high (that's at about the same level as the top of the lower right rectangle). First we'll look at the area shown by the large rectangle in the upper left, then we'll look at the area shown approximately by the smaller rectangle in the upper left, an area which goes farther left than the rectangle indicates. After that, we'll zoom to the lower left rectangle, then to the lower right.
This is the one area where I could see the somewhat diffuse contact of the breccia with the carbonate wall rock. Contacts elsewhere were obscurred or I missed them (very possible — there is a lot to see!). I have no idea whether the contact is always gradational like this, or whether it is in some places sharp or some places irregular. Fracturing and incipient brecciation — sometimes in the form of crackle breccia, sometimes in the form of calcite veins or small masses — extends upward into the wall rock about 4 or 6 feet past the contact. The calcite veins might tell quite a story about any stresses affecting the rock during the fracturing and brecciation episode. (I'm taking the view that a tectonic origin is likely for this breccia, although one non-geological website was postulating a cave collapse origin.)
Moving into the breccia from the gradational contact (the smaller of the upper left rectangles), we see that breccia fragments decrease in size toward the more solid rock in the upper right, and it looks like it would be fairly easy to put the fractured rock back together in the zone right near that boundary. 
Now to the lower left rectangle. The large white calcite mass in the left center is shaped roughly like a large breccia fragment and appears to show some sub-horizontal grooves. I think this calcite mass is a thin matrix zone that once cemented a fragment, now eroded, to the rest of the breccia mass. The upper edge of the large calcite mass bends outward, towards us, as if it were about to wrap around a large rock fragment; the lower contact with dark rock is quite sharp, possibly the edge of the now eroded fragment. The grooves may be a tectonic feature or may be something else, for example, grooving related to large boulders going downstream during huge flash floods. I favor the tectonic idea, but haven't examined the area enough to back up this hypothesis.
Now on to the rectangle in the lower right. This part of the breccia is composed of subangular to rounded fragments of the white calcite that forms the matrix of the bulk of the breccia. Note that the white to cream-colored calcite is fractured and cut by hematitic veinlets; also note that many of these fractures formed prior to calcite brecciation but that some didn't. These calcite fragments are floating in a dark mass that is composed, at least in part, of sub-rounded to rounded fragments of carbonate wall rock and other carbonate material, including white unfractured calcite fragments, and possibly including carbonate rock flour. This is a dark breccia matrix for the brecciated calcite that is usually the light-colored matrix of the darker breccia fragments. 
If you want to see this breccia, besides driving one way into Titus Canyon on a long, narrow, one-way (open only to east-to-west traffic), sometimes rocky dirt road that may require 4WD or a high-clearance vehicle (I'd want both to dodge the rocks sticking out of the road uphill from Leadfield), you can drive to the Titus Canyon alluvial fan from the Death Valley side and walk in from there. It looks like it's about 1.5 to 2 miles in, and probably a magnificent hike. (Google Maps Location and additional photos).
Because breccia is one of my favorite rock types, I've got a few more photos of the Titus Canyon megabreccia from our Death Valley and Mojave trip in 2009. In this first photo, the large breccia fragment in the lower left of the canyon wall is about 5 to 6 feet high (scale here and here). Looking carefully at this face of the Titus Canyon narrows, you can see that the breccia extends quite a ways up the canyon wall, into the darkly stained area where textures are difficult to see. The often angular clasts of the dark gray carbonate rock are angular to subrounded, and they are usually matrix supported by white to very pale orange to cream-colored calcite. In places, some of the fragments could easily be put back together, as if they were part of a jigsaw puzzle; in other places, the breccia pieces don't seem to fit.
This one face of the canyon wall — man's hand on the right for scale — shows a lot of the white calcite that often forms the breccia matrix. There is seemingly more matrix than usual, although a large fragment or two could lie just behind the rock face, changing the apparent fragment:matrix ratio.
We'll zoom in on the area almost obscurred by the hand in the previous photo. Above the two large dark fragments, we see a large mass of the white calcite.
Looking more closely at the area above the two dark fragments, the main calcite mass can seen to be fractured and brecciated. But there's a complication: a heterolithologic breccia of dark carbonate rock and white calcite is shooting through the rock just to the right of my hand. This second breccia has subrounded to rounded clasts, and appears to be cemented by a reddish, hematitic material (possibly consisting of ground up pieces of both the original dark wall rock fragments and the secondary calcite cement).
This photo of the breccia provides some scale, by way of my truck, for the next photo, which is just to the left of this one.
What I see here is a relatively narrow banded zone that may be a calcite-cemented or calcite-filled fault, dipping off toward the left. Is it a fault? The answer appears to lie upward, requiring rock climbing, or finding a way around to the top from some other direction. Note the diffuse contact between breccia below and fairly solid rock above, just left of the banded zone. We'll zoom in on that...soon.
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.
