Western North America microcontinent bigger than previously thought?

[Travis]

I grew up with the idea that the area west of the San Joaquin Fault would eventually split off and become a micro continent.

But lately I've stumbled upon the idea that this might not be so. That the rift might actually be further east in an area known as Walkers Lane of which Death Valley serves as part.

I find this an intriguing prospect as the land area that would eventually rift away from North America will be much larger.

How formalized is this hypothesis in the geologic community? Would this account for volcanism in the Sierra Nevada such as that around Mono Lake including the giant volcano at Long Valley?

 

[Dinwar]

You're getting into one HELL of an argument!

The issue is that there's a lot going on. There's the San Andreas, Garlock, and San Joaquin fault systems, which are basically pushing the Coastal Ranges north and west. They're also pushing the Los Angeles Basin and areas south of that to the north and west. But these faults (due to complex geometry I don't pretend to understand) are twisting the Transverse Ranges, making them go east/west rather than north/south like all the other mountain ranges in the region.

Then you have the rift ini the eastern Mojave Desert (basically running right through Las Vegas). Depending on who you talk to this is either a failed rift, or one that just happens to be quiet right now. That's where a lot of the argument is going on: what, exactly, is goinig on beneith Las Vegas?

I don't pretend to be a geophysicist or an expert in rifting or tectonic theory, but I have looked into this in some depth due to the number of fossil surveys I've had to do out there. Here's my opinion: There are two separate peices (or more) splitting off the North American continent. There's the stuff west of the San Andreas, and then there's the stuff between the San Andreas and roughly Las Vegas. The first is moving northwest, towards Alaska, while the second is moving essentially due west (and the east side of LV is moving east). The continent is ripping itself apart, and will fragment into a number of pieces, along that rift. We're already seeing some of this--the Salton Sea (well, the ancient one--the modern one is due to a dam failure) and Gulf of California are examples of a split growing between the microcontinent and the rest of North America.

If you want to know the details I suggest geting a copy of Dibblee's 1965 book. It's not the latest and greatest, but the basis FOR the latest and greatest. It's a good place to start.

 

[CORed]

IANAG, but my understanding is that the whole Basin and Range Province, which encompasses a pretty good chunk of Western US, is being stretched. There is also the Rio Grande Rift, which runs from southern Colorado through most of New Mexico. The western part of North America has very complex geology. It's kind of hard to tell where it might eventually come apart.

 

[casebro]

What, no faults along the Mississippi River?

 

[Dinwar]

What, no faults along the Mississippi River?

No big, active ones. The Mississippi is a failed arm of a rift system, as I recall (a lot of big rivers are, as they provide a nice low spot for lots of water to run to), but it's a few tens of millions of years old and more or less quiet now. The issue on that side of the continent is the large chunks of continent well below the surface that occasionally shift a bit. There ARE faults, but not on the scale you see in the Great Basin and certainly not active or recently active rifting.

 

[Travis]

You're getting into one HELL of an argument!

The issue is that there's a lot going on. There's the San Andreas, Garlock, and San Joaquin fault systems, which are basically pushing the Coastal Ranges north and west. They're also pushing the Los Angeles Basin and areas south of that to the north and west. But these faults (due to complex geometry I don't pretend to understand) are twisting the Transverse Ranges, making them go east/west rather than north/south like all the other mountain ranges in the region.

Then you have the rift ini the eastern Mojave Desert (basically running right through Las Vegas). Depending on who you talk to this is either a failed rift, or one that just happens to be quiet right now. That's where a lot of the argument is going on: what, exactly, is goinig on beneith Las Vegas?

I don't pretend to be a geophysicist or an expert in rifting or tectonic theory, but I have looked into this in some depth due to the number of fossil surveys I've had to do out there. Here's my opinion: There are two separate peices (or more) splitting off the North American continent. There's the stuff west of the San Andreas, and then there's the stuff between the San Andreas and roughly Las Vegas. The first is moving northwest, towards Alaska, while the second is moving essentially due west (and the east side of LV is moving east). The continent is ripping itself apart, and will fragment into a number of pieces, along that rift. We're already seeing some of this--the Salton Sea (well, the ancient one--the modern one is due to a dam failure) and Gulf of California are examples of a split growing between the microcontinent and the rest of North America.

If you want to know the details I suggest geting a copy of Dibblee's 1965 book. It's not the latest and greatest, but the basis FOR the latest and greatest. It's a good place to start.

I might have to put in a request for that book. What is the title?

 

[esquel]

One might quibble about just how active the New Madrid fault zone is. It jiggles frequently, but usually in the under 2 range on the Richter Scale. Every few years, it has a swarm of larger than normal quakelets, and geologists all eyeball the area uneasily, just in case it's setting up for a repeat of the big series around 1812. The main worry is that no one on that side of the Rockies is at all prepared for large, shake-down-your-house quakes, which the New Madrid could conceivably deliver.

e.t.a. The New Madrid Fault Zone is a failed rift zone complex of faults centered around the 'bootheel' in Missouri, running very roughly through the valley of the Mississippi. Millions of years ago, North America tried to split itself apart at that point, and failed.

 

[This is The End]

What, no faults along the Mississippi River?

No big, active ones.

Beeeeeep.

The New Madrid fault is big. And it definitely is active. It is right smack dab along the Mississippi and it affects 7 states.

http://en.wikipedia.org/wiki/New_Madrid_Seismic_Zone

the New Madrid Fault Line, is a major seismic zone and a prolific source of intraplate earthquakes (earthquakes within a tectonic plate)

<snip>

The New Madrid fault system was responsible for the 1811–1812 New Madrid earthquakes and may have the potential to produce large earthquakes in the future. Since 1812 frequent smaller earthquakes were recorded in the area.

<snip>


The zone had four of the largest North American earthquakes in recorded history, with moment magnitudes estimated to be as large as 8.0,

 

[AlBell]

What, no faults along the Mississippi River?

New Madrid in 1811-1812. Four distinct episodes, last one purported to involve thrusting.

 

[This is The End]

New Madrid in 1811-1812. Four distinct episodes, last one purported to involve thrusting.

That was just the very big one though. That's relatively frequent in geological time scales....

I've personally been in at least 3 or 4 in the area over 4.0. And several over 3.0.

I consider that "active".

 

[Nihilianth]

No big, active ones. The Mississippi is a failed arm of a rift system, as I recall (a lot of big rivers are, as they provide a nice low spot for lots of water to run to), but it's a few tens of millions of years old and more or less quiet now. The issue on that side of the continent is the large chunks of continent well below the surface that occasionally shift a bit. There ARE faults, but not on the scale you see in the Great Basin and certainly not active or recently active rifting.

I was wondering about that, because there was a fairly sizeable earthquake last year centered in Virginia. I felt it all the way up in North-Central Pennsylvania, and learned that the epicenter was in a fault line that goes through Virginia into WV. I was like: "Fault line? In VIRGINIA? " Guess you learn something new everyday....

 

[Travis]

If that fault line failed I wonder what it would be like if this Western North America rift system fails at some point. Would there be a giant great plain west of the Rockies? A much extended Gulf of California reaching almost all the way to Canada?

 

[Dinwar]

I might have to put in a request for that book. What is the title?

"Areal Geology of the Western Mojave Desert, California." It's dense--Dibblee didn't make ANY attempt to make the topic fun or engaging. But there's a huge amount of information in it, including discussions of the horst-and-graben development in the western Mojave (which is identical to that going on under Las Vegas).

The New Madrid fault is big. And it definitely is active. It is right smack dab along the Mississippi and it affects 7 states.

Fair enough. It's not nearly as active as the ones in California, though--in 24 years of living in Ohio it slipped a bit once, compared to four earthquakes in the last two years in California. And 3.0s and 4.0s don't count out here--until it gets to 4.5 or so people don't even notice. I wasn't using the word in the technical sense. You are right, of course, that on geologic scales it certainly is active.

If that fault line failed I wonder what it would be like if this Western North America rift system fails at some point. Would there be a giant great plain west of the Rockies? A much extended Gulf of California reaching almost all the way to Canada?

Probably not. What we'd probably end up with is a series of island arcs, with ocean of varying depth between them. As the crust is pulled apart out west it's twisting the rocks along a(n essentially) north/south axis. If you look at Google Earth you see a bunch of mountain ranges in southern California, Nevada, Utah, etc. running north-south. It's kind of hard for me to explain without drawing it, but essentially when the crust pulls apart it doesn't do so smoothly, but rather at weap points that became major faults. The rock slips at those faults, dipping downard on the side away from the axis of the rift (and consequently rising relative to everything else on the side close to the axis--this is how the Spring Mountains outside of Vegas formed). This creates that very linear series of mountains and deep (over 3 miles in some cases) valleys you see out West. As those valleys expand eventually they'll reach the ocean ("eventually" being on geologic timescales). Then they fill up with water, making new gulfs and then becoming part of the Pacific. That's what happening (essentially) in the Gulf of California, for example.

 

[Travis]

I've read some speculation that the Atlantic will stop widening and that subduction will open up again on the eastern North American margin. If that happens and the western margin become passive what happens with this microplate?

 

[CORed]

I've read some speculation that the Atlantic will stop widening and that subduction will open up again on the eastern North American margin. If that happens and the western margin become passive what happens with this microplate?

If subduction starts up on the Atlantic side, and the North American reverses course to move east, there will have to be rifting somewhere on the west side. It's hard to say whether the continental rifting taking place in in western North America would continue, splitting part of North America off, or whether new rifts would open in the Pacific.

 

[Dinwar]

If subduction starts up on the Atlantic side, and the North American reverses course to move east, there will have to be rifting somewhere on the west side.

I'm not so sure. On the one hand, tectonics isn't as simple as that--just becuase a subduction zone opens up on one side doesn't mean that subduction won't continue on the other. Of course, on the other hand there's this huge rift system that may still be active, and the continent may rip apart anyway. I certainly don't know enough to guess what would happen.

 

[Foster Zygote]

Obviously, it's Lisa's fault.

 

[Soapy Sam]

One difficulty in discussing plate tectonic futures is that while the physical processes (crumpling, tearing and buckling) are simple in theory, the timescale of operation is radically unlike any process we normally experience.

A fault movement that shifts one side of a fault a metre to the left in two minutes seems straightforward- one step in an ongoing movement.
But if, over the entire span of human history, that fault moved six times, three to the left, two to the right and once up and down, what actually is the stress regime? Is it constant, or are the forces involved themselves changing over a few millennia?

If we watch ice form on a windblown lake, over a period of days, (or skin on a simmering crock of soup over a few minutes) it is clear that the stress fields involved are in fact constantly changing. Inertia. Hysteresis. The strain patterns (deformation) reflect the changes, but time-lagged, because stress is already locked up at different points and that must released before the constantly changing forces can affect the newly deformed rocks the way they "want to now". The inertial lag in objects the size of tectonic plates is enormous.
Until we can measure relict stresses across immense areas , at surface and at depth, at pressures and temperatures we don't yet know, in rocks with stengths we don't know, we will not be able to say where any area will move next.

All of that will surely happen, if our civilisation survives long enough- but I'm betting on several decades at least before we know nearly enough about pre-existing stress and tectonic dynamics to make precise predictions on any scale of use to us.

We will know where America is going when it gets there. Of course, by then, "there" won't be there any more and likely neither will we.

One measurement at a time. We'll get there.

 

[jj]

Well, North America, except for the Canadian Shield which is partially excepted, is accreted. There are faults everywhere, even in Northern Ohio, and lots of places you wouldn't expect. The Rockies and the Basin and Range are a collection of more faults than anyone knows about. Looking at some of the large mountains you can see faulting along a half-dozen planes. The Southeast has a variety of faults. The west coast has so many faults it's worse than the Rockies.

Then it has Long Valley and Yellowstone, and maybe something going on in Cascadia.

So North America is no Australia, with one nice solid bit of rock under it. It's a collection continent, made up of a whole lot of former continents, with both rifts and subduction faults in unexpected places. I have to say that perhaps that's why it was so uninhabited until after the last ice age. It doesn't go kaboom very often,but when it does...

 

[Soapy Sam]

Sounds a lot like Americans, now you mention it.