The effects of internal heating and large scale variations on tectonic bi-stability in terrestrial planets

New student paper out! Matt Weller, publishing in Earth and Planetary Science Letters, finds that planets migrate through tectonic states over time as their surface temperatures change. A planet even can have multiple stable tectonic states over time! [article]

Abstract

We use 3D mantle convection and planetary tectonics models to explore the links between tectonic regimes and the level of internal heating within the mantle of a planet (a proxy for thermal age), planetary surface temperature, and lithosphere strength. At both high and low values of internal heating, for moderate to high lithospheric yield strength, hot and cold stagnant-lid (single plate planet) states prevail. For intermediate values of internal heating, multiple stable tectonic states can exist. In these regions of parameter space, the specific evolutionary path of the system has a dominant role in determining its tectonic state. For low to moderate lithospheric yield strength, mobile-lid behavior (a plate tectonic-like mode of convection) is attainable for high degrees of internal heating (i.e., early in a planet’s thermal evolution). However, this state is sensitive to climate driven changes in surface temperatures. Relatively small increases in surface temperature can be sufficient to usher in a transition from a mobile- to a stagnant-lid regime. Once a stagnant-lid mode is initiated, a return to mobile-lid is not attainable by a reduction of surface temperatures alone. For lower levels of internal heating, the tectonic regime becomes less sensitive to surface temperature changes. Collectively our results indicate that terrestrial planets can alternate between multiple tectonic states over giga-year timescales. Within parameter space regions that allow for bi-stable behavior, any model-based prediction as to the current mode of tectonics is inherently non-unique in the absence of constraints on the geologic and climatic histories of a planet.

Earth Science ESCI 334 Field Trip

2015-March-ESCI334-NMESCI 334 is one of the Earth Science department’s capstone undergraduate courses. The field trip takes us to a location near San Ysidro, New Mexico, because of the easy access to phenomenal rock exposures, which span approximately one billion years of geological time. Students map in detail rocks that were folded and faulted during the late Cretaceous Period (approximately 70 million years ago), as a consequence of the intense mountain building along the western margin of North America during that time.

During the field campaign, students integrate and apply knowledge they acquired throughout their undergraduate earth science core curriculum. The goal is for students to achieve their full potential in exploring and discovering the geology underfoot. Throughout the field trip students build their confidence in working independently. They make their own scientific observations, which culminate in an interpretive report that transforms their observations into geological understanding.

During the field trip students form bonds and friendships with one another that often extend beyond their studies at Rice. This sort of camaraderie and friendship is in part facilitated by our lodging arrangement. This year the group of students was rather large, but we were able to find a vacation rental on the outskirts of Albuquerque, which had two houses on a single property. Although quarters were tight, this allowed us to share meals and socialize in the evenings, after a hard day’s work in the field.

RUGS Students Visit 1 Billion-year-old Serpentinite

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1 Gy serpentinite. From left to right: Elli Ronay, Xun Yu, Detao He (behind Xun), Larisa LaMere, Sriparna Saha, Michael Farner, Lexi Malouta, Rachel Marzen, Jackie Rios, Adeen Denton, Tierra Moore, Emily Paine, and Cin-Ty Lee (taking the picture!).

How do you get water back into the mantle?  Serpentinites, of course!  The main source of water in magmatic arcs like the Cascades comes from altered oceanic lithosphere.  Interaction of the seafloor results in extensive hydration of the crust and lithospheric mantle, resulting in the conversion of olivine-rich rocks (peridotite) to a green phyllosilicate-bearing rock known as serpentinite.  Studying serpentinites is the key to understanding the global water cycle, at least on million year timescales. What were subduction zones like billions of years ago?  There are many examples of Phanerozoic serpentinites, but really old serpentinites are rare.  It turns out that we have some ancient serpentinites right here in Texas, in the one billion year Grenvillian orogenic belt that cuts across our state.

Green, green olivine
Peridot is when you’re on show.
Drops of water,
Sooner or later,
You become serpentine.

So the Rice Undergraduate Geology Society (RUGS) organized a one day trip out to central Texas today to look at these 1 billion year old serpentinites. We started out at 6 am in the morning and hightailed out to the Coal Creek serpentinite. We met one of our professors Cin-Ty Lee out there – he had arrived several hours earlier to search for a rare Mexican bird called a Striped Sparrow. A couple of faculty from UT Austin also joined us. The day turned out to crystal clear, with mild temperatures. We could not have asked for a better day weather-wise.


We arrived at the serpentinite outcrop. We immediately noticed that the quarried sections were light green, but the weathered surfaces were red, a common feature of ultramafic terranes. There was almost no vegetation growing on the serpentinite surfaces, in stark contrast to the surrounding amphibolite rocks, which had small live oaks growing on them. We spent a couple hours scrambling over the quarry, trying not to roll our ankles, all the while pondering what type of peridotite protolith this was. Was it oceanic lithosphere, fore-arc lithosphere, sub-arc lithosphere, or back-arc lithosphere? How would we tell? Does the serpentinite represent a suture between the collision of two continents? What we all agreed upon was that this serpentinite looked old! No hints of fresh olivine anywhere, but everywhere, there were hints of serpentinizing fluids in the form of syndeformation veins. We also found a couple of local samples that appeared as if they were originally troctolites based on relic cumulate texture.

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Serpentinites with magnetite-rich veins


After having lunch on these green rocks, we decided to head to Enchanted Rock to look at granites up close and personal. Unfortunately, when we arrived, the park had reached its capacity for the day and they were not letting in anymore visitors until 3 PM. So we went back towards where we had just come from and stopped to look at an unconformity where Proterozoic granites and migmatites were overlain by Paleozoic limestones. A hundred million years missing beneath our feet! We spent a half hour crawling over some beautiful exposed migmatites and discussed what conditions are needed to melt the lower crust.

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Syn-magmatic diking in migmatite


Daylight was running out, so we thought we would take one more shot at getting into Enchanted Rock State Park. We arrived there with only about 2 hours left of sunlight and a long line of cars inching their way through the entrance. We eventually got in with enough time to climb to the top of one of the granite domes. It was a nice way to end the day! Participants (see our group shot above) included from left to right: Elli Ronay, Xun Yu, Detao He (behind Xun), Larisa LaMere, Sriparna Saha, Michael Farner, Lexi Malouta, Rachel Marzen, Jackie Rios, Adeen Denton, Tierra Moore, Emily Paine, and Cin-Ty Lee (taking the picture!).

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running up Enchanted rock.