Great Range - Wikipedia style...

[MattC]

Thanks for the replies, guys. Interesting stuff. Unfortunately the book Moonrock mentions appears to be out of print, but maybe I'll luck out and find a used copy sometime. I knew anorthosite was the primary type of rock and not much else, so this has been informative.

Matt

 

[moonrock]

I'm familiar with the veins on Giant. The veins are comprised of plagioclase feldspar megacrysts. Those veins were formed when hydrothermal fluids dissolved plagioclase from surrounding anorthosite and precipitated the plagioclase in fractures.

If this formed in fractures, it sounds like the hydrothermal activity came after the anorthosite emplacement. Regional or local metamorphism ?

 

[Willie]

Regional or local metamorphism?

 

[moonrock]

As a deep magma body starts to cool, specific mineral types crystallize first, specifically iron-magnesium rich silicates (olivine, pyroxenes) and calcium-rich feldspar, gradually depleting the liquid of those elements. If the FeMg minerals settle out, and the remaining liquid remains mobile, the liquid and growing feldspar crystals can migrate elsewhere (upwards) and form other rocks (like anorthosite), which are very different from the original magma composition.

The Adk anorthosite apparently formed in two stages and distinct textures: a coarser rock containing (settled-out) large crystals of calcium-rich feldspar (labradorite), and a finer grained rock that formed at shallower depth.

Actually, (in case any petrologists are reading) I got this partly backward: The magma originates from partial MELTING of deep, formerly solid crust : The iron/mg minerals are left behind 'cuz they have a higher melting temperature; the anorthosite component is a hot slush of liquid and feldspar crystals that then migrates upward from the parent rock.

The earlier process IS generally how magma differentiates itself into various minerals, and can even produce segregated rock types. But the anorthosite is part of a suite of igneous rock types whose origins are complexly interrelated and occurred in multiple pulses. I hate to fudge the details.

But getting back on topic:
I think the views from Sawteeth and Haystack benefit from their "nonconformist" locations, Agreed ??

 

[bubba]

But getting back on topic:
I think the views from Sawteeth and Haystack benefit from their "nonconformist" locations, Agreed ??

As long as they are not 'spurs" of another mountain!!

What about Whiteface and Esther - being different types of mtns?? That true?

 

[moonrock]

I am looking at Jaffe's (elusive) book.

Whiteface and Esther summits are both GABBROIC anorthosite, which simply means they contain > 10% of darker minerals. So are the Dix and Seward ranges, Sawteeth, Rocky Peak, Porter and Cascade.

Most of the highest peaks are Anorthosite, tho Saddleback, Gothics, the Wolfjaws and Big Slide summits are listed as (metamorphosed) anothosite gneiss - again, slid up over the underlying rock by a low angle thrust fault, before the present mountains and valleys were eroded out of the pile-up.

Interestingly, Giant Mtn summit is SYENITE, on top of Gabbroic Anorthosite. Syenite is like granite but too starved of silicon to form quartz crystals (it sometimes makes do by forming "silica-light" versions of feldspar). Syenite dikes also cut through the mountain at around 2000 and 3000 ft elevation, which might be those light veins you saw.

 

[Willie]

In places faulting thrust giant, solid slabs of the coarser crystalline anorthosite up from depth to overlie the finer rock. An example is Big Slide: the top of the peak is a resistant remnant of the overthrust, which "capped" the underlying rock as Johns Brook Valley formed. The profile of the mountain is assymetrical because of the low angle (upwards north-to-south) of the fault, which vertically separates the two.

Most of the highest peaks are Anorthosite, tho Saddleback, Gothics, the Wolfjaws and Big Slide summits are listed as (metamorphosed) anothosite gneiss - again, slid up over the underlying rock by a low angle thrust fault, before the present mountains and valleys were eroded out of the pile-up.

I've observed many outcropping high-angle normal faults in the High Peaks, but I never observed a low-angle thrust fault. Can you point me to an outcropt that reveals a low-angle thrust fault? Perhaps a description of an outcrop in printed material?


EDIT: I'm confused.

According to the Dix Mountain Wilderness Area Unit Management Plan Amendment, Big Slide's shape was created by glaciers, which I always thought to be true:

"As the main continental glacier retreated, smaller mountain glaciers remained in the high peaks region. These smaller glaciers concentrated erosion within stream valleys and sharpened the landscape. Glacial retreat accentuated steep valley walls into “U” shaped valleys and naturally tended to form cliffs on mountaintops and on the sides of steep slopes. This is responsible for the ramp-and-cliff pattern on Algonquin and Big Slide." See Dix Mountain Wilderness Area Unit Management Plan Amendment (January 2004) at p. 7​

 

[moonrock]

Look at alavigne's second photo, of Big Slide viewed from the NE

http://alavigne.net/Outdoors/ImageGallery/BigSlide2003/index.jsp

The low angle thrust (or reverse) fault would be somewhere at/below the base of the cliff on lower left, planing gently downward and right into the peak. I looked for photos of outcrop exposures but could not find any.

A similar fault slices through/beneath the summits of Saddleback-Wolfjaws.
It is illustrated in Jaffe's book, on a sketch of the range from the NE. Imagine gluing two different pieces of wood together, then sanding away all of the upper piece except for a few separate knobs.

Given the depth that fault movement occurred, the adjacent rock would likely be sheared, possibly flow-folded and even melted to glass (mylonite). In that case, the outcrop itself could be continuous across the change.

One of my professors (from Utrecht) used to call them "truss vaults", to mild amusement of the undergrads.

MR

 

[moonrock]

A similar fault slices through/beneath the summits of Saddleback-Wolfjaws.
It is illustrated in Jaffe's book, on a sketch of the range from the NE.
MR

My Bad: I looked back at Jaffe and the thrust fault was ABOVE the current tops of the peaks. But the upper "coarse" thrust sheet DOES include the summits of Basin, Haystack and Marcy.
The asymmetry of Big Slide summit is mostly influenced by internal layering in the summit formations, though it has been sheared and mylonized partway down.

Enough memory f*rting for now.
I gotta read Jaffe again - esp now that's it's a "rare book".

 

[MattC]

Thanks again, guys. And I just requested that book through inter-library loan.

Matt

 

[Willie]

I actually went to the Chicago Public Library, obtained a library card, and checked out Geology of the Adirondack High Peaks Region: A Hiker’s Guide by Howard W. and Elizabeth Jaffe (1986). My purpose was to gain insight into the authors’ interpretation of the structural evolution of the High Peaks. In my opinion, the authors’ interpretation is outdated and fatally flawed for several reasons, but I will focus only on one - plate tectonics.

Although the authors seemingly acknowledge plate tectonic theory in the book’s introduction, they ignore it as they interpret the structural evolution of the High Peaks. For example, the authors’ draw cross-sections showing thrust faults on Big Slide and a nappe on Marcy, Haystack and Basin. The cross-section of Big Slide (fig. 28) depicts a low-angle thrust fault with displacement from the NW to the SE. The cross-section of the Great Range (Fig. 48) depicts the so-called “Basin Thrust” and “Great Range Nappe” with displacement from SW to NE (the authors place a question mark next to the nappe in the drawing). The authors do not explain the cause of the perpendicular thrusting, perhaps because it is unexplainable in under plate tectonic theory. Additionally, according to Figure 2B, the authors suggest that the Great Range Nappe formed at some point between 5 and 60 million years ago. Here’s a news flash! No major orogenic event occurred in the Adirondacks during that period. In my opinion, the authors’ interpretation of the structural evolution of the High Peaks is not supported by plate tectonic theory.

 

[SAR-EMT40]

Is this like one of the coolest threads ever?

Very cool information. Great discussion.

Keith

 

[MattC]

I actually went to the Chicago Public Library, obtained a library card, and checked out Geology of the Adirondack High Peaks Region: A Hiker’s Guide by Howard W. and Elizabeth Jaffe (1986). My purpose was to gain insight into the authors’ interpretation of the structural evolution of the High Peaks. In my opinion, the authors’ interpretation is outdated and fatally flawed for several reasons, but I will focus only on one - plate tectonics.

Although the authors seemingly acknowledge plate tectonic theory in the book’s introduction, they ignore it as they interpret the structural evolution of the High Peaks. For example, the authors’ draw cross-sections showing thrust faults on Big Slide and a nappe on Marcy, Haystack and Basin. The cross-section of Big Slide (fig. 28) depicts a low-angle thrust fault with displacement from the NW to the SE. The cross-section of the Great Range (Fig. 48) depicts the so-called “Basin Thrust” and “Great Range Nappe” with displacement from SW to NE (the authors place a question mark next to the nappe in the drawing). The authors do not explain the cause of the perpendicular thrusting, perhaps because it is unexplainable in under plate tectonic theory. Additionally, according to Figure 2B, the authors suggest that the Great Range Nappe formed at some point between 5 and 60 million years ago. Here’s a news flash! No major orogenic event occurred in the Adirondacks during that period. In my opinion, the authors’ interpretation of the structural evolution of the High Peaks is not supported by plate tectonic theory.

Wow, maybe that's why it's out of print? Was 1986 the original date of publication, or was that a new edition of a really old book? Hasn't plate tectonics been almost universally accepted in the geology community since the 60s?

And lastly, in your opinion does this undermine the whole book, or just ideas about the orogenic events that created the Adirondacks? (Although that's quite a lot right there) Is there still useful info in the book about rocks, glaciation or other topics?

Matt

 

[Willie]

Was 1986 the original date of publication, or was that a new edition of a really old book? Hasn't plate tectonics been almost universally accepted in the geology community since the 60s?

And lastly, in your opinion does this undermine the whole book, or just ideas about the orogenic events that created the Adirondacks? (Although that's quite a lot right there) Is there still useful info in the book about rocks, glaciation or other topics?

1986 is the original publication date, but the authors reference very old material and the cross-sections appear to be many decades old, which suggests to me that they were draw pre-plate tectonics and simply copied into this book. For example, during the years that preceded plate tectonics, geologists attempted use the “nappe” theory, which was developed in the Alps to explain folding and thrusting of sedimentary rocks, to explain complicated structures in mountain chains around the world, no matter the lithology. Nappes, however, are not needed to explain Adirondack geology and, in light of plate tectonic theory, is simply wrong.

My criticism is not aimed at the entire book; rather, it’s focused on the structural geology aspects of the book (e.g. the so-called Big Slide Thrust, the Basin Thrust, and the Great Range Nappe). The other parts of the book (e.g. descriptions of rocks, mineralogy, glaciation, etc.) are good for purposes of obtaining a basic understanding of High Peak geology.

 

[moonrock]

the authors reference very old material and the cross-sections appear to be many decades old, which suggests to me that they were draw pre-plate tectonics and simply copied into this book. For example, during the years that preceded plate tectonics, geologists attempted use the “nappe” theory, which was developed in the Alps to explain folding and thrusting of sedimentary rocks, to explain complicated structures in mountain chains around the world, no matter the lithology. Nappes, however, are not needed to explain Adirondack geology and, in light of plate tectonic theory, is simply wrong.

For example, the authors’ draw cross-sections showing thrust faults on Big Slide and a nappe on Marcy, Haystack and Basin. The cross-section of Big Slide (fig. 28) depicts a low-angle thrust fault with displacement from the NW to the SE. The cross-section of the Great Range (Fig. 48) depicts the so-called “Basin Thrust” and “Great Range Nappe” with displacement from SW to NE (the authors place a question mark next to the nappe in the drawing). The authors do not explain the cause of the perpendicular thrusting, perhaps because it is unexplainable in under plate tectonic theory. Additionally, according to Figure 2B, the authors suggest that the Great Range Nappe formed at some point between 5 and 60 million years ago.

I flipped through Jaffe last night to refresh my (faulty) memory of what was written there.
I had partially confused the Great Range depictions with that more vague sketch of Big Slide (from a distance, underlain by three low angle thrusts). But upon review I did not see anything else about Big Slide except descriptions including shearing and mylonization in outcrop below the summit cliff (pp 70s). I'll have to look again.

I assume by nappe you mean an overthrust sheet from a failed recumbent fold. In the NC Apps, there is no controversy about thrust sheets; they're stacked like pancakes. Of course an overthrust is not required to produce evidence of shearing; as you know, it can also happen within the limbs of a fold.

Did the authors actually STATE that the displacements were NW-SE and SW-NE, respectively ? Or was that merely implied by the orientations of the cross sections ?
If, say, sections with those orientations were made through a WEST-EAST overthrust, then they could produce similar-looking sections with similar apparent shear-plane angles (steeper than the actual dip).

I agree: never heard of major Paleozoic-Cenozoic compressional tectonism in the Adirondacks, even during the appalachian orogenies. Modern intraplate stress would not figure, at most contributing to minor seismicity along old faults. If they said it was Cenozoic, that's out of left field.

We know the Appalachian orogen formed in multiple collisional events, separated by periods of uplift/erosion.
And on a mid-80s RPI Field Geology trip (along the Northway !) State Geologist Yngvar Isachsen showed us Grenville outcrops and rock specimens that clearly had been folded 2-3 times and directions.

Jaffe reports that anorthosite was emplaced (entensionally) following the initial Grenville compressive event. Then it was partially metamorphosed to gneiss, presumably during a subsequent collisional event. Multiple collisions (including overthrusts) during Mid-late Proterozoic - even if interspersed with extensional events - would not be inconsistent with PT theory.

But I REALLY need to re-read more of Jaffe

 

[moonrock]

If, say, sections with those orientations were made through a WEST-EAST overthrust, then they could produce similar-looking sections with similar apparent shear-plane angles (steeper than the actual dip).

OOPS !: SHALLOWER than the actual dip !

 

[Willie]

I assume by nappe you mean an overthrust sheet from a failed recumbent fold.

No, when I use the word nappe, I mean nappe.

In the NC Apps, there is no controversy about thrust sheets; they're stacked like pancakes.

Pennsylvania and Maryland, too. I have drawn many balanced cross-section of thrust sheets, from the basement to the surface, starting from the Blue Ridge and continuing through the Great Valley province, the Valley and Ridge province, and the Appalachian Plateau. I’m aware of thrust sheets.

Of course an overthrust is not required to produce evidence of shearing; as you know, it can also happen within the limbs of a fold.

Of course, but Jaffe and Jaffe suggest, for example, the Great Range Nappe.

Did the authors actually STATE that the displacements were NW-SE and SW-NE, respectively ? Or was that merely implied by the orientations of the cross sections ?
If, say, sections with those orientations were made through a WEST-EAST overthrust, then they could produce similar-looking sections with similar apparent shear-plane angles (steeper than the actual dip).

I interpreted Fig. 28. The authors state that “the cone of Big Slide has ... been thrust to the E in several thin slices...” The authors also state that “the anorthosite of Table Top has been thrust to the East over the gabbroic anorthosite gneiss core of Big Slide....” Okay, I’ll give you 45 degrees.
In figure Fig. 48, the authors label the diagram with a NE on the left side and a SW on the right side. I assume that means northeast and southwest. Also, look at Plate 7A.

We know the Appalachian orogen formed in multiple collisional events, separated by periods of uplift/erosion.
And on a mid-80s RPI Field Geology trip (along the Northway !) State Geologist Yngvar Isachsen showed us Grenville outcrops and rock specimens that clearly had been folded 2-3 times and directions.

Do you mean the Appalachians formed in multiple collisional events? Those are the Taconic, Acadian, and Alleghanian orogenies.

Jaffe reports that anorthosite was emplaced (entensionally) following the initial Grenville compressive event. Then it was partially metamorphosed to gneiss, presumably during a subsequent collisional event. Multiple collisions (including overthrusts) during Mid-late Proterozoic - even if interspersed with extensional events - would not be inconsistent with PT theory.

Jaffe and Jaffe suggest the Great Range Nappe formed during the Phanerozoic. No tectonic event occurred during the Phanerozoic Eon that would explain nappes in the Adirondacks.

 

[moonrock]

No, when I use the word nappe, I mean nappe.

Of course, but Jaffe and Jaffe suggest, for example, the Great Range Nappe.

They seem to use the word more loosely, while acknowledging that its original Alpen context involved folding that is hard to prove in the Adirondacks. Personally, I'd just call it the Great Range Overthrust, and move on.
They described the shearing on Big Slide as like nappes "but on a smaller scale", which to me sounds like a mere analogy.

I interpreted Fig. 28. The authors state that “the cone of Big Slide has ... been thrust to the E in several thin slices...” The authors also state that “the anorthosite of Table Top has been thrust to the East over the gabbroic anorthosite gneiss core of Big Slide....” Okay, I’ll give you 45 degrees.
In figure Fig. 48, the authors label the diagram with a NE on the left side and a SW on the right side. I assume that means northeast and southwest. Also, look at Plate 7A.

I still maintain that NE and SW in Fig 48 merely indicate the SECTION orientation, along the Great Range itself. As you indicated, Plate 7A and text indicate an eastward thrust, and the text (pp 74-75) indicates eastward displacement at Big Slide as well.
However (now that I have the book in front of me), I see that Big Slide is east of the (inferred) Basin thrust fault. But I see no reason why the shearing at Big Slide couldn't be a manifestation of the same tectonic event. The idea of Tabletop anorthosite overthrusting Big Slide's "core" also seems consistent, as long as it's eastward, like they indicate.

Do you mean the Appalachians formed in multiple collisional events? Those are the Taconic, Acadian, and Alleghanian orogenies.

That's exactly what I meant. As you know "Appalachian" was formerly used interchangably with "Alleghanian", which just caused confusion.

Jaffe and Jaffe suggest the Great Range Nappe formed during the Phanerozoic. No tectonic event occurred during the Phanerozoic Eon that would explain nappes in the Adirondacks.

I don't read that from Jaffe and Jaffe. In Fig 2, Event 8 is the overthrust, Event 9 is late Precambrian (late Proterozoic) erosion and Event 10 is (Phanerozoic) sedimentation, extentional tectonics and later uplift. The Event 10 cross section shows the thrust fault in relation to the present day Great Range profile, but I read the arrows along the fault to indicate only the Precambrian offset, not more recent movement. The "60-5 MY" note refers only to erosion and uplift.

And now I have to bone up on geology of the Smokies, where I'll be backpacking this weekend ! Bit too early for foliage down here, though, so I'll have to try and get back there in early November.

 

[Willie]

And now I have to bone up on geology of the Smokies, where I'll be backpacking this weekend ! Bit too early for foliage down here, though, so I'll have to try and get back there in early November.

Have a good weekend, moonrock. I've enjoyed this thread, but I think I've had enough geology for one week. I hope the readers enjoyed the thread, too.

 

[WinterWarlock]

Have a good weekend, moonrock. I've enjoyed this thread, but I think I've had enough geology for one week. I hope the readers enjoyed the thread, too.

Well, this is one I want to print and re-read, because, yes, it is wicked awesome. Thanks to all our rock-hounds out there...