remarkable regions

Alaska: a gold rush of along strike variations

Alaska:  a gold rush of along strike variations

Every 8 weeks we turn our attention to a Remarkable Region that deserves a spot in the scientific limelight. After exploring the Mediterranean and the ancient Tethys realm, we now move further north and across the Pacific to the Aleutian-Alaska subduction zone. This post was contributed by Kirstie Haynie who is a PhD candidate at the department of geology at the University at Buffalo, State University of New York, in the United States of America.

Given that Alaska is a remarkable region, I decided to walk up to strangers and ask them what comes to mind when they hear the word “Alaska”. Indeed I received some confusing looks and laughs, but everyone I asked had something to say. Some people alluded to popular TV shows set in Alaska, such as Gold Rush, Bush People, and Alaska: the Last Frontier, while others spoke about the cold weather, dog mushing, Eskimos, fishing and hunting, and the Trans-Alaska pipeline. A few of the answers I received referenced the beauty and wilderness of the large snow capped mountains, glaciers, and the Northern Lights (Aurora Borealis): all emblematic of the largest state in America. But to me, Alaska is more than just a pretty landscape and a place to fish. It is a region riddled with geologic mysteries and rich in along strike variations.

The Aleutian-Alaska subducton zone marks a North American-Pacific plate boundary where subduction varies greatly along strike (Figure 1). At the western end of the subduction zone, the Aleutian volcanic islands are the result of oceanic-oceanic subduction while in the eastern part of the subduction zone there is oceanic-continental collision where the Pacific plate descends beneath the North American plate. The age of the subducting sea floor increases laterally from around 30 Ma in the eastern subduction corner to 80 Ma at the end of the Aleutian volcanic arc (Müller et al., 2008). Slab dip changes drastically from 50° to 60° in the west and central Aleutians to flat slab subduction under south-central Alaska (Ratchkovski and Hansen, 2002a; Lallemand et al., 2005; Jadamec and Billen, 2010). This leads to a variation in the slab pull force, which is a main driving force of subduction caused by the weight of dense slabs sinking into the mantle (Morra et al., 2006).

Figure 1: Tectonic map of Alaska modified from Haynie and Jadamec (2017). Topography/bathymetry is from Smith and Sandwell (1997) and Seafloor (SF) ages are from Müller et al. (2008). Blue lines are the slab contours of Jadamec and Billen (2010) in 40 km intervals; the thick black line is the plate boundary from Bird (2003); and the thinner black lines are faults from Plafker et al. (1994a). The location of Denali is marked by the orange hexagon. Holocene volcanoes are given by the pink triangles (Alaska Volcano Observatory). The purple polygon is the outline of the Yakutat oceanic plateau (Haynie and Jadamec, 2017). WB – Wrangell block fore-arc sliver; JdFR – Juan de Fuca Ridge.

There is also a distinct change in margin curvature from convex in the west to concave in the east. At the end of the eastern bend, the Alaska part of the subduction zone is truncated by a large transform boundary, the Fairweather-Queen Charolette fault, which gives rise to a corner-shaped subduction-transform plate boundary (Jadamec et al., 2013; Haynie and Jadamec, 2017). Here, convergence is oblique with an average velocity of 5.2 cm/year northwest (DeMets and Dixon, 1999). Seismic studies (Page et al., 1989; Ferris et al., 2003; Eberhart-Phillips et al., 2006; Fuis et al., 2008) show that thicker than normal oceanic crust lies off-shore in the subduction corner. This thick oceanic material has been identified as the Yakutat oceanic plateau (Plafker et al., 1994a; Brocher et al., 1994; Bruns, 1983; Worthington et al., 2008; Christeson et al., 2010; Worthington et al., 2012). Even though oceanic plateaus tend to resist subduction (Cloos, 1993; Kerr , 2003), the Yakutat plateau is currently subducting beneath the Central Alaska Range to depths of 150 km (Ferris et al., 2003; Eberhart-Phillips et al., 2006; Wang and Tape, 2014). It is also colliding into south-east Alaska (Mazzotti and Hyndman, 2002; Elliott et al., 2013; Marechal et al., 2015) where the largest coastal mountain range on Earth, the Saint Elias Mountains, are located (Enkelmann et al., 2015).

With regards to surface deformation, in addition to Denali (the tallest mountain in North America), other notable along strike variations reside within the broad deformation zone of south-central Alaska. For example, a normal volcanic arc occurs over the Aleutian part of the subduction zone and above the Alaska Peninsula. However, above the flat slab there is a gap in volcanism followed by the presence of the enigmatic Wrangell volcanoes (Rondenay et al., 2010; Jadamec and Billen, 2012; Martin-Short et al., 2016; Chuang et al., 2017). These volcanoes are marked by a range of morphologies as well as adakitic geochemical signatures (Richter et al., 1990; Preece and Hart , 2004), which have a petrogenesis that may be attributed to slab melting (Defant and Drummond , 1990; Peacock et al., 1994; Castillo, 2006, 2012; Ribeiro et al., 2016). Analogue (Schellart , 2004; Strak and Schellart , 2014) and 3D numerical models (Stegman et al., 2006; Piromallo et al., 2006; Jadamec and Billen, 2010, 2012) predict that toroidal flow can produce upwellings around the edge of a slab that may have implications for melting of the slab and the formation of adakites. However, the formation of the Wrangell volcanoes is still debated.

Also located above the subducting plateau and flat slab is the Wrangell block fore-arc sliver, which exhibits northwest motion and counterclockwise rotation (Cross and Freymueller, 2008; Freymueller et al., 2008; Bemis et al., 2015; Waldien et al., 2015; Jadamec et al., 2013; Haynie and Jadamec, 2017). This sliver is bounded in the north by the arcuate shaped Denali fault, which illustrates a lateral change in slip rates that increases towards the center of the fault (Haynie and Jadamec, 2017; Haeussler et al., 2017). 3D high-resolution geodynamic models show that the flat slab drives motion of the Wrangell block fore-arc sliver (Jadamec et al., 2013; Haynie and Jadamec, 2017) and contributes to fault parallel motion along the eastern Denali fault and convergence along the apex of the fault (Haynie and Jadamec, 2017) (Figure 2). However, when model predictions of the Wrangell block motion and the difference in Denali fault parallel motion are compared with observations, model predictions are lower, suggesting that the flat slab alone is not sufficient enough to explain the broad deformation zone of Alaska (Haynie and Jadamec, 2017). Thus, it is thought that the neotectonics of south-central Alaska are predominantly driven by the subduction-collision of the buoyant Yakutat oceanic plateau (Bird , 1988; Plafker et al., 1994b; Fitzgerald et al., 1995; Ratchkovski and Hansen, 2002b; Bemis and Wallace, 2007; Chapman et al., 2008; Haeussler , 2008; Jadamec et al., 2013; Lease et al., 2016; Haynie and Jadamec, 2017). 4D numerical modelling of this process is currently underway.

Figure 2: Top: map of south-central Alaska (zoomed in from Figure 1) with model predicted velocities (blue arrows) from Haynie and Jadamec (2017) plotted on top. Bottom: percent of slab contribution from Haynie and Jadamec (2017) models to observed Denali fault slip rates (modified from Haynie and Jadamec (2017)). Results from Haynie and Jadamec (2017) show that the slab drives northwest and counter-clockwise motion of the Wrangell block fore-arc sliver and contributes to an average of 20-28% of motion along the Denali fault. The flat slab exerts the largest contribution to motion along the eastern segment of the fault, where surface motion parallels the fault, and also along the central segment of the fault, where the slab is driving the Wrangell block into the North American backstop and subducting obliquely to the fault.


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The lost Tethyan seaways: A deep-Earth and deep-time perspective on eastern Tethyan tectonics

The lost Tethyan seaways: A deep-Earth and deep-time perspective on eastern Tethyan tectonics

Every 8 weeks we turn our attention to a Remarkable Region that deserves a spot in the scientific limelight. Following from the first entry which showcased the Eastern Mediterranean, we move further east, and back in time, to the realm of the Tethys. The post is by postdoctoral researcher Sabin Zahirovic of the EarthByte Group and Basin GENESIS Hub, The University of Sydney.

Sabin Zahirovic

The southern and southeastern region of Eurasia (Fig. 1) represents one of the most tectonically complex areas in the world, with active deformation that appears as flurries of sometimes-deadly earthquakes, as well as the slower processes of mountain building. This region truly represents geophysical extremes, with the world’s tallest mountain ranges (namely, the Himalayas), and the planet’s deepest surface depressions (namely, the Mariana Trench), which collectively demonstrate the ongoing tectonic processes that are shaping, and re-shaping, our planet’s surface. Embedded in every rock, grain, mineral, and fossil in the region is the record of ancient tectonic activity, with a mosaic of exotic terranes sutured onto the continental margin. The Tethys was a vast oceanic domain at the southern margin of Eurasia and parts of its eastern history are now preserved in sutures onshore. The sutures are often peppered with ophiolites (Fig. 2), which are fragments of ancient ocean basins and gateways, the rest of which have been lost to subduction and now reside deep in the mantle. The opening and closure of these ocean basins has also been implicated in fundamental shifts of oceanic circulation and long-term sea level, which highlights the importance of better understanding the tectonic evolution of our planetary surface and deep churning interior.

Figure 1. Shuttle Radar Topography Mission data of the eastern Tethyan region displayed in the GPlates web portal, with 50X vertical exaggeration of elevation. (EarthByte Group and Scripps Institution of Oceanography)

Figure 2. Regional map highlighting the sutures (pink and blue lines) demarcating the terranes of the eastern Tethyan tectonic domain, as well as the ancient ocean basins that have since been consumed by subduction. Figure from Zahirovic et al. (2016b).

The first step to rewinding the tectonic clock requires undoing seafloor spreading in preserved ocean basins. Coincidentally, the geoscience community is celebrating the 50th anniversary of plate tectonics this week, with the understanding of seafloor spreading being a critical component of the plate tectonics paradigm. Through the work of numerous scientists such as Fred Vine, Harry Hess, Drummond Matthews, Dan McKenzie, amongst others, the ideas of seafloor spreading began to take shape. Another visionary who has received less acknowledgement is Marie Tharp, who painstakingly collected and manually plotted sonar measurements from voyages criss-crossing the Atlantic. Marie pieced together the morphology and topology of the mid-oceanic ridge systems (Fig. 3), which were mysterious bathymetric features at the time. When Marie superimposed earthquake epicentres on her maps, and suggested that this could be the missing piece of the continental drift puzzle, her significant insights were initially dismissed. However, Marie’s perseverance and determination laid much of the foundations in our understanding of these bathymetric features representing regions of seafloor spreading, from which a rush of subsequent work established the principles of plate tectonics.

Figure 3. Marie Tharp’s ground-breaking bathymetric map highlighting the elevated mid-oceanic ridge systems, which were later proved to be regions of seafloor spreading (© Marie Tharp 1977). Photograph inset of Marie in 2001 (Columbia University).

Following decades of data collection, including magnetic polarity reversal mapping of the oceans, the age of the oceanic crust and other features of the seafloor fabrics were compiled into global digital community models (e.g., Müller et al., 1997; Matthews et al., 2011). With these two ingredients, one can generate digital models of plate tectonic reconstructions, where the seafloor spreading of the ocean basins can be reversed to reveal the ancient configuration of continents (e.g., the reconstruction of Seton et al., 2012 using open-source community plate reconstruction software, GPlates, (Boyden et al., 2011)). However, due to the conservation of surface area (i.e., the Earth is not expanding), large gaps emerge that represent ancient subducted ocean basins.

As the Atlantic, Southern and Indian oceans opened, it occurred at the expense of the youngest (the Neo-Tethys), which was subducted along the southern margin of Eurasia. This process left a vast chain of arc volcanoes and orogens, stretching from the Alps in Europe to Sundaland in Southeast Asia (Fig. 1). However, only patchy evidence remains of how the continental terranes traversed the Tethyan ocean basins (Fig. 2), with existing methods largely relying on sedimentary affinities, fossil co-occurrences, and paleo-latitudinal estimates from paleomagnetic data (e.g., Metcalfe, 1994; Torsvik and Cocks, 2004). With the advent of modern seismic techniques (e.g., Li et al., 2008), it was possible to “image” the Earth’s mantle using seismic tomography (akin to a medical scan, Animation 1). This led teams of researchers to interpret the mantle structure (Fig. 4), and infer the history of subduction in the absence of preserved data on the surface (e.g., Hafkenscheid et al., 2006; van der Voo et al., 1999). In order to test interpretations of the India-Eurasia convergence that consumed the eastern Neo-Tethyan ocean basin, efforts were then launched to try and reproduce the India-Eurasia mantle structure using numerical models of mantle convection (e.g., Jarvis and Lowman, 2005).

Figure 4. Schematic interpretations of the India-Eurasia subduction history (A) using inferences from seismic tomography (van der Voo et al., 1999) (B), which were then tested using simple numerical experiments of mantle convection (Jarvis and Lowman, 2005) (C). Figures modified from Zahirovic et al. (2016b).

Animation 1. East-west sweep through the eastern Tethyan mantle, highlighting cold subducted slabs (blue) representing ancient Tethyan oceanic lithosphere (tomography model from Li et al., 2008).

With advances in community software platforms to model the evolution of entire plates and their boundaries through time, as well as a near-exponential increase in supercomputing resources, it became easier to model the coupled plate-mantle system in a time-evolving 3D spherical shell (e.g., using tools such as Aspect, CitcomS, and others). These advances enabled us, our colleagues, and the wider geo-community, to track subducting slabs from a complex and evolving global network of plate boundaries, including those associated with the eastern Tethyan tectonic domain. The new models provided an additional insight into the interaction of the deep Earth and surface processes, especially in such a tectonically-complex region like Southeast Asia.

For more than a decade, research has highlighted that Australia’s northern continental margin and the Sundaland continental promontory were low-lying and partially-flooded regions, likely due to the influence of sinking lithospheric plates from the eastern Tethyan slab graveyard (DiCaprio et al., 2009; DiCaprio et al., 2011; Spasojevic and Gurnis, 2012). What the models of mantle flow have revealed is that this effect, known as “dynamic topography”, is a transient signal through space and time, depending on the position of the continents in relation to large-scale mantle upwellings or downwellings. In the case of the eastern Tethys, these mantle downwellings were ephemeral, affecting the relative contribution of global sea level change and regional uplift or subsidence that drove shoreline retreat and advance over the continents. For Southeast Asia, we now think that the whole region was elevated and entirely emergent from about 80 to 40 million years ago, largely due to a hiatus in subduction along southern Eurasia between ~80 and 60 million years ago (Fig. 5). These notions complement traditional ideas of tectonic topography (where the surface experiences uplift or subsidence due to collisional or rifting processes), and allow us to consider the additional role of mantle flow (upwellings and downwellings contributing to several hundred meters in elevation change) over geological time in shaping the surface evolution of our planet.

Figure 5. Palaeogeographic reconstructions highlight that Southeast Asian topography was very different in the past, one dominated by a contiguous and emergent landmass about 80 to 40 million years ago, despite higher sea levels than today. Numerical models of the eastern Tethyan region suggests that the absence of subduction between 80 and 60 million years ago led to a “dynamic rebound” of the Sundaland continental promontory. However, the region became flooded again from about 40 million years ago because of a resumption in subduction, which led to regional subsidence, even with falling long-term sea levels. Figure adapted from Zahirovic et al. (2016a).

The plate tectonic reconstructions (Animation 2), along with the mantle flow models, have added significant insights into the evolution of the plate-mantle system. However, one emerging area of study is the exploration of how the shifting tectonic regimes have influenced other Earth systems – whether it be climate, ocean circulation, or biological evolution. In terms of the eastern Tethys, further research will help us uncover the role of tectonics and mantle flow in understanding the deep carbon cycle (at the plate-mantle scale), which has the potential to reveal the flux of carbon between shallow and deep Earth reservoirs, and their influence on deep-time climate (e.g., Jagoutz et al., 2016; Kent and Muttoni, 2013). For example, as the Neo-Tethys ocean basin was consumed, huge amounts of CO2 were emitted through a vast network of convergent plate boundaries that hosted arc volcanoes, leading to warmer greenhouse conditions. However, once India crashed into these subduction systems, the buoyant continental crust jammed and shut down subduction, turning off a major input of CO2 into the atmosphere. In addition, the mountain building processes in the Alpine-Himalayan mountain belt sequestered enormous amounts of CO2 from the atmosphere through chemical weathering of silicate rocks, influencing (and perhaps controlling) long-term cooling of the planet in the last 45 million years. These components collectively highlight the need for more integrative work that unpacks the complexity of regions like the eastern Tethys in order to understand the interaction of physical, chemical, and biological processes that have shaped planetary evolution.

Images from Animation 2 – please click the link below to download the movie file (10MB)


Animation 2 (please click the link above to download and view). The latest plate tectonic reconstructions of the eastern Tethys consider both the evolution of the continents, as well as the oceanic plate that carried it, much like a conveyor belt on the Earth’s surface. The reconstructions are embedded in global models with evolving plate boundaries (GPlates software), and are easier linked to numerical models of mantle convection (Zahirovic et al., 2016b), which provide an additional avenue to test our interpretations of past continental and oceanic arrangements

Note from author: This summary is by no means comprehensive, and represents a personal perspective of plate tectonics and eastern Tethyan geodynamics.


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