Strangers in a Strange Land: I'm feeling detached...is that normal? Looking at Death Valley's faults

Strangers in a strange land is my latest blog series, dealing with the geology of one of California's most unique environments, the Basin and Range Province and the Mojave Desert. I visited both in the last few months, and I have been inviting the readers of this blog to learn the basic principles of geology through the eyes of my community college students. The last few posts have concerned the nature of faulting. Sixty years ago, as far as anyone was concerned, there were essentially four kinds of faults, left and right lateral, normal and reverse. There was a variation of a reverse fault, one with a low angle plane of movement called a thrust. Thrusts result from compressional forces, and are commonly seen the Appalachians, the Alps and the Himalayas, all of which are produced at convergent plate boundaries.

Thrust faults generally push older rocks over younger rocks, and a number of Mesozoic-aged thrust faults are found in the mountains of Death Valley. Levi Noble, one of the first geologists to map Death Valley, found many of them. As noted in the last post, he also found a series of unusual "thrusts" that were distinguished by having younger rock on top of older sequences. A nearly horizontal young rock-on-old rock fault can be seen crossing the middle of the photo above.

Another geologist, H. D. Curry, who worked the region in the late 1930s noticed some very strange structures in the Black Mountains on the east side of Death Valley. He called the huge dome-like mountains turtleback faults. That's the Copper Canyon turtleback in the picture at the top of this post. The turtlebacks (there are three obvious ones in the Black Mountains, and a number of somewhat cryptic ones scattered through other mountain ranges across the region) are made of ancient Proterozoic-aged metamorphic rock, and their surfaces are fault planes. When rocks are found above the fault planes, they are invariably younger. The enigmatic young-on-old 'thrusts' and turtlebacks seemed to be closely related. 

Other geologists started to question the idea that these faults were compressional. Even though Noble was initially sure that the faults were thrusts, over the years he began to accept that his first perceptions might possibly be invalid. The 'thrusts" were associated with numerous normal faults, and it was known by then that the Basin and Range province was produced by extensional forces in middle and late Cenozoic time. Finding the odd exception to the rule is not unusual, but by the 1970s, geologists were finding these enigmatic fault systems all across the Basin and Range province. As they realized the extensional nature of these faults, they knew they had discovered a counterpart to the compressional thrust faults. These low-angle normal faults were soon being called detachment faults, and the dome-like systems of ancient metamorphic rocks were termed metamorphic core complexes.

The picture above shows the exposure of the Badwater Turtleback where it plunges into the subsurface in the canyon above Natural Bridge in the central part of Death Valley. It is an astounding feeling to lay one's hands on the surface between young volcanic rocks and the ancient rock from deep in the continental crust. Recognition that these structures were an entirely new kind of fault zone did not provide answers to all of the questions raised by their discovery. The nature of detachments and metamorphic core complexes is still a field of ongoing research and controversy.

The exact nature of the turtlebacks in Death Valley is not a settled issue. Are the turtlebacks like the other core complexes, or are they a more unique local phenomena involving steeper normal faults? You can check out this report by Miller and Pavlis for a review of the issues involved, or pick up the excellent guide to the geology of Death Valley by Miller and Wright.

Diagram of a 'typical' detachment fault from Spencer and Reynolds, 1989, Middle Tertiary Tectonics of Arizona
and Adjacent Areas, Arizona Geological Society Digest 17, p. 539-574.

When one stands on the top of the Black Mountains and looks across the incredible Death Valley graben to the Panamint Mountains on the far side, one can barely comprehend the implication of the discovery of detachment faulting and metamorphic core complexes. The Panamints were once on top of the Black Mountains, or nearly so. And only a short time ago in the geologic sense, just a few million years before the present.

Death Valley Days: The Second Day - We Dodge a Bullet

The biggest, baddest storms still find heartbreak in the barren jagged peaks of the Basin and Range Province, and especially Death Valley. Last night we awaited the onslaught of winds and a 100% chance of rain, but the mountain barriers of the Panamint and Cottonwood Mountains absorbed most of the storm's moisture, and we had just a spritz of rain in our camp at Stovepipe. Furnace Creek got a bit more, 0.14 inches, but all in all we dodged a very big bullet, aside from some inconveniently heavy winds that destroyed one of the student's tents overnight. Morning arrived with a wonderful moonset over the Cottonwoods.

As we hit the road, we realized the storm really was past, as sunshine lit up the Panamint Mountains on the east side of Death Valley. If you are noticing the snow, it's not an unusual occurrence in the park, considering the Panamints top out at over 11,000 feet in elevation. The valley floor in the foreground of the picture is below sea level. Death Valley has some of the highest relief of any place in the United States (the difference between the highest and lowest points). The rugged slopes are composed of Paleozoic and Proterozoic sedimentary rocks, mostly carbonates, with a healthy mix of intrusive granites and older metamorphic rocks

It would be inaccurate to say that the flower blooms have started in earnest, but we found a few here and there, and they added a nice splash of color to the desert.

We made stops at some fault exposures east of the park near Shoshone (the 'Charlie Brown outcrop'), and headed over the Black Mountains into the southern reaches of the Death Valley graben. We checked out the Amargosa Chaos exposures at Exclamation Point, the Pahrump Group near Ashford Mills, the fault scarps, offset cinder cone and wave cut terraces near Shoreline Butte, and I took...no pictures! I was busy...eh...teaching and stuff.

We made a stop at Mormon Point and took in a couple of Death Valley's enigmatic features, the turtleback faults. These are massive dome-shaped fault surfaces whose origins are debated, but they are composed of deep crustal rocks that have been metamorphosed into gneiss and schist. The rugged exposures reveal collisions of land masses around 1.7 billion years ago, and are the oldest rocks in the park.

Mudflows and flashfloods have scattered large boulders of the ancient rock across the valley floor and are a treat to explore. One huge boulder (below) contained huge crystals of black hornblende 3-4 inches long.

We of course had to do the touristy thing and visited the lowest point in the western hemisphere at Badwater. The actual lowest point is out on the salt flats a mile or two away, but what's a foot or two between friends? People were walking out onto the freshest salt exposures.

The afternoon sun was sinking fast, but we had one more thing we wanted to do...

It's also a touristy thing too, but we wanted to see the flaming colors of the sunset on the Artists Palette. We drove up Artist Drive as fast as we were able, and we didn't quite get to the Palette, but we did stop and get a nice panorama of the other volcanic rocks at the north end of the Black Mountains.

Conditions are a lot calmer tonight, so hopefully no rain or heavy winds. Two nights of that and I'll have a tired class, and we have lots of great stuff to do tomorrow!