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tuolumne_tradster
Trad climber
Leading Edge of North American Plate
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Aug 10, 2010 - 12:07am PT
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Dipper: I'll be interested to see if Minerals offers an interpretation. Can you take a close up photo of your sample and post it?
RE your question about Pt Reyes, here's a USGS animation that shows 30 million years of movement along the San Andreas fault and its predecessors. It is a compilation of an enormous inter-disciplinary data set including paleomagnetic data from the Transverse range (watch it rotate ~ 90 degrees clockwise) and the correlation of Pinnacles to the Neenatch Fm ~ 300 km to the south. You can see that Pt Reyes and Bodega Head, both composed of granite (Salinian Block), originate from the southern Sierras. The stripes off shore are sea floor magnetic anomalies.
http://earthquake.usgs.gov/monitoring/deformation/tectonics/western-na.mov
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tuolumne_tradster
Trad climber
Leading Edge of North American Plate
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Aug 10, 2010 - 01:02am PT
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The location of right lateral slip along the western margin of the N American plate has migrated or "jumped" eastward (landward) with time. The previous incarnations of the modern San Andreas Fault are often referred to as the proto-SAF. You are correct that at about 12 mya, most of the right lateral slip is taking place mainly along the modern trace of the SAF.
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dipper
climber
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Topic Author's Reply - Aug 11, 2010 - 03:03pm PT
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TT,
Very cool animation. Thanks for posting.
Here are two close-ups of one sample. Opposite sides of the same piece.
I am off to the mtns. for a few days. Thanks for any ideas you might share.
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tuolumne_tradster
Trad climber
Leading Edge of North American Plate
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Aug 11, 2010 - 04:31pm PT
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thanks for the close-ups...looks like quartz, feldspar (light minerals) and and the dark mineral is possibly an iron oxide maybe mangetite? so I'm guessing some kind of layered quartz-rich igneous rock
I'm curious what Minerals interpretation will be.
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tuolumne_tradster
Trad climber
Leading Edge of North American Plate
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Aug 11, 2010 - 11:16pm PT
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Geo bump
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Minerals
Social climber
The Deli
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Yeah, Geo Bump!
Hey, I missed your posts from August, Dipper, and apologize for not responding. Just stumbled across this while looking for an earlier post of mine, er... granites.
Tradster, I agree with your initial interpretation, on the previous page.
Dipper, it looks like banded marble – coarse-grained and recrystallized during contact metamorphism from the intrusion of the surrounding granitic magmas. Looks like another screen of at least two different metamorphic rock types in the outcrop photo (image 6).
The “salt-lick” reference would fit with a sedimentary protolith, no? The white mineral is calcite and the dark mineral is probably garnet or vesuvianite (idocrase) or some other common skarn mineral. In your post on the previous page, image 5 shows banding of the dark mineral and boudinage – the dark bands are broken up into segments. Boudinage forms during ductile deformation of the rock mass, due to the difference in mechanical competence of different materials/minerals. Image 4 shows the typical surface weathering texture of marble. Yup, 40-grit or coarser seems about right.
The close-up photos on this page definitely look like coarse-grained marble to me.
Here’s the real test… If the above photos are of a sample that you have collected, take a pocketknife to the white mineral and try to scratch it with the tip of the knife. If it scratches, it’s calcite. If it doesn’t scratch and the mineral is harder than the knife, then it’s quartz and/or feldspar.
Hope that helps! Cheers!
Now then… where are those Wild Dike photos…? That leucocratic septa has got to be a partial melt product…
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Spider Savage
Mountain climber
SoCal
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Dipper's last photo looks like calcite served up on matrix of decomposed granite. But the fellow above know's way more than I do.
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dipper
climber
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Topic Author's Reply - Dec 10, 2010 - 09:10pm PT
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The image you reference depicts the Arch Rock area of Point Reyes National Seashore.
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Jaybro
Social climber
Wolf City, Wyoming
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Dec 11, 2010 - 12:09am PT
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Wild Dykes?
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Spider Savage
Mountain climber
SoCal
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Dec 11, 2010 - 12:26am PT
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Calcite is pretty mushy. And disappointing.
Quartz is nice.
Papa used to carry HCL in a little green bottle in the ashtray of his state of Idaho issued 1963 Scout. I've got a bottle in the garage but don't use it much.
Someone post another picture. This is getting boring.
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tuolumne_tradster
Trad climber
Leading Edge of North American Plate
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Dec 11, 2010 - 12:42am PT
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I usually scratch the white rock with a key or knife...if the metal comes off on the rock, it's usually quartz...if the rock is easily scratched by the metal, it's usually calcite
Here's Minerals in action describing the origin of Ladder Dikes
Jaybro...these are the wildest dikes I've seen in a long time
Deformed Cambrian Quartzite near May Lake
Here's an uplifted sea stack in Franciscan Melange on the Sonoma Coast near Jenner, CA. If you look closely, you can see a couple of olistostromes (exotic blocks found in submarine deposits resulting from slumping or gravity sliding)
Here's a closeup of the Green Schist olistostrome (dk green oval shaped block just below center)
here's a closeup of the Blue Schist olistostrome (just left of center)
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tuolumne_tradster
Trad climber
Leading Edge of North American Plate
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Dec 21, 2010 - 01:38am PT
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geobump
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dipper
climber
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Topic Author's Reply - Dec 21, 2010 - 02:08am PT
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Still have not scratched those earlier samples....
Check out the feature smack dab in the middle in the below image:
Now check out the up close and personal:
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Spider Savage
Mountain climber
SoCal
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Dec 21, 2010 - 04:40am PT
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That cave in "Nam is pretty cool!
Dipper's feldspar blob above is fascinating. I picture that big chunk floating in the deep plastic half a billion years ago and wonder how and why.
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ydpl8s
Trad climber
Santa Monica, California
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Dec 21, 2010 - 11:43am PT
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Marble? Did somebody say marble?
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tuolumne_tradster
Trad climber
Leading Edge of North American Plate
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Dec 21, 2010 - 04:09pm PT
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dipper: very cool photo of what appears to be an aplite dike?
reminds me of this outcrop near the summit of Mt Whitney
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Minerals
Social climber
The Deli
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Dec 21, 2010 - 04:22pm PT
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Wow! That’s some crazy looking rock, Dipper. Really neat. 3-D checkerboard!
Yeah, looks like it could be a pegmatite dike that is mostly, or all K-spar (orthoclase). You can see that there is a series of dikes in the second photo. The rock fractured along the dike, leaving the planar surface exposed. The checkerboard pattern may be a result of the cleavage angles in the K-spar, and formed as the rock fractured.
Another possibility is that the dikes are composed of aplite (fine-grained) and the complex fracture pattern within the dikes developed prior to the formation/fracturing of the planar surface that is now exposed. Granitic rock types of differing grain size behave differently under cooling and tectonic stresses – one rock type may be only slightly jointed while an adjacent type is completely chopped up.
It also looks like there is a little bit of modal layering (bands of light and dark minerals) in the bottom of the second photo, if you look closely. This is a somewhat common feature in granitic rocks.
The oldest granitic rock in the Sierra Nevada Batholith is about 220 million or so years old while the quartzite near May Lake in Tradster’s third photo above is about half a billion years old.
Edit: Didn’t see Tradster’s post before I posted but was thinking of that photo too. Cool stuff.
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scuffy b
climber
Three feet higher
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Dec 21, 2010 - 04:52pm PT
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Are the roof pendants further south in the Sierra roughly the same age
as the May Lake/Snow Peak quartzites?
I have another question about those. In that new book we are told the
May Lake metamorphics migrated a long way from the South.
Were they formerly south of the big roof pendants?
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Minerals
Social climber
The Deli
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Dec 21, 2010 - 06:58pm PT
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The metamorphic roof pendants of the Sierra vary in age. The quartzite at May Lake is related to the Zabriskie Quartzite in Death Valley and the Tapeats Sandstone in the Grand Canyon. The metavolcanic rocks in the Tioga Pass area are younger (Triassic and Jurassic) but the metasedimentary rocks in that area are closer in age to the quartzite at May Lake. I believe that the metavolcanic rocks of the Ritter Range pendant are Triassic as well (without bothering to check…).
The metasedimentary rocks at May Lake, Benson Lake, and Snow Lake, etc. (western margin of the Tuolumne Intrusive Suite), are all part of the Snow Lake block, a huge slice of Earth’s crust that was transported approximately 400 km northward along the Mojave-Snow Lake Fault. This fault was an intra-arc strike-slip fault that was active somewhere in the time frame between 148 Ma (Independence Dike Swarm age) and maybe 120 or 130 Ma (I forget the exact age of the granitic unit that cuts the fault…). All of the younger metamorphic rocks in the Tioga Pass area lie to the east of what was once the trace of the Mojave-Snow Lake Fault. The quartzite of the Snow Lake block was transported from the Mojave Desert area, so yes, these rocks were formerly south of the eastern roof pendants.
Here’s a quick little diagram, hoisted from the net:
An intra-arc strike-slip fault forms within an active magmatic arc (chain of volcanoes with granitic rock forming underneath) because plate convergence at the subduction zone/trench is not perpendicular, but oblique. The magmatic arc acts as a structural weakness within the crust and a strike-slip fault develops, in order to accommodate the oblique convergence forces at the trench. If it weren’t for oblique convergence at the trench, the quartzite at May Lake would still be sitting in the Mojave Desert area. Does that make sense?
Here’s an abstract on the Mojave-Snow Lake Fault from my professors:
http://www.agu.org/journals/ABS/1990/TC009i006p01609.shtml
Here’s a great resource for checking out more of the roof pendants in the Sierra:
http://geomaps.geosci.unc.edu/quads/quads.htm
Don’t pay attention to the Jurassic age of the metaseds on the western side of the Tuolumne Intrusive Suite (Tuolumne quad) – that age is incorrect. Many new interpretations have been made regarding the various rock units on these Sierra quads since they were published, so don’t take the maps as 100% correct. Science is always evolving.
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