Ars Sierra

A quick note that Scott Johnson has posted (published) his piece at Ars Technica on the Sierra Nevada that has been in the works for months. It is worth a look (you don’t often see science journalism that matures over that length of time).

GG has a few minor notes, but the piece does convey something of the disagreements bouncing around in the field without too much jargon-y overhead.

The standard story a couple of decades ago—one that is no longer subscribed to—started with that volcanically active, Andes-like range that stood over 80 million years ago. This version of Sierran history, Yosemite Park Geologist Greg Stock told Ars, “had those volcanoes shutting off, maybe by 60 million years ago, and then there was a period of prolonged erosion where the Sierra Nevada was eroded down to just ‘a series of low, rolling hills’—those are the words you’ll often see used.”” Presumably Greg (and many others) remember things that aren’t quite what was in the literature. If you go back and reread papers by Lindgren about a century ago or a review paper in 1966 by Christensen, there was significant relief inferred to exist in the mid-Tertiary Sierra. These workers recognized that there were peaks rising from the lowlands–Table 2 of Christensen shows mid-Tertiary relief of several thousand feet in several areas. Arguably GG made the closest thing to a statement like this in idealizing the situation for some simple numerical calculations in a 2004 paper. Maybe the low rolling hills stuff came from popularizations or just a sloppy overgeneralization of the lower relief thought to exist in the middle Tertiary. Anyways, it isn’t really the bone of contention it is often made out to be. However, the idea that there was tectonic quiescence and gradual reduction of relief through the early to middle Tertiary is still kicking around, so this sentence perhaps exaggerates things.

The deep guts of the volcanic system are much more exposed in the south. ” Actually, no. There is no Cenozoic volcanic system to the south remotely comparable to what happened to the north. The closest you get are the volcanic necks that brought up the xenoliths 8-12 Ma, and these are not very deeply eroded.

The post indicates that it is only the southern Sierra where the mantle lithosphere has been removed. The minimal thickness of mantle lithosphere is not only under the southern High Sierra, it extends north to at least about Tahoe; things are a bit murkier north of there. So while the south and north are different, the business of removing mantle lithosphere seems to make it into what most would call the northern Sierra.

If the delamination hypothesis is right, the southern Sierra should have been popping up rapidly for the last 3 million years—and they should still be growing today“. Actually the models LePourheit and Saleeby published (referred to a bit later in the piece) predict the Sierran crest should be subsiding (check out fig. 13 of their 2012 Geosphere paper). More likely, the conversation refers to observations Saleeby and other have made in the southwesternmost Sierra that suggest rapid uplift; although Jason likes to interpret this as an extension of the delimitation hypothesis, the particulars of that uplift are not directly part of the numerical models made to this point.

““The idea that you get mountain uplift by [removing some of the] lower crust has been kicking around since around 1980.” This was quoting yours truly, who either misspoke or misremembered (or was misquoted, which seems least likely), not that this is a big deal.  In the Sierra, the foundering mantle+lower crust story started with the collection of xenoliths and their discussion at one of the last meetings of the Southern Sierra Continental Dynamics project; that was in the mid-1990s.  Before that there was considerable speculation that the mantle lithosphere (not crust) was removed under the Sierra (at least back to Tom Crough in 1977, and arguably back to 1970 with work by Dean Carder).

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