A late AGU response: Making the Laramide

One of GG’s colleagues reported that there was discussion of his talk well after he had left the room (having to leave to collect a spouse from the airport). On the very off chance anybody who cares finds this, here is something of a response to what GG was told was being discussed. A lot of this should be in a 2011 paper by GG and others in Geosphere.

First a note on the talk, which was to argue that the presence of foreland basement-cored mountain ranges (like the southern Rockies) need not require a “flat slab,” which is a slab that travels horizontally along the base of the continental lithosphere. “Foreland basement cored mountain ranges” is shorthand for mountains created well inboard of the more typical mountains formed by shortening of the continental crust near a subduction zone; these mountains typically arise in places where the thickness of sedimentary rocks is thin and so when the thrust faults develop, they are relatively steep and bring the crystalline crust below up, where it is exposed by erosion.  The main mechanical explanation for such mountains was that the oceanic slab was scraping along against the bottom of the continent.  A simple balance of stresses requires that the horizontal stresses increase the farther inland the slab goes, so the stresses are highest where the slab finally turns downward. However, this mechanism will shear the continental lithosphere; one numerical model of this process in 1988 removed all the continental lithosphere in the western U.S., a prediction not supported by observation. In the talk, GG pointed out an alternative, outlined in that 2011 paper, and noted that the physical mens of generating mountains like the southern Rockies is not well established, so the presence of such mountains need not reflect the presence of a flat slab.

The main point was, to use the observation of the mountains to infer what the slab was doing requires an understanding of how the mountains were made; a simple reliance on a single modern example is inadequate. (Not mentioned in the talk is that there are several profound differences between the Laramide and the modern Sierras Pampeanas, the three most relevant being the less deformed Colorado Plateau, which lacks an analog in South America, the Pierre Shale basin in northern Colorado and SE Wyoming, while the pre-uplift Sierras Pampeanas had very little sediment accumulation, and the obliquity of the subduction in North American vs. near normal subduction in South America.  That the South American margin today seems to erode the upper plate while that in North American seemed to accumulate material could also be relevant).

<Phew>

As GG understands it, the question was related to how this could possibly be consistent with the magmatic history of the region.  Apparently part of the discussion centered on the South American example in the Sierras Pampeanas, which do overlie a flat slab.  GG was not commenting on this orogen, but does note that the physical means of making these mountains is actually poorly tested (some years ago, Brian Isacks, who knows a thing or two about that orogen, argued that the mechanism was actually increased stress from the trench and not a basal traction). But what about the Laramide?

OK, a lot of this is in that 2011 paper but maybe not quite all.  There are three main features to keep in mind: the magmatic arc in California basically died by 80 Ma (and probably a bit earlier), during the Laramide there was very limited magmatism, mainly some strange igneous rocks in Nevada and NW Arizona (peraluminous, or two-mica, granites) and magmas associated with the Colorado Mineral belt, pretty limited plutons more or less in a NE trending belt across Colorado, and finally the emergence of very vigorous, dominantly silicic magmatism across much of the western U.S. starting about 35 Ma. Let’s take these one by one.

Shutdown of the arc: In South America today, this nearly perfectly coincides with flat slabs. But subduction in South America is nearly perfectly trench-normal.  The western U.S. about 80 Ma looks a lot different. Plate reconstructions show significant northward motion of the subducting plate, an inference consistent with structures found within the dying volcanic arc. This might mean that things were different in the western U.S.  Second, the only place we know we had extremely shallow (possibly flat-slab) subduction was in the Mojave Desert, where the lowest crust and upper continental mantle were removed and replaced with oceanic affinity material. A bit to the north, though, we find the whole of the magmatic arc down into the underlying mantle was still present at 12 Ma, long after the Laramide orogeny and its supposed episode of flat-lab subduction.  How this could have survived being juxtaposed with a subducting slab right at its base for some 30-50 million years is a bit of a mystery: if there was sufficient coupling to create the stresses needed to make the southern Rockies, this material should have been removed; if there was not sufficient coupling, then the problem is how to make the southern Rockies at all. in the 2011 paper, we suggested that disrupting the flow of fresh asthenosphere into the arc could prevent the creation of new magmas.

Syn-Laramide magmatism.  If there was a flat slab against the base of the continental lithosphere, then there was no way to make melts out of the mantle: everything was being cooled from below as cold ocean floor was placed against the continent. Yet magmas in the Iretiba pluton as well as many of the igneous bodies in the Colorado Mineral Belt require some melts to emerge from the mantle.  Admittedly these are small volumes, but they are not zero. This suggests there was still some asthenosphere present in the region.  Unfortunately the crustal contamination on all these magmas is so severe that getting a geochemical fingerprint of the source has proven impossible so far, but right now the only mechanism to generate these melts in a flat slab system would be from frictional melts, which is probably a very hard mechanism to get working at those depths with the materials that were present (and, again, a lot of friction means a lot of lithosphere is being removed).

Post-Laramide magmatism.  This has been suggested to basically reflect storage of meltable material during the Laramide that, once exposed to a normal thermal regime, melts rather dramatically and produces the massive eruptions seen in the San Juan Mountains of Colorado and across much of western Utah and Nevada. There seem to be two main ingredients for this to occur: a subnormal geothermal during the Laramide and the addition of water (or other fluids) to the mantle under the continent either during or after the Laramide. Do either of these depend on a flat slab? Well, no. The hot areas of a back arc are driven by the asthenospheric counterflow [there’s a mouthful–that is the tendency of the flowing mantle beneath the lithosphere to be drawn toward the trench as the slab subducts–when it reaches the slab, it turns and flows downward with the slab]. If the counterflow is shutdown for any reason, the geothermal in the backarc drops down.

Now it could be that the questioner had other geochemical points in mind, and so maybe these points miss the objection. This idea certainly spawns a host of possible tests–what does happen to the asthenospheric wedge in this situation, does it cool enough, how will the oblique subduction affect mantle flow, etc. But it didn’t sound like any of that was the basis of the objection raised.

Anyways, GG is sorry to have missed the discussion.  But sometimes things get in the way.

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