Archive | geology RSS for this section

Which Question to Answer?

There is a survey circulating within AGU asking for suggestions for the most important questions or challenges facing geoscience.  This is kind of a regular thing (NSF gathers meetings around similar questions), but GG wonders if this is a productive exercise.

First, most important to whom? If we are talking the public at large, then you are almost certainly talking about geohazards from climate change to hurricanes to tsunamis to earthquakes to landslides.  Better predicting or mitigating these hazards are probably the things that society most wants.  Close behind are some more traditional concerns like locating mineral deposits.

If these are the class of most important problems we should pursue, then it might well make sense to encourage scientists to focus on these. And for what it is worth, there is a lot of effort directed toward these ends.

But are these the most important questions at a more abstract level?  A lot of the work on hazards isn’t addressing more general principles, it is applying specialized knowledge to particular situations. The basic physics of most landslides has been well known for a long time.  The conditions that produce tsunamis are pretty well understood as well. So maybe there are things we really have no solid grasp of that might be worth getting at.

When you shift to this style of questioning, things necessarily break apart by discipline or study topic.  Who is to say that determining the presence or absence of century-scale atmospheric oscillations is more or less important than resolving the physical state and composition of material near the core-mantle boundary? Is learning when the Andes rose up more important than when the Tibetan Plateau went up? Or the Rocky Mountains?  GG is at a loss; he makes his own calls, of course, but of the numerous issues in earth science that remain unclear, how would you choose a subset that really are the  “most important”? And having confronted that ambiguity, what do you gain from answering the question?

Keeping in mind that abstract or non-directed science is funded because it produces unexpected insights that can be of great but unanticipated utility, how do you pick winners? GG is of a mind that trying to get some community to settle on a set of questions is probably not the most effective way of getting really juicy new knowledge.  Having everybody pile on, say, calculating dynamic topography would probably produce far more chaos than insight while starving other experiments that might be just as valuable. And yet Congress might bridle at giving out money without some kind of master goal (perhaps this is why NASA has been rather successful in its probe initiatives: saying we are going to look for life on Mars or on Europa or Titan sounds sexy even if the probes also get to do a lot of other, less sexy, things).

If we sidestep Congress wanting some clear mileposts, what might be the most effective way to get somewhere?  Probably a good way is in fact how many NSF programs work at present: on a case-by-case basis, proposal by proposal. If some proposal comes in that has nothing to do with the community’s wish list of problems but is well thought out and makes a good case that its problem is significant, why should it be rejected in favor of some crank-turning me-too middling thing that is pointed at that wish list? GG would say it shouldn’t.  Committees are notorious for compromised and pasteurized repackaging of some advocates’ favorites (the old saw of a camel being a horse made by a committee comes to mind). So maybe we should bypass the group-think in making target lists and just try to follow the problems that really engage us. Some of us will choose well, which is the best we can hope for.

So, for instance, GG phrases his interests in the western U.S. as stating that this orogen is the largest non-collisional orogen on Earth.  It is arguably the most poorly understood feature of its size. Does this make studies of this more important than, say, untangling the slip history of major faults in Southern California? Not necessarily–but it is better than saying that this research addresses point 1(b) section 4 of some summary document.

Slab Tectonics?

So Howard Lee over at Ars Technica took a swing at how our understanding of global tectonics has been changing over the past 40 or 50 years and wrote a lengthy article on it.   It is full of quotes and assertions that really don’t hang together very well, making a certain geophysicist kind of grumpy.  It doesn’t seem that any of the scientists quoted were really saying anything wrong, but the assembly in the article, which doesn’t seem to recognize the discrepancies nor fully master the techniques being used, can lead to a sense of “WTF?”

Read More…

The 85 Ma Trainwreck: Mojave Mojo

One of the strange things about the 85 Ma trainwreck (which we have discussed components of here and here and here and here) is the central place that the Mojave Desert seems to play.  For the most part we might look to the 2003 paper by Jason Saleeby for elevating the Mojave to star status, tapping it as the entry point of an oceanic plateau (specifically the hypothesized conjugate to the Shatsky Rise now in the northwest Pacific) that he proposed as the main actor in driving the Laramide Orogeny.

SaleebyNarrowSlab

Saleeby (2003) vision of the key elements in the Laramide orogeny, with the shallowly subducting part of the slab corresponding to the highly disrupted geology of the Mojave Desert with the slab then proceeding to the north-northeast into Wyoming.

So what is really peculiar about the Mojave c. 75-85 Ma?  After working through parts of the literature and getting some education at GSA this past week, GG would suggest these are possible candidates:

  • Emplacement of subducted sediments against the middle/lower crust after experiencing greenschist grade metamorphism.
  • Extension that permitted this juxtaposition to rise to the surface, either shortly after (as in the Salinian Schist) or much later (as the Orocopia Schist)
  • An extraordinarily unusual pattern of late Cretaceous magmatism (83-72 Ma) including metaluminous plutons that lack the temporal eastward shift seen elsewhere

There are two pertinent questions for these: (1) how localized are these things really, and (2) do these point towards an impacting plateau and a broader Laramide implication?

Read More…

Nature, Nurture and the Anthropocene

Kind of a few thoughts bouncing about the internet about land and how it should managed and how that relates to the challenged term of the Anthropocene epoch.  Collected, they indicate how confused we are about nature and humanity’s impacts on nature. Let’s work our way backwards through time in pondering this.

First up, a story about a transfer of management from the federal government to a Native American tribe. High Country News covers this transfer in Oregon, where the Cow Creek band of the Umpqua Tribe took over control of land just in time for a considerable fraction to burn in a forest fire. They had yet to implement a management plan, but they know the path they want to follow:

[Michael] Rondeau explained that the management of Cow Creek Band of Umpqua Tribe of Indians reservation lands would reflect Indigenous values: an example separate from either industry or conservation groups. “We don’t believe in locking up the forests and allowing them to ‘remain natural,’ because it never was,’” Rondeau said. “For thousands of years, our ancestors used fire as a tool of keeping underbrush down, so that the vegetation remains healthy and productive.”

As the article points out, this places the tribe at odds with many environmentalists, a conflict that actually goes pretty far back–though maybe not quite as far as some would have it:

“The conservation movement began as a way for settlers to justify the seizure of Indigenous lands under the pretext that Native peoples didn’t know how to manage them,” says Shawn Fleek, Northern Arapaho, who is director of narrative strategy for OPAL Environmental Justice Oregon. “If modern conservation groups don’t begin their analysis in this history and struggle to address these harms, it becomes more likely they will repeat them.”

This is an interesting take on frontier justice, for while conservationists were indeed complicit in accepting the status quo that followed removal of Native peoples, given the opposition from locals to withdrawal of lands from private use, it seems a reach to imagine gold miners triggering armed conflict under the banner of conservation.

At the heart of this dispute is the question of what exactly do we mean by “nature”? Read More…

The 85 Ma Trainwreck: Deformation

In previous entries, we’ve examined the emplacement of oceanic/forearc affinity (POR) schists and the igneous activity from a similar timeframe. Here we will consider what was deforming when and how. There are four main pieces to this puzzle: the termination of ongoing thrusting of the Sevier, Eastern Sierra belts, and Maria belts, the emergence of thrusting in southeastern Arizona and New Mexico, the appearance of extensional faulting, and the beginnings of Laramide shortening in the Colorado Plateau and Southern Rockies.

The outline version is that thrusting in the north-south trending Sevier foreland fold-and-thrust belt shutdown by about 90 Ma in southern Nevada but continued for another 30+ million years farther north. Northwest-trending retroarc thrusts probably continued to be active in southeastern California until 80 Ma and possibly 75 Ma. Rock uplift and extensional shear zones between ~75 and ~65 Ma in several localities may reflect extension of the crust in the Sevier hinterland, but some kind of intra-continental convection is hard to rule out (e.g., Wernicke and Getty, 1997). Closer to the coast, right-lateral strike-slip deformation in the dying Sierran arc reflects some obliquity to convergence at the plate margin. As time passes, Laramide-style basement-cored uplifts begin to emerge, perhaps including structures very close to the Sevier thrust front in the Kingman arch and associated uplift of the southwestern Colorado Plateau. Thrusting appears to have accompanied magmatism in expanding eastward across southern Arizona.

There is a lot here and yet GG is confident he’s missed some important papers–feel free to point some out in the comments.

Read More…

Nevada oceanfront property?

All the attention on the Ridgecrest earthquakes has returned the Eastern California Shear Zone and its rather more obscure relative, the Walker Lane. Among the news articles out there is a pointer back to an article on the notion that the plate boundary will shift in to these fault systems sometime in the geologic future. This then results in western Nevada eventually facing an ocean–or, more extravagantly, Salt Lake City as a coastal town. So let’s clarify our terms, understand when and why these faults have emerged, and then set out to consider what it takes to move a chunk of continent onto an oceanic plate.

First off, there are two named parts to any potential new plate boundary (well, actually, they are already taking up about a quarter of the plate motion): the Eastern California Shear Zone and the Walker Lane. The Eastern California Shear Zone is a well-defined feature in the Mojave Desert south of the Garlock fault and named recently in 1990 by Dokka and Travis. The Walker Lane, in contrast, has had multiple incarnations: it was originally defined by Locke et al. (1940) on the basis of more confused topography along a swath of ground running from Las Vegas through the Walker Lake area to north of Reno.  At times it was thought to be as old as Jurassic. John Stewart redefined it in a 1988 book chapter, and subsequent workers have now generally drawn the Walker Lane as kind of hugging the east side of the Sierra out about 150-200 km into the Basin and Range, removing the Las Vegas Valley Shear Zone from being within the Walker Lane and, generally, dating the strike-slip motion within the last 9 million years, with ~4 Ma seemingly a good guess (Andrew and Walker, 2009).

The northern edge is more of a mystery and the names for it more inconsistent.  The Central Nevada Seismic Zone takes off from the Walker Lane to the northeast as a more dominantly normal fault system. Strike-slip faulting cuts into the Sierra on a Northern California Fault Zone (Wesnousky, 2005) and might connect to the coast (e.g., Unruh et al., 2003) or towards the Cascades arc (e.g., Waldien et al., 2019), and some have carried deformation northward through central and eastern Oregon and Washington (e.g., Pezzopane and Weldon, 1993). If we really are making a new transform boundary, its northern extent sure isn’t obvious.

So why is strike-slip faulting a relatively late arrival in this area where deformation extends back 15 or 25 million years?

Read More…

The 85 Ma Trainwreck: Igneous Activity

This follows an overview and a discussion of Pelona/Orocopia/Rand schists.

Hike the John Muir trail and you find yourself constantly in landscapes formed from the c. 85 Ma intrusions that marked the end of one of the longest stretches of volcanic arc activity in the past several hundred million years. For a long time, this was taken as simply the end of an era; the absence of activity that followed was the big message of something strange going on. But increasingly there are signs that this final pulse was itself out of the ordinary and may well have carried the seeds of its own demise.

Looking beyond the Sierra, chaos quickly reigns. While some papers and textbooks describe an orderly eastward shift of the volcanic arc, the rocks on the ground present a more confusing account. While the arc did shift to the east after 85 Ma in southern Arizona and eventually part of New Mexico, no such orderly shift is evident farther north. Magmatic activity actually approached the trench in the Mojave Desert just before volcanoes popped up in Colorado, skipping much intervening terrain. The few plutonic rocks in Nevada and Utah look like melts derived from the crust, not typical arc rocks.

IgneousU-PbAges

Uranium-lead dates from NAVDAT database of igneous rocks plus additional reports from Barth et al. (2004), Cecil et al. (2012), Chapman et al. (2018), Gonzales (2017), Gonzales et al. (2015), Leveille and Steiger (2016), Nadin et al. (2016, summarizing older work), Needy et al. (2009), and Vikre et al. (2014)

This crazy quilt of magmatism then takes a toll on geodynamic models. The shutdown of the Sierra has been taken as a sign of the slab flattening under North America–but the clearest evidence of the slab flattening is under the Mojave, where igneous activity continued well after flat slab deformation began nearby. Models that predict that slab flattening should shut off igneous activity have trouble with some of the intrusions in the Colorado Mineral Belt–and their absence to the north and south. And if impinging oceanic plateaus are responsible, just how does that timing work out?

We can try to tear this apart in some ways.  Here let’s consider these questions:

  • Why was there a flare up of activity in the Sierran arc near 85 Ma?
  • What could have shut down the Sierran arc?
  • What do we know about the relationship of magmatism to schist emplacement in the Mojave region?
  • Are the two-mica (peraluminous) granites really just crustal melts?
  • What can we say about the Colorado igneous activity?

Read More…