Meadow making

UPDATE: they have a paper in the November 2014 GSA Today on this, which provides more detail than the tlk.

Another interesting talk at GSA came from Richard Becker, a PhD student with Basil Tikoff. Tikoff’s group had identified a peculiar set of fractures in the Tuolumne Meadows area, what they called tabular fracture clusters that are kind of shattered zones up to about a meter wide.  Tuolumne Meadows is a rather unique piece of real estate, usually called the largest  subalpine meadow in the Sierra, and it stands in stark contrast to the rugged terrain immediately to the north in headwaters of many of the tributaries to the Tuolumne River. The previous workers contended that these fractures were caused by the release of fluids as the last phase of the Tuolumne Intrusive suite was crystalizing. In this talk, Becker suggested that the peculiarly gentle topography of the area was because of pervasive sets of these fractures, which weakened the rock and also introduced a structural anisotropy that glaciers might exploit in different ways. As the mapped extent of these fractures suggests that Tuolumne Meadows overlies some of the densest arrays of these fractures (much of the area is covered by till), it seems plausible that these play an important role in creating the broad gentle area of the meadows.  Becker suggested this was not simply a product of the rock type, noting that a similar end-stage intrusion farther south lacked such gentle topography and also lacked many such fractures.

Be-10 ages from Dühnforth et al, Geology, 2010, around Tuolumne Meadows.

Be-10 ages from Dühnforth et al, Geology, 2010, around Tuolumne Meadows. POTD is Pothole Dome (west edge of meadows on the road),LEMD is Lembert Dome.

This is an interesting idea, but there are some issues that crop up.  Glaciers are really good at digging well below any sills in their path, unlike rivers that cannot cut below a base level (this is why there are fjords and deep alpine lakes); if this rock is really that weak, shouldn’t Tuolumne Meadows be seriously scooped out? Maybe it is and there is a lot more sediment there than seems apparent.  Similarly, if this weakness extended out some distance, the glaciers would clearly have carved down deeply, as is seen just to the west in the Grand Canyon of the Tuolumne.

Ironically it seems like an absence of erodibility, not an excess of it, that defines this area, as was documented by Dühnforth et al.in Geology in 2010 (image above). 10Be dates predating the last glacial episode (outlined in blue) mean that only 2-3 m of rock could have been removed in the last glacial episode. Presumably this doesn’t say anything about how much of the fracture-riven rock was removed, but it does suggest that glaciers are probably not creating Tuolumne Meadows.

There are other features in the Sierra that resemble the topography of Tuolumne Meadows, none nearly as glaciated.  Monache Meadows in the southern Sierra is a broad upland on the South Fork of the Kern River that has very little relief (and much of the area around it is similarly subdued despite high elevations).  Horseshoe Meadows is somewhat smaller and a bit bumpier but also kind of odd looking.  While there are many canyon flanks that are very gentle (look at the area south of the Clark Range on the north side of the San Joaquin River, or the Chagoopa Plateau along the Kern River), on main stem rivers the other interesting candidate is the region near Courtright Reservoir on the North Fork of the Kings River.

Perhaps the answer will turn out to be rivers rather than glaciers; perhaps the fracture sets greatly air river erosion in ways that create these broad low relief areas, but glaciers traveling across these zones have little traction and so, unless the bedrock is more fractured in a traditional way, do little damage.

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