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Metamorphic Rocks & Structures

Quartzite with pervasive lineation, Santa Catalina Mountains, Tucson, Arizona

Age & Formation: 
Precambrian Neoproterozoic Apache Group, possibly the Dripping Springs Quartzite, ~1.1 Ga.
The smooth concave “underbelly” is the preserved shape of a fold. The lineation pattern on the quartzite surface reflects the orientation direction of the fold. The lineation pattern is a mold of a now-missing, soft, cleaved rock layer, probably a phyllite, which before metamorphism was mud in contact with a sand later, now metamorphosed to quartzite. Seeing only these lines tells a lot about the folding.
Whereabouts: 
Collected this in 2006 on Pontatoc Ridge in the Santa Catalina Mountains, Tucson, AZ. I am attracted to this rock mainly because of what is missing, and the smoothly curved nature of the folding.
Rock Image: 
quartzite_pervasive_still.jpg
QTVR URL: 
quartzite_pervasive.mov

Quartz-fiber pressure shadow on fractured pebble, northwestern Arizona

Age & Formation: 
Mazatzal Quartzite, Mesoproterozoic in age, ~1.5 Ga.
The purple object is part of a quartize pebble. The pebble was fractured and pulled apart in a tectonic strain environment that ‘demanded’ profound stretching. This occurred at significant depth under temperature and pressure conditions of low-grade regional metamorphism. As the pebble fragments slowly pulled away from one another, the intervening area continuously ‘filled’ with quartz-sericite crystal fibers, forming ‘beards’ growing from the fractured faces of the pebble. The fibers grew in the sheltered shadow region of the hard pebble fragments.
Whereabouts: 
I believe I collected this in Sue Beard’s thesis area in northwestern Arizona near the Cottonwood Cliffs. This was back in ~1983-84. This is one of my favorite rocks, …remarkable for being a pressure shadow you can hold in your hand, and use to discuss how pressure dissolution operates.
Rock Image: 
quartz_fiber_still.jpg
QTVR URL: 
quartz_fiber.mov

Argillite, shaped in the form of ellipsoidal linear structure, Central Andes, Peru

Age & Formation: 
Devonian Excelsior Formation, ~375 Ma.
This is a linear structure shaped like a projectile. The rock formation was originally a mudstone. But when the mudstone was subjected to intense folding and experienced profound strain, the mudstone became progressively metamorphosed to argillite. Strain was especially intense in cores of folds, where the rock experienced not only flattening but profound stretching and extension parallel to the axes of the major folds. It was this stretching that caused parts of the rock body to be transformed into elongate, projectile-like linear structures.
Whereabouts: 
Yauli Dome area, central Peru. I collected this while working with Lou Lepry and Ralph Rogers in Peru in ~1980, where Lou was doing his thesis work near Morococho and Ralph was doing PhD work at Cerro de Pasco. It was a terrific day, working at ~16,000 ft and bundled up in a parka. The ellipsoidal projectiles were lying on the ground all over, as if we had stumbled upon an ammunition storage bunker. I like the ballistic shape of this structure, and how it speaks to an environment of constrictional flow during deformation. I wish I had collected more of these things.
Rock Image: 
argillite_ellipsoidal_still.jpg
QTVR URL: 
argillite_ellipsoidal.mov

Jaspellite, Tower-Soudan mine area, northern Minnesota

Age & Formation: 
Paleoproterozoic age, ~1.7 Ga
Banded iron formation (BIF) is characterized by alternating iron-rich and silica-rich laminae. Jaspelite is a particularly spectacular variety, rich in red hematite. Jaspelite is very interested structurally, for folds and faults are so conspicuous, given the prominence of contrasting layers. Banded iron formations and jaspelites are restricted to ancient times, and many believe that their origin required an atmosphere deficient in oxygen.
Whereabouts: 
I collected this in 1992 on a Carleton College geology field trip to northern Minnesota. I was a visiting professor at Carleton at the time. I wish I had a room full of jaspelite specimens, because colors, textures, and geometries are so rich; and the mineralogy (for a sedimentary rock) so unusual.
Rock Image: 
rock45_R.jpg
QTVR URL: 
jaspelite.mov

Chloritic alteration, slickenlines, and grooves on fault, Mesabi Range, northern Minnesota

Age & Formation: 
Mesoproterozoic, ~1.7 Ga.
I found this rock along a quartz vein, and concluded that it represents “hot slickenlines,” i.e., preserved tracks of fault movements under conditions of elevated temperature and pressure and lots of fluid flow. I believe that the dark material is chloritic, i.e., a chloritic alteration typical of low-grade regional metamorphism.
Whereabouts: 
I collected this in 1992 on a Carleton College geology field trip to northern Minnesota. I like the fact that it reflects a deeper hotter level of faulting and shear.
Rock Image: 
chloritic_alteration_still.jpg
QTVR URL: 
chloritic_alteration.mov

Tight folds in gneiss, north Norway

Age & Formation: 
I do not know the original age of the rocks that became so deformed and metamorphosed, but we do know that the deformation occurred during the Caledonian orogeny, between Ordovician and early Devonian, roughly 490 to 390 Ma.
Such tight folds and shear structures are typical of what was produced during thrusting and formation of fold and fault nappes. The deformation took place under conditions of high-grade metamorphism, and this is what makes the internal deformation so ductile, and spectacular.
Whereabouts: 
I have forgotten the exact location, but somewhere in the region between Narvuk and Tromso in north Norway. I was there in 1985 with Steve Naruk, where he was doing part of his PhD work. I like the fact that this rock is marked by such penetrative deformation: nothing escaped the influence of tectonic straining of the country rock.
Rock Image: 
rock48_R.jpg
QTVR URL: 
rock_09.mov

Sheath folded quartz vein, Santa Catalina Mountains, Tucson, Arizona

Age & Formation: 
Unknown.

This appears to be part of a sheath fold. The rock itself is a set of quartz-rich layers. Sheath folds form under conditions of heavy-duty simple shear (non-coaxial deformation). First there develops a strongly asymmetrical fold oriented perpendicular to the shortening direction. But with continued shearing the hinge of the fold can flare out in the direction of tectonic transport, into a form that resembles a wind sock. The fold assumes the form of a cylinder, closed at one end. Cross sections through such folds look like ‘eyes.’

Whereabouts: 
A neighbor came over to our house one morning and handed this to me (in ~1994). She found it while looking for golf balls along a desert course. It obviously had been transported downslope from the Santa Catalina Mountains. She thought it might be a fossil bone. I ‘pronounced’ it a sheath fold, and hope that I am right. It is not often that you can hold a sheath fold, and most of its puzzling geometry, in your hand.
Rock Image: 
sheath_folded_quartz_still.jpg
QTVR URL: 
sheath_folded_quartz.mov

Intrafolial folds in quartz sericite schist, Bathurst, New Brunswick, Canada

Age & Formation: 
rock from the Tetagouche Group of Ordovician age, ~420 Ma.
When I was doing my Ph.D. work at The University of Michigan, I read all I could on bedding transposition during folding and regional metamorphism. Thus it was gratifying to find clear examples of transposition in my dissertation area, the Caribou (massive sulfide) mine area. Transposition is a process by which original bedding becomes tightly folded, and axial planar schistosity develops. As shortening continues, the schistosity becomes better developed, the folds become isoclinal, and the limbs of the fods become subparallel.
Whereabouts: 
I collected this rock in 1969 in a region of very poor outcrop in the Caribou mine area. Because the ‘bush’ in this region is so thick, and outcrops so scant, the mapping involved following picket lines surveyed along NS and EW grid lines. In outcrop I could make out the possibility that the quartz sericite schists were internally intrafolially folded, which then was confirmed when I slabbed and polished the rock.
Rock Image: 
intrafolial_folds_quartz_still.jpg
QTVR URL: 
intrafolial_folds_quartz.mov

Isoclinal intrafolial fold in massive sulfide, Bathurst, New Brunswick, Canada

Age & Formation: 
Tetagouche Group of Ordovician age, ~420 Ma.
This is the prize item from my dissertation research. I already have shared my interests in transposition (see description of intrafolial folds in quartz sericite schist). The first time I spotted transposition structures in my dissertation research was the summer AFTER the Caribou mine was sealed and shut (too much in the way of acid water despoiling local streams).
Whereabouts: 
But how would I marshall more evidence? I went immediately to the mouth of the adit where there was an accumulation of tons of massive sulfide ore, that had been removed from the mine for purposes of crushing and metallurgical testing. I thought to myself, “Maybe I can spot a specimen of ore rock that reveals transposition.” All the rocks were dark grey to black, but within 15 minutes I picked up a chunk that looked like it could be a winner. That same afternoon I slabbed it (I had made up a portable diamond-bladded saw for work that summer), and discovered what you see here: a beautiful isoclinal intrafolial fold of sulfide ore, complete with axial plane schistosity. This find and what it represented was an important part of interpreting the ore deposit as a syngenetic massive sulfide deposit, originally deposited as a ‘strange’ rock type on the seafloor bottom in a volcanic arc setting.
Rock Image: 
isoclinal_still.jpg
QTVR URL: 
isoclinal.mov

Kink fold in quartz sericite schist, Bathurst, New Brunswick, Canada

Age & Formation: 
Quartz sericite schist from the Tetagouche Group of Ordovician age, ~420 Ma.
There is a huge literature on kink folding, with details on various mechanisms and processes of formation. The main ‘take away’ is that kink folding is the natural and inevitable result of tectonic loading of rocks containing penetrative schistosity or cleavage. The loading must be reasonably ‘end on,’ and when this takes place the schist or phyllite or slate is able to shorten through kink folding, a semi-brittle phenomenon.
Whereabouts: 
My dissertation area in the Bathurst district was loaded with kink folds. For example, in the mine adit where I carried out a full summer of mapping, the entrance area alone (300 m long) was marked by steeply dipping quartz sericite schists. I had both cross sectional views of the schist, as well as broad exposures in the plane of schistosity. The latter exposures were marked by myriad lineations, expressions of the kinked hinges of the kink folds. I arrived for this mapping equipped with a ‘super compass’ that I had made to rapidly determine the trends and plunges of kink fold hinges and strikes and dips of individual kink fold limbs and axial surfaces. Very time consuming! During the first morning of mapping I covered a distance of less than 20 meters, and began to wonder how I would ever complete my work in a summer!
Rock Image: 
kink_fold_still.jpg
QTVR URL: 
kink_fold.mov

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