Sierra Nevada uplift

geologic controversy - Rise of the Sierra Nevada

The Sierra Nevada range of eastern California is one the most famous ranges in the world, but the timing and processes that led to its elevated topography is a major question mark. Christopher Henry (2009) summarizes this controversy and explains that most researchers once considered Sierran uplift to have mostly occurred during the last 10 million years, but more recent studies argue for much earlier uplift in the Mesozoic. The issue is related to the proposed “Nevadoplano” of DeCelles (2004), who argued that the present day Great Basin region (mostly Nevada) was a high plateau during the Mesozoic, much like the modern day Altiplano of the central Andes. The Sierra Nevada region would have formed the flank of this great plateau. Mulch et al. (2006) and Cassel et al. (2009) supports an early rise model with stable isotope evidence that suggest the Sierra Nevada was near its current elevation by at least the Eocene and Oligocene. Major batholithic volcanism also occurred at this time and is consistent with thickened crust. In contrast, Ducea and Saleeby (1996) propose the Sierra Nevada was uplifted in the late Cenozoic following the removal of a dense eclogitic root; citing thermobarometry of mantle xenoliths. Both sides seem to have support look for more work to be done on this issue.

Figure 1 of Henry (2009) showing modern day topography overlain by flow direction of ancient rivers.

Sources

Henry, C. D., 2009, Uplift of the Sierra Nevada, California, Geology,

Cassel, E.J., Graham, S.A., and Chamberlain, C.P., 2009, Cenozoic tectonic and topographic evolution of the northern Sierra Nevada, California, through stable isotope paleoaltimetry in volcanic glass: Geology, v. 37, p. 547–550.   

DeCelles, P.G., 2004, Late Jurassic to Eocene evolution of the Cordilleran thrust belt and foreland basin system, western U.S: American Journal of Science, v. 304, p. 105–168, doi: 10.2475/ ajs.304.2.105.

Ducea, M.N., and Saleeby, J.B., 1996, Buoyancy sources for a large, unrooted mountain range, the Sierra Nevada, California: Evidence from xenolith thermobarometry: Journal of Geo- physical Research, v. 101, p. 8229–8244, doi: 10.1029/95JB03452.

Mulch, A., Graham, S.A., and Chamberlain, C.P., 2006, Hydrogen isotopes in Eocene river gravels and paleoelevation of the Sierra Nevada: Science, v. 313, p. 87–89. 

viscous death in subduction zones

Fun geology term: viscous death

What is controlling water concentrations in arc magmas? 

Water is the weirdest and most important molecule there is. Besides sustaining life on our planet, water affects every part of the evolution of an arc volcano. Water's presence in the deep mantle dramatically lowers its melting temperature, resulting in magmatism in an otherwise cold subduction zone. Water also lowers the viscosity of magmas allowing them to rise, and later expands as a gas fueling explosive eruptions. Despite its importance to the transfer of heat and matter throughout nearly every system on our planet, little is known about the filtering effects of subduction zone water. This is changing with the rise of improved measurements for water concentrations in magmatic inclusions, tiny bits of volcanic glass that were trapped as liquids in growing crystals (usually olivine), prior to degassing and eruption. The new data seem to point to a surprisingly narrow range of water concentrations within each arc, and a fairly uniform global average of about 4 weight percent.

This uniformity implies a process that regulates the water concentration. Plank et al., 2103 suggest two hypotheses.
1) Arc magmas stall at around the same depth (~6 km) beneath the Earth’s surface water is saturated within the melt and melt inclusions form at relatively constant water concentrations. Such uniformity in upper crustal process, however, seems unlikely.

2) Water concentration strongly controls the amount of melting in the mantle, maintaining a general proportion of water to melt: more water = more melting.

The answer will carry important implications on the amount of water that is transferred from the mantle to the surface, and how much magma ultimately gets stuck and never erupts. For example, magmas with more water leads to an earlier crystallization of amphibole, which subsequently lowers the water content of the remaining melt. Lower amounts of water increase the melt viscosity and also raises the melting temperature, bringing on further crystallization. Thus magmas with more water may actually never erupt, but rather succumb to a viscous death. This leads to the counterintuitive idea that plutonic rocks are actually wetter than volcanic rocks. 

Figure 4 from Plank et al. (2013) showing the fairly constant water concentrations of arc magmas. 

Source:

Terry Plank, Katherine A. Kelley, Mindy M. Zimmer, Erik H. Hauri, Paul J. Wallace, Why do mafic arc magmas contain ∼4 wt% water on average?, Earth and Planetary Science Letters, Volume 364, 15 February 2013, Pages 168-179, ISSN 0012-821X, (http://www.sciencedirect.com/science/article/pii/S0012821X1200670X)

Earth's plate tessellation

Fun geologic process of the day: The numbers and sizes of Earth’s tectonic plates are controlled by styles of mantle convection

Earth’s two largest tectonic plates (Pacific and Africa) are on exact opposite sides of the globe (antipodes) and it is perhaps not a coincidence. The numbers and sizes of all of the earth’s plates aren’t random either, but instead are controlled by movements in the mantle. Morra et al. (2013) analyzed the position and size of earth’s plates over the last 200 million years and suggest the systematic tessellation (the gap-free arrangement of tiles, or plates) of Earth’s surface is a consequence of mantle convection styles that alternate between being subduction driven (top-down) and deep mantle driven (bottom-up).

Source: Gabriele Morra, Maria Seton, Leonardo Quevedo, R. Dietmar Müller, Organization of the tectonic plates in the last 200 Myr, Earth and Planetary Science Letters, Volume 373, 1 July 2013, Pages 93-101, ISSN 0012-821X,(http://www.sciencedirect.com/science/article/pii/S0012821X13002021).

Petit spots

Fun geology term for the day: petit spot
 

Hirano et al. (2013) use the term to “petit spot” to describe young (~8 Ma to 50 ka), small (<1 km³) basaltic eruptions on the downgoing plate just before it enters the Japan trench. As the lavas are on the wrong side of the trench for subduction related volcanism, and thousands of km from a divergent margin, the authors suggest some unusual explanations for their presence: bending (flexure) of the downgoing plate just before it arrived to the trench created fractures which allowed asthenospheric melts to rise to the surface. High ⁴⁰Ar/³⁶Ar ratios are attributed to an upper mantle source. The problem of why the asthenosphere melted in the first place is not delved into too much, although they suggest that these petit spots provide evidence for the long-debated hypothesis that the low velocity zone in the asthenosphere is a reservoir of long-lived partial melts.

 

Source: Volcanism in Response to Plate Flexure, 2006, Naoto Hirano, Eiichi Takahashi, Junji Yamamoto, Natsue Abe, Stephanie P. Ingle, Ichiro Kaneoka, Takafumi Hirata, Jun-Ichi Kimura, Teruaki Ishii, Yujiro Ogawa, Shiki Machida, Kiyoshi Suyehiro, Science 313, 1426 (2006); DOI: 10.1126/science.1128235

Unexplained volcanism: Saudi Arabia

The Arabian Peninsula has been diverging from Africa for the past 30 million years, following the arrival of the Afar hot spot. Decompression melting has been providing the lavas that create new oceanic crust in the opening Red Sea, but a suite of volcanoes in western Saudi Arabia that have been erupting for the past 12 million years is probably too far to the east of the divergent margin to be directly related. What then is causing this volcanism? (check out western Saudi Arabia in Google maps to see the impressive black lava surrounded by the vast desert landscape). The Afar hot spot would be a good candidate, but it is also thought to be a bit too far to the south, near the diverging triple junction. Perhaps lateral flow of the hot spot plume is driving the melting, or maybe a separate plume directly beneath Saudi is involved. Bob Duncan and Abdullah Al-Amri (2013) set out to address this question by studying Harrat Lunayyir volcanic field, which last erupted about 1000 years ago and occasionally experiences seismic activity suggesting that it is still active. Their 40Ar/39Ar age determinations show that the field is very young (less than 600 thousand years) with lavas characterized by intraplate geochemistry (perhaps ruling out the divergent margin or other extension related melting). The lack of age progression, along with isotopic considerations, also seems to rule out lateral flow from the Afar plume. More study is necessary to understand the cause of volcanism this region.

Source: Robert A. Duncan, Abdullah M. Al-Amri, Timing and composition of volcanic activity at Harrat Lunayyir, western Saudi Arabia, Journal of Volcanology and Geothermal Research, Volume 260, 15 June 2013, Pages 103-116, ISSN 0377-0273, (http://www.sciencedirect.com/science/article/pii/S0377027313001479).

orogeny and volcanism in Tibet

Fun geologic relationship of the day: orogenic plateaus and volcanic triggers

The processes occurring beneath the world’s great plateaus such as Tibet and the central Andes are difficult to decipher. How the deep crust and mantle lithosphere are behaving, and how they interact with the underlying lithosphere has major implications for how the plateaus grow and why they contain widespread volcanism. In one of the first major studies of volcanism in Tibetan, Turner et al. (1996) interpreted highly potassic lavas to indicate melting of biotite-rich, garnet bearing mantle lithosphere that had been metasomatized (enriched in volatiles and hydrous fluids) during earlier (pre-collisional) periods of subduction. Evidence includes negative Nb-Ta and Ti anomalies, fractionated LREE/HREE, high K contents, and isotopic considerations. Turner et al. (1996) suggest that the hydrated mantle lithosphere most likely melted due to temperature increases related to convective removal of its lower regions, as proposed by England and Houseman (1988, 1989). In this model, the collision between India and Eurasia (beginning ~52 million years ago) thickened the cold, relatively dense lower mantle lithosphere until it detached and was replaced by warm, buoyant asthenosphere. This influx of asthenosphere then raised the plateau, and led to gravitational collapse of its edges. Turner et al. proposed that the newly exposed (undetached) mantle lithosphere heated up and melted, resulting in the plateau volcanism. They further suggest that the age of the lavas (< 13 Ma) puts a time stamp on the removal process, as shown in this figure. 

 Figure 13 of Turner et al. (1996).

The timing of plateau uplift has major implications concerning the evolution of our atmosphere and the life it harbors, as a rising plateau may lead to increased global cooling. However, it is very difficult to pin down the dates of plateau uplift (it does not necessarily correlate with crustal thickening alone) and is the focus of numerous studies.  

An alternatives to the convecting thinning / mantle lithosphere melting model discussed in this post was addressed on this blog earlier, and el volcán tranquilo will, no doubt, return to this controversy repeatedly.  

References

Turner, S., Arnaud, N., Liu, J., Rogers, N., Hawkesworth, C.,Harris, N., Kelley, S., Van Calsteren, P. & Deng, W. (1996). Post-collision, shoshonitic volcanism on the Tibetan plateau: implications for convective thinning of the lithosphere and the source of ocean island basalts. Journal of Petrology 27(1), 45--71.

Molnar, P., P. England, and J. Martinod, Mantle dynamics, the uplift of the Tibetan Plateau, and the Indian monsoon, Reviews of Geophysics, 31, 357-396, 1993.

England, P. C. & Houseman, G. A.; 1988. The mechanics of the Tibetan plateau. Philosophical Transactions of the Royal Society of London A326, 301-319.

England, P. C. & Houseman, G. A., 1989. Extension during continental convergence, with application to the Tibetan Plateau. Journal of Geophysical Research 94, 17561-17579.

hidden plate tectonics

--> Fun geology term for the day - hidden plate tectonics

The Tibetan plateau is the largest topographic feature on Earth and is related to the ongoing collision between India and Eurasia that began some 55 million years ago. Construction of the plateau began earlier with the accretion of ocean island crustal blocks to the Eurasian continent in the Jurassic, although the extreme topography likely wasn’t created until the Cenozoic following the collision with India (see my earlier post about the debate over the timing of uplift). How and when the plateau crust became thickened and elevated is a major debate in geology. Tapponnier et al. (2001) suggest that the collision with India resulted in intracontinental subduction (“hidden plate tectonics”) of mantle lithosphere of the previously accreted terranes.

Image from figure 3 from Tapponier et al. (2001). The authors caution that the proposed continental subduction has not yet been imaged geophysically. 

Tapponier et al. (2001) suggest that the terranes act as coherent blocks, contradicting other ideas of plateau growth that suggest the Tibetan mantle lithosphere behaves more fluidly. They challenge an earlier hypothesis put forth by mainly by Philip England and Gregory Houseman of a “soft Tibet” that involves: a) the entire lithosphere thickening as a viscous sheet, b) eventual removal and sinking of this dense lithosphere into the asthenosphere, c) subsequent buoyant rise, and extension of the  plateau.

Image from figure 9 of Molnar et al. (1993) showing convective removal of the Tibetan lithosphere.

Another simple model of lithospheric removal beneath Tibet. From Harrison et al., 1992

The debate often centers on determining of the timing of plateau thickening vs. uplift and in the interpretation of the Late Miocene to recent faulting that appears to be forcing the plateau crust eastward. Tapponier et al. (2001) suggest that the observed extension by normal faulting due to gravitational collapse is negligible, and that the lateral extension of the plateau is largely occurring along strike slip faults, which are more in line with the continental subduction hypothesis. How the plateau is deforming has strong implications on the causes for the volumetrically minor, but widespread potassic volcanism on the plateau. Tapponnier et al. further argue that the post-collisional volcanism is localized in three different belts that line up well with the continental subduction model, and not so much with wholesale removal of the lithospheric mantle. How continental subduction promotes melting is not addressed in this paper, but later researchers do and will be discussed here shortly.    

Citations: 

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T. M. Harrison, P. Copeland, W. S. F. Kidd, A. Yin, Science 255, 1663 (1992).

Molnar, P., P. England, and J. Martinod, Mantle dynamics, the uplift of the Tibetan Plateau, and the Indian monsoon, Reviews of Geophysics, 31, 357-396, 1993.

Tapponnier, P., Xu, Z.Q., Roger, F., Meyer, B., Arnaud, N., Wittlinger, G., and Yang, J.S., 2001, Oblique stepwise rise and growth of the Tibet plateau: Science, v. 294, p. 1671–1677.