IGR: Deep mantle roots and continental emergence: implications for whole-Earth elemental cycling, long-term climate, and the Cambrian explosion

Deep mantle roots and continental emergence: implications for whole-Earth elemental cycling, long-term climate, and the Cambrian explosion

Cin-Ty A. Lee, Jeremy Caves, Hehe Jiang, Wenrong Cao, Adrian Lenardic, N. Ryan McKenzie, Oliver Shorttle, Qing-zhu Yin & Blake Dyer
International Geology Review 2017, http://dx.doi.org/10.1080/00206814.2017.1340853

Elevations on Earth are dominantly controlled by crustal buoyancy, primarily through variations in crustal thickness: continents ride higher than ocean basins because they are underlain by thicker crust. Mountain building, where crust is magmatically or tectonically thickened, is thus key to making continents. However, most of the continents have long passed their mountain building origins, having since subsided back to near sea level. The elevations of the old, stable continents are lower than that expected for their crustal thicknesses, requiring a subcrustal component of negative buoyancy that develops after mountain building. While initial subsidence is driven by crustal erosion, thermal relaxation through growth of a cold thermal boundary layer provides the negative buoyancy that causes continents to subside further. The maximum thickness of this thermal boundary layer is controlled by the thickness of a chemically and rheologically distinct continental mantle root, formed during large-scale mantle melting billions of years ago. The final resting elevation of a stabilized continent is controlled by the thickness of this thermal boundary layer and the temperature of the Earth’s mantle, such that continents ride higher in a cooler mantle and lower in a hot mantle. Constrained by the thermal history of the Earth, continents are predicted to have been mostly below sea level for most of Earth’s history, with areas of land being confined to narrow strips of active mountain building. Large-scale emergence of stable continents occurred late in Earth’s history (Neoproterozoic) over a 100–300 million year transition, irreversibly altering the surface of the Earth in terms of weathering, climate, biogeochemical cycling and the evolution of life. Climate during the transition would be expected to be unstable, swinging back and forth between icehouse and greenhouse states as higher order fluctuations in mantle dynamics would cause the Earth to fluctuate rapidly between water and terrestrial worlds.

Nature Comm: Lithospheric foundering and underthrusting imaged beneath Tibet

Lithospheric foundering and underthrusting imaged beneath Tibet

Min Chen, Fenglin Niu, Jeroen Tromp, Adrian Lenardic, Cin-Ty A. Lee, Wenrong Cao & Julia Ribeiro

Nature Communications 8, Article number: 15659 (2017), doi:10.1038/ncomms15659

Long-standing debates exist over the timing and mechanism of uplift of the Tibetan Plateau and, more specifically, over the connection between lithospheric evolution and surface expressions of plateau uplift and volcanism. Here we show a T-shaped high wave speed structure in our new tomographic model beneath South-Central Tibet, interpreted as an upper-mantle remnant from earlier lithospheric foundering. Its spatial correlation with ultrapotassic and adakitic magmatism supports the hypothesis of convective removal of thickened Tibetan lithosphere causing major uplift of Southern Tibet during the Oligocene. Lithospheric foundering induces an asthenospheric drag force, which drives continued underthrusting of the Indian continental lithosphere and shortening and thickening of the Northern Tibetan lithosphere. Surface uplift of Northern Tibet is subject to more recent asthenospheric upwelling and thermal erosion of thickened lithosphere, which is spatially consistent with recent potassic volcanism and an imaged narrow low wave speed zone in the uppermost mantle.

EPSL: Coupled magmatism-erosion in continental arcs: reconstructing the history of the Cretaceous Peninsular Ranges batholith, southern California through detrital hornblende barometry in forearc sediments

Hehe Jiang, Cin-Ty A. Lee

Continental magmatic arcs are characterized by voluminous flare-ups accompanied by rapid arc unroofing and sedimentation in the forearc basin. Such magmatism and erosion may be dynamically linked and influence the long-term evolution of crustal thickness. To evaluate these effects, we conducted a case study in the Peninsular Ranges batholith (PRB) in southern California, where mid-Late Cretaceous (125-75 Ma) emplacement of felsic plutons coincided with a major pulse of arc-derived sediments into the adjacent forearc basin. We compiled zircon U-Pb ages in the PRB plutons and estimated magmatic addition rates from exposed areas of plutons with different ages. To obtain erosion rates, sandstone samples of known depositional age from the PRB forearc basin were investigated. Major element compositions of detrital hornblendes were determined by electron probe microanalysis and used to calculate emplacement depths of eroded plutons using Al-in-hornblende barometry. These results were combined with laser ablation ICPMS based U-Pb ages of accompanying detrital zircons to estimate the integrated erosion rate by dividing the detrital hornblende emplacement depth by the lag time between peak detrital zircon age and depositional age. Both magmatic addition and erosion rates are between 0.1-2 km/Myr. Magmatic addition peaked at 100-90 Ma, followed by a long, protracted period of erosion between 90-50 Ma. Mass balance and isostatic modeling suggests that due to high magmatic influx, more than 30 km integrated crustal growth and 5 km elevation increase was achieved shortly after peak magmatism. The data and models suggest that erosion was driven by magma-induced crustal thickening and subsequent surface uplift, with an erosional response time of 3-6 Myr. Prolonged erosion after the cessation of magmatism resulted in gradual smoothing of the topography and significant removal of the excess crustal thickness by late Eocene time. The short erosional response times inferred from this study suggest that erosion and magmatism are intimately linked, begging the question of whether the thermal state, metamorphism and rheology of crust in continental arcs is controlled in part by the interplay between erosion and magmatism. We speculate that syn-magmatic erosion, through its effects on the thermal structure of the crust, may also play a role in modulating the depth of pluton emplacement.

Earth and Planetary Science Letters, Volume 472, 15 August 2017, Pages 69-81. doi: 10.1016/j.epsl.2017.05.009

April 15, 2017 – Birding High Island and Anahuac with Pete Vail

When: April 15, 2017

Where: Birding at High Island and Anahuac NWR with Pete Vail

What to expect: peak of bird migration, thousands of shorebirds, hundreds of colorful songbirds, and lunch on a salt dome rimmed by oil wells!

What to bring: binoculars (if you have them, but not absolutely necessary), hat, sunscreen, water, lunch, full gas tank, friends.  All are welcome. No experience necessary.

Organizers: Cin-Ty Lee and Martha Lou Broussard

Itinerary

Stop 1.

Meet at 10 AM at Anahuac NWR Nature Store (NOTE THAT THERE ARE SEVERAL REFUGE AREAS, SO PAY CLOSE ATTENTION TO DIRECTIONS BELOW)

From Houston, take I-10 east.  Get off on EXIST 812 (TX-61/Hankamer/Anahuac).  Turn right (south) on TX-61, which will eventually become FM 562.  At the Y intersection, turn left onto Whites Ranch Rd (FM1985). From the T intersection, continue for about 3 miles to Anahuac NWR sign on your right. Turn right and go south for 2.5 miles to nature store.

For iphone or google maps, insert these coordinates 29.614916, -94.535613 or “Friends of Anahuac Refuge Nature Store” (default on iphone will take you somewhere else). MAP

We will do the Shoveler Pond loop and look for various waterbirds.

Stop 2.

12 PM – Lunch at High Island, First Baptist Church

1368 Weeks Ln, High Island, TX 77623

From Anahuac, return to FM 1985 and turn right (east). Continue until you reach TX 124. Turn right (south) to High Island. Cross intracoastal canal and then veer left on Weeks Road to the church.

The First Baptist Church holds a BBQ every Saturday of April.  Those of us who want to eat BBQ can eat there, others can bring their own lunches.  We will picnic outside of the church.

 

Stop 3.  High Island

1 PM – High Island Boy Scout Woods

From the church, continue south on Weeks Rd. Turn right on 7th and then an immediate left on Dunman. At the intersection of Dunman and 5th, there is a parking lot. Park here and walk to Boy Scout woods (handicap parking after turning left on 5th).

2 PM – High Island Smith Oaks Heron Rookery

Retrace route to Weeks Rd, but instead of going back to the church, turn right on Old Mexico Road until you reach Smith Oaks headquarters.

 

Contact Cin-Ty Lee or Martha Lou Broussard for more details, ride-sharing, etc. Please let us know if you are coming so that we don’t leave anyone behind at the meeting spot.

Cin-Ty Lee – 713 348 5084 (ctlee@rice.edu)

Martha Lou Broussard (mlbrou@rice.edu)

 

 

 

Episodic nature of continental arc activity since 750 Ma: A global compilation

Episodic nature of continental arc activity since 750 Ma: A global compilation

Wenrong Cao, Cin-Ty Lee, Jade Star Lackey

Earth and Planetary Science Letters (2017) 461:85-95

http://dx.doi.org/10.1016/j.epsl.2016.12.044

Continental arcs have been recently hypothesized to outflux large amounts of CO2 compared to island arcs so that global flare-ups in continental arc magmatism might drive long-term greenhouse events. Quantitative testing of this hypothesis, however, has been limited by the lack of detailed studies on the spatial distribution of continental arcs through time. Here, we compile a worldwide database of geological maps and associated literature to delineate the surface exposure of granitoid plutons, allowing reconstruction of how the surface area addition rate of granitoids and the length of continental arcs have varied since 750 Ma. These results were integrated into an ArcGIS framework and plate reconstruction models. We find that the spatial extent of continental arcs is episodic with time and broadly matches the detrital zircon age record. Most vigorous arc magmatism occurred during the 670–480 Ma and the 250–50 Ma when major greenhouse events are recognized. Low continental arc activity characterized most of the Cryogenian, middle–late Paleozoic, and Cenozoic when climate was cold. Our results indicate that plate tectonics is not steady, with fluctuations in the nature of subduction zones possibly related in time to the assembly and dispersal of continents. Our results corroborate the hypothesis that variations in continental arc activity may play a first order role in driving long-term climate change. The dataset presented here provides a quantitative basis for upscaling continental arc processes to explore their effects on mountain building, climate, and crustal growth on a global scale.

Dr. Albert Bally awarded a Doctor Honoris Causa from the University of Fribourg (Switzerland)

GCURS 2016 – Gulf Coast Undergraduate Research Symposium

Rice Earth Science was pleased to sponsor the ESCI section of the 2016 Gulf Coast Undergraduate Research Symposium (GCURS) on Saturday, October 22.  The department hosted 17 outstanding presenters from earth science departments across the country.  Enjoy some photos from this year’s event!

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EPSL: Critical porosity of melt segregation during crustal melting: Constraints from zonation of peritectic garnets in a dacite volcano

Critical porosity of melt segregation during crustal melting: Constraints from zonation of peritectic garnets in a dacite volcano

Xun Yu* and Cin-Ty Lee

*visiting student

Earth and Planetary Science Letters
Volume 449, 1 September 2016, Pages 127–134

 

The presence of leucogranitic dikes in orogenic belts suggests that partial melting may be an important process in the lower crust of active orogenies. Low seismic velocity and low electrical resistivity zones have been observed in the lower crust of active mountain belts and have been argued to reflect the presence of partial melt in the deep crust, but volcanoes are rare or absent above many of these inferred melt zones. Understanding whether these low velocity zones are melt-bearing, and if so, why they do not commonly erupt, is essential for understanding the thermal and rheologic structure of the crust and its dynamic evolution. Central to this problem is an understanding of how much melt can be stored before it can escape from the crust via compaction and eventually erupt. Experimental and theoretical studies predict trapped melt fractions anywhere from <5% to >30%. Here, we examine Mn growth-zoning in peritectic garnets in a Miocene dacite volcano from the ongoing Betic–Rif orogeny in southern Spain to estimate the melt fraction at the time of large-scale melt extraction that subsequently led to eruption. We show that the melt fraction at segregation, corresponding approximately to the critical melt porosity, was ∼30%, implying significant amounts of melt can be stored in the lower crust without draining or erupting. However, seismic velocities in the lower crust beneath active orogenic belts (southern Spain and Tibet) as well as beneath active magmatic zones (e.g., Yellowstone hotspot) correspond to average melt porosities of <10%, suggesting that melt porosities approaching critical values are short-lived or that high melt porosity regions are localized into heterogeneously distributed sills or dikes, which individually cannot be resolved by seismic studies.

 

NATURE GEOSCIENCE: Two step rise of atmospheric oxygen

Two-step rise of atmospheric oxygen linked to the growth of continents

Cin-Ty A. Lee, Laurence Y. Yeung, N. Ryan McKenzie, Yusuke Yokoyama, Kazumi Ozaki, & Adrian Lenardic

Nature Geoscience (2016) doi:10.1038/ngeo2707

Earth owes its oxygenated atmosphere to its unique claim on life, but how the atmosphere evolved from an initially oxygen-free state remains unresolved. The rise of atmospheric oxygen occurred in two stages: approximately 2.5 to 2.0 billion years ago during the Great Oxidation Event and roughly 2 billion years later during the Neoproterozoic Oxygenation Event. We propose that the formation of continents about 2.7 to 2.5 billion years ago, perhaps due to the initiation of plate tectonics, may have led to oxygenation by the following mechanisms. In the first stage, the change in composition of Earth’s crust from iron- and magnesium-rich mafic rocks to feldspar- and quartz-rich felsic rocks could have caused a decrease in the oxidative efficiency of the Earth’s surface, allowing atmospheric O2 to rise. Over the next billion years, as carbon steadily accumulated on the continents, metamorphic and magmatic reactions within this growing continental carbon reservoir facilitated a gradual increase in the total long-term input of CO2 to the ocean–atmosphere system. Given that O2 is produced during organic carbon burial, the increased CO2 input may have triggered a second rise in O2. A two-step rise in atmospheric O2 may therefore be a natural consequence of plate tectonics, continent formation and the growth of a crustal carbon reservoir.