New data on Sierra uplift

[mokelumnekid]

Abstract from an interesting paper from the journal Geology 2012 (be nice to get Giantbrookie's take on this work)

"Contemporary uplift of the Sierra Nevada, western United States, from GPS and InSAR measurements"

Modern space geodesy has recently enabled the direct observation of slow geological processes that move and shape Earth's surface, including plate tectonics and crustal strain accumulation that leads to earthquakes. More elusive has been the direct observation of active mountain growth, because geodetic measurements have larger uncertainties in the vertical direction, while mountain growth is typically very slow. For the Sierra Nevada of California and Nevada, western United States, the history of elevation is complex, exhibiting features of both ancient (40–60 Ma) and relatively young (<3 Ma) elevation. Here we exploit the complementary strengths of high-precision three-component point positions from the GPS and blanket coverage line-of-sight measurements from interferometric synthetic aperture radar (InSAR) to show that contemporary vertical motion of the Sierra Nevada is between 1 and 2 mm/yr. The motion is upward with respect to Earth's center of mass and with respect to a relatively stable eastern Nevada, indicating generation of relief and uplift against gravity. Uplift is distributed along the entire length of the range, between latitude 35°N and 40°N, and is not focused near localized, seismically imaged mantle downwellings. These results indicate that the modern episode of Sierra Nevada uplift is still active and could have generated the entire modern range in <3 m.y.

[TehipiteTom]

Saw a story about this in the Chronicle, and wondered what the geologists here would have to say about it. Interesting stuff.

[maverick]

Hi MK,

Check you PM please. Thanks

[giantbrookie]

Hi folks. I've been pretty swamped lately with finals and end of semester and the usual, plus I've had the unhappy experience of a good friend of mine destroying his shoulder in a fall while on what should have been a glorious field trip with me Monday. In any case, here's my two cents and this will give a few previews of a paper I submitted that is currently in review with Geosphere.

The InSAR/GPS data indicating rapid current uplift of the Sierra is reasonably consistent with geologic evidence but it is just a tad puzzling in detail. Calculations of late Cenozoic uplift from tilted marker horizons suggest about 1-2 km of uplift (note this does NOT account for the entire elevation of the range--there was some preexisting topography with a lot more in the south), with the start up time ranging from between 5-10 Ma in the San Joaquin R. drainage to about 3 Ma in the northern Sierra (this really changes the picture from what we knew at the time I wrote the 2001 paper with Sawyer and had the whole thing starting up at about 5 Ma). If we take 2 km of uplift and about 8 Ma start up (probably more reasonable than the young limit) in the San Joaquin we have a time averaged uplift rate of about 0.25 mm/yr. For the Feather River we have, say 1.8 km/3 Ma for an averaged rate of 0.6 mm/yr. The latter is closer to the 1-2 mm/yr modern rate but there are some catches here. The last 600 ka appear to show a deceleration of incision (and thus inferred uplift rate) in the Feather River, although it is still quite high. In the central and southern Sierra the deceleration of incision is much more pronounced as recorded by Greg Stock's studies using dated cave deposits--incision in Kings Canyon has slowed to nearly zero in the last few hundred thousand years. It is now abundantly clear that geology shows a north-south difference in timing of uplift with the most active and youngest uplift at the northern end of the range and probably the southernmost part of the range.

Accordingly, the data does not seem surprising, but I do not know how far it can be interpreted in terms of being representative of uplift rates over geologically significant time spans. It has taken many years to be able to measure the much smaller vertical rates of movement compared to the horizontal ones. One thing seems to be getting clearer with time, though, regarding modern day rate measurements: For years there seemed to be discrepancies between GPS/geodetic/etc. horizontal modern rates and geologic rates averaged over hundreds of thousands or millions of years. The more field geology has been conducted the smaller the discrepancies have become. With the Sierra, I am fairly confident that we know the initiation timing and the amount of uplift well enough to say that the long-term averaged rate (since initiation of uplift) is less than 1 mm/yr. What we don't know, though, is whether or not the rate within the last few thousand or ten thousand years is in the 1-2 mm/yr range. I am not aware of any geomorphic evidence of this, though.

[tomba]

Wouldn't it be natural to expect recent acceleration of uplift given the melting of glaciers after the last ice age, and the decreased weight causing a rebound?

[Jimr]

The last 600 ka appear to show a deceleration of incision (and thus inferred uplift rate) in the Feather River, although it is still quite high. In the central and southern Sierra the deceleration of incision is much more pronounced as recorded by Greg Stock's studies using dated cave deposits--incision in Kings Canyon has slowed to nearly zero in the last few hundred thousand years.

Just so I'm clear (a rare occurrence), Is this implying that uplift rates are adjusted to account for the counteracting erosion rates? If I'm reading this right, evidence is suggesting that the uplift rate in Kings Canyon seems to be approaching the rate of erosion from the Kings River cutting into and deepening the canyon.(?) A near equilibrium?

Jeez! My head wants to dive deep. I've spent the last hour (ok, it's now 1.5 hours) writing and deleting questions that could probably be answered in the i-surf zone.

[giantbrookie]

The last 600 ka appear to show a deceleration of incision (and thus inferred uplift rate) in the Feather River, although it is still quite high. In the central and southern Sierra the deceleration of incision is much more pronounced as recorded by Greg Stock's studies using dated cave deposits--incision in Kings Canyon has slowed to nearly zero in the last few hundred thousand years.

Just so I'm clear (a rare occurrence), Is this implying that uplift rates are adjusted to account for the counteracting erosion rates? If I'm reading this right, evidence is suggesting that the uplift rate in Kings Canyon seems to be approaching the rate of erosion from the Kings River cutting into and deepening the canyon.(?) A near equilibrium?

Jeez! My head wants to dive deep. I've spent the last hour (ok, it's now 1.5 hours) writing and deleting questions that could probably be answered in the i-surf zone.

Uplift rates are not adjusted to account for erosion/incision rates, but incision can be taken as a signal for ongoing uplift. Although incision of a stream in bedrock can result from an increase in discharge or an increase in gradient, considering the various data available for the Sierra it appears that stream incision in the Sierra has been driven by uplift and this uplift is in fact of tectonic origin (in this case, this would be faulting). The data shows that stream incision has not quite kept pace with incision (ie uplift rates are faster than the downcutting rates of stream). That having been said, the reduction in incision rates, recorded in the Kings and other central/southern Sierran streams, over the last million years or so, appears to reflect a decrease in uplift rate (with a permissible solution that uplift has stopped). I hope this helps. I good brew helps with funky reference frame problems (have returned from some rounds at a brew festival).

[rlown]

guessing we're safe to hike out most of our lives before seeing 15'ers in the Sierra Range then.