GD
Geodynamics

Remarkable Regions

Remarkable Regions – The Kenya Rift

June 13, 2018
Remarkable Regions – The Kenya Rift
Image Credit: modified by Anne Glerum after NASA Earth Observatory/Joshua Stevens.

Every 8 weeks we turn our attention to a Remarkable Region that deserves a spot in the scientific limelight. After looking at several convergent plate boundaries, this week the focus lies on part of a nascent divergent plate boundary: the Kenya Rift. The post is by postdoctoral researcher Anne Glerum of GFZ Potsdam.

Of course an active continental rift is worthy of the title “Remarkable Region”. And naturally I consider my own research area highly interesting. But after seeing it up-close and personal on a recent 10-day trip organized by the University of Potsdam, Roma Tre and the University of Nairobi (stay tuned for the travel log, or read that of the University of Potsdam), I must say, the Kenya Rift is a truly beautiful and fascinating region.

Figure 1. Topography (Amante and Eakins 2009) and kinematic plate boundaries (Sarah D. Stamps based on Bird 2003) of the East African Rift System (EARS). Plate boundary colors schematically indicate the western and eastern branches of the EARS.

Constituting one segment of the 5000 km long East African Rift System (EARS, Fig. 1), the Kenya Rift is host to an amazing landscape, wildlife and people, all of which somehow tie back to continental rifting processes. Although the youngest rifting phase in Kenya commenced in the Miocene, the east African region as a whole has been shaped by rifting episodes since Permian times (Bosworth and Morley 1994). The present active rift system runs from the Afar region in the north all the way south to Mozambique and is split into a western and an eastern branch that run around the Archean Tanzanian Craton (Chorowitz 2005, see Fig. 1). Generally speaking, the western branch is more seismically active, but deprived of magmatism, compared to the eastern branch, of which the Kenya Rift is part (Chorowitz 2005). Three processes characterize the EARS (Burke 1996) as well as the Kenya Rift specifically: normal faulting, volcanism and uplift.

Uplift

The Tanzanian Craton together with the enveloping western and eastern EARS branches constitutes the broad, uplifted area coined the East African Plateau (~1200 m elevation, Strecker 1991; Simiyu and Keller 1997, Fig. 2). The onset of uplift of this plateau can be constrained to the Early Miocene with the help of one of the longest phonolitic lava flows on Earth (> 300 km, Wichura et al. 2010; 2011) and a whale that stranded inland 17 Ma (and was only recently found again after going missing for 30 years, Wichura et al. 2015). Plume-lithosphere interaction is thought responsible for the uplift (e.g. Wichura et al. 2010), although there is disagreement about the continuity of the low seismic velocity anomalies seen in the east African upper mantle and whether they are connected to the lower mantle. For example Ebinger and Sleep (1998), Hansen et al. (2012), Sun et al. (2017) and Torres Acosta et al. (2015) advocate for one East African superplume, while Pik et al. (2006) distinguish separate lower and upper mantle plumes and Davis and Sack (2002) and Halldórsson et al. (2014) consider a lower mantle plume splitting in the upper mantle.

Figure 2. Topography (Amante and Eakins 2009) and fault traces (GEM) of the central EARS. Triangles indicate off-rift volcanoes, dotted grey lines the three segments of the Kenya Rift.

Magmatism and volcanism

The northward motion of Africa over this hot mantle anomaly has been thought the cause of a north-to-south younging trend in the age of the ensuing EARS volcanism and rifting (e.g. Ebinger and Sleep 1998; George et al. 1998; Nyblade and Brazier 2002), although more recent studies arrive at a more spatially disparate and diachronous rifting evolution (Torres Acosta et al. 2015 and references therein). In general, massive emplacement of flood-phonolites preceded the onset of rifting in Kenya around 15 Ma (Torres Acosta et al. 2015). With ongoing rifting, and localization of faulting towards the rift axis, volcanism also migrated towards the center of the rift. Since the Miocene, massive amounts of volcanics have thus been emplaced (144,000-230,000 km3, MacDonald 1994; Wichura et al. 2011). Moreover, dyking also accommodated a significant part of the extension, with 22 to 26 % of the crust in the rift valley being composed of dykes (MacDonald 2012). Not surprisingly, the highlands directly around the rift valley, the Kenya Dome (Fig. 2) formed through a combination of volcanism and uplift (Davis and Slack 2002) with elevations of up to 1900 m.

The composition of rift magmatism is bimodal, showing phonolites and trachytes on the one side and nephelinites and basalt on the other, predominantly resulting from fractional crystallization of a basaltic source. The low viscosity of these magmas allows the young volcanoes in the volcano-tectonic axis to reach significant heights (see Fig. 3; MacDonald 2012). The most impressive volcanoes are to be found outside of the rift however (Fig. 2), with Mnt. Elgon reaching 4321 m and Africa’s highest mountains Mnt. Kenya and Mnt. Kilimanjaro reaching up to 5200 m and 5964 m, respectively (Chorowitz 2005).

Figure 3. View on the crater rim of the 400 ky old Mnt. Longonot volcano in the tectono-magmatic rift axis, at 2560 m asl. Courtesy of Corinna Kallich, GFZ Potsdam.

Normal faulting

The Kenya rift itself is composed of 3 asymmetric segments, distinguished by sharp changes in their orientation (Chorowitz 2005, Fig. 2). The 2300-3000 m high Elgeyo, Mau and Nguruman escarpments result from the steep Miocene east-dipping border faults in the west, while the antithetic border faults on the eastern side formed later during the Pliocene (Strecker et al. 1990). The older border faults formed along preexisting foliation generated by the Mozambique Belt orogeny in the late Proterozoic (Shackleton 1993; Hetzel and Strecker 1994). A change in strike of this foliation from NNE in the northern and southern Kenya rifts to NW determined the change in orientation in the central Kenya rift (Strecker et al. 1990). Consequently, different generations of faults in the northern and southern rift segments run parallel, while in the central segment, the Pleistocene change in extension direction from ENE-WSW/E-W to the present-day WNW-ESE/NW-SE directed extension results in obliquely reactivated border faults and younger, en echelon arranged left-stepping NNE-striking fault zones along the rift axis (Strecker et al. 1990). Extension is transferred between the different zones by coeval normal and strike-slip faulting or dense sets of normal faults.

Figure 4. View of lake Magadi and the Nguruman escarpment. Lake Magadi is a saline, alkaline lake, commercially mined for trona. Courtesy of Corinna Kallich, GFZ Potsdam.

Human evolution

The uplift, volcanism and normal faulting together have set the stage for human and animal evolution. For example, the shift in hoofed mammals from eating predominantly woods to grazing species evidences that the large-scale uplift modified air circulation patterns resulting in aridification and savannah-expansion at the expense of forested areas (Sepulchre et al. 2006; Wichura et al. 2015). The rift basins enabled the formation of large lakes, which were subsequently compartmentalized by tectonic and volcanic morphological barriers (Fig. 4). On the short-term, lake coverage varied due to tectonically induced changes in catchment areas, drainage networks and outlets. Maslin et al. (2014) actually found a correlation between this ephemeral lake coverage and hominin diversity and dispersal. Lake highstands link with the emergence of new species and allowed the spread of hominins north and southward out of east Africa. Remarkable, or what!

References:
Amante, C. and Eakins B. W., 2009. NOAA Technical Memorandum NESDIS NGDC-24. National Geophysical Data Center, NOAA.
Bosworth, W. and Morley, C.K., 1994.  Tectonophysics 236, 93–115.
Burke, K., 1996. S. Afr. J. Geol. 99 (4), 339–409.
Chorowitz, J., 2005. J. Afr. Earth Sci. 43, 379-410.
Davis, P. M. and Slack, P. D. 2002. Geophys. Res. Lett. 29 (7), 1117.
Ebinger, C.J. and Sleep, N.H., 1998. Nature 395, 788-791.
George, R. et al., 1998.  Geology 26, 923–926.
Halldórsson, S. A. et al., 2014. Geophys. Res. Lett. 41, 2304–2311,
Hansen, S. E. et al., 2012.  Earth Planet. Sc. Lett. 319-320, 23-34.
Hetzel, R., Strecker, M.R., 1994. J. Struct. Geol. 16, 189–201.
Macdonald, R. et al., 1994a. J. Volcanol. Geoth. Res. 60, 301–325.
Macdonald, R., et al., 1994b. J. Geol. Soc. London 151, 879–888.
MacDonald, R., 2012. Lithos 152, 11-22.
Maslin, M. A. et al., 2014. Quaternary Sci. Rev. 101, 1-17.
Nyblade, A. A. and Brazier, R. A., 2002. Geology 30 (8), 755-758.
Pik, R. et al., 2006. Chem. Geol. 266, 100-114.
Sepulchre, P. et al., 2006. Science, 1419-1423.
Shackleton, R.M., 1993. Geological Society, London, Special Publications 76, 345–362.
Simiyu, S.M., Keller, G.R., 1997. Tectonophysics 278, 291–313.
Strecker, M., 1991. Das zentrale und südliche Kenia-rift unter besonderer berücksichtigung der neotektonischen entwicklung, habilitation, Universität Fridericiana.
Sun, M. et al., 2017.  Geophys. Res. Lett. 44, 12,116–12,124.
Torres Acosta, V. et al., 2015. Tectonics 34, 2367–2386.
Wichura, H. et al., 2010. Geology 38 (6), 543–546.
Wichura, H. et al , 2011. The Formation and Evolution of Africa: A Synopsis of 3.8 Ga of Earth History, eds. D. J. J. Van Hinsbergen, S. J. H. Buiter, T. H. Torsvik, C. and Gaina, S. J.
Wichura, H. et al., 2015. P. Natl. Acad. Sci. USA 112 (13), 3910-3915.

Remarkable Regions – The India-Asia collision zone

December 20, 2017
Remarkable Regions – The India-Asia collision zone
Image Credit: modified by Anne Glerum after NASA Earth Observatory/Joshua Stevens.

Every 8 weeks we turn our attention to a Remarkable Region that deserves a spot in the scientific limelight. This week we zoom in on a particular part of the eastern Tethys as Adina Pusok discusses the India-Asia collision zone. She is a postdoctoral researcher at the Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, UCSD, US.

Without doubt, one of the most striking features of plate tectonics and lithospheric deformation on Earth is the India-Asia collision zone, largely comprised of the Himalayan and Karakoram mountain belts and the Tibetan plateau. What makes this collision zone so remarkable? For one, Tibet is the largest, highest and flattest plateau on Earth with an average elevation exceeding 5 km, and it includes over 80% of the world’s land surface higher than 4 km. Then, the bordering Himalayas and the Karakoram Mountains include the only peaks on Earth reaching more than 8 km above sea level.

It makes one wonder, how can such a mountain belt and high plateau form? Most of the major mountain belts and orogenic plateaus on Earth are found within the overlying plate of subduction and/or collision zones (e.g. the Alps, the Andes, the American Cordilleras etc.). When an ocean closes and two continental plates meet at a destructive (subduction) boundary, the continents themselves collide. Such collisions result in intense deformation at the edges of the colliding plates. Neither continent can be subducted into the mantle due to the buoyancy of continental crust, so the forces that drive the plate movement prior to collision are brought to act directly on the continental lithosphere itself. At this stage, further convergence of the plates must be taken up by deforming one or both of the plates of continental lithosphere over hundreds of kilometres [Figure 1]. Mountain belts can form under these circumstances.

Figure 1 Global map of surface velocities and the second invariant of strain rate (from Moresi [2015]). The surface velocities show the location and extent of plates, and the strain rate map highlights the fact that most of the deformation is concentrated at plate boundaries (high strain rates), while the continental interiors have little or no deformation (low strain rates). In some places, deformation occurs over broader regions, especially following mountain belts. These boundaries are called diffuse plate boundaries. The white rectangle roughly indicates the extent of the India-Asia collision zone.

The Himalayas and the Tibetan plateau are no different. Following the closure of the Tethys ocean (see earlier blog post), the Indian continent collided with Eurasia around 50 million years ago (e.g., Patriat and Achache [1984]), thus giving rise to this anomalously high region. This tectonic boundary is complex and changes character along its length. The Tibetan plateau is a collage of continental blocks (terranes) that were added successively to the Eurasian plate during the Paleozoic and Mesozoic [Figure 2]. The boundaries between these terranes are marked by scattered occurrences of ophiolitic material, which are rocks characteristic of oceanic lithosphere. The Himalayas represent the traditional accretionary wedge formed by folding and thrusting of sediments scraped off the subducting slab.

Figure 2 Simplified tectonic map of Tibet and surrounding region showing approximate boundaries of the major terranes, suture zones, and strike-slip faults (from Ninomiya and Bihong [2016]). Blocks and terranes: ALT-EKL-QL: Altyn Tagh–East Kunlun–Qilian terrane; BS: Baoshan terrane; HM: Himalayan terrane; IC: Indo-China block; KA: Kohistan Arc terrane; LA: Ladakh arc terrane; LC: Lincang–Sukhothai–Chanthaburi Arc terrane; LST: Lhasa terrane; NCB: North China Block; NQT: North Qiangtang terrane; QT: Qiangtang terrane; SP: South Pamir terrane; SPGZ: Songpan-Ganze terrane; SQT: South Qiangtang terrane; TC: Tengchong terrane; TSH: Tianshuihai terrane; WB: West Burma terrane; WKL: West Kunlun terrane. Suture zones: BNS: Bangong-Nujiang Suture; EKLS: East Kunlun Suture; ITS: Indus-Tsangbo Suture; JSS: Jinsha Suture; LSS: Longmu Tso–Shuanghu–Menglian–Inthanon Suture; WKLS: West Kunlun Suture. Basins: QB: Qaidam Basin; KB: Kumkol Basin. Faults: ALT: Altyn Tagh Thrust; ALTF: Altyn Tagh Fault; KKF: Karakorum Fault; LMST: Longmen Shan Thrust; MFT: Main Frontal Thrust; NQLT: North Qilian Thrust; RRF: Red River Fault; SGF: Sagaing Fault; XXF: Xianshui River–Xiaojiang Fault.

Interestingly, the India-Asia collision orogen is not just the youngest and most spectacular active continent collision belt, it is also the most studied research area on Earth. Studies on this region span a wide range of topics and methods for over more than 100 years. I am not sure if it is the fascination with the highest mountain on Earth (Mt. Everest was actually climbed for the first time as late as 1953 by Tenzing Norgay and Edmund Hillary), similar to our fascination for exploring the Moon, Mars and the other planets in our Solar System nowadays, or the hope that studying the youngest orogeny will help us decipher the older ones (soon to realize different mountain belts evolve differently).

To understand the magnitude of the work done in the past 100 years, a simple search of the keywords “India Asia collision” on Google Scholar yielded ~90k results, and a more focused geosciences search on Web of Science (where I filtered the results to those from geophysics, geochemistry, geology, geosciences multidisciplinary only) yielded >1600 results for the same keywords (other keywords: “Himalaya” > 5600 results, “Tibet” > 6500 results, “India Asia” > 2200 results). These numbers can be intimidating to a new student taking on the topic, but it is a topic worth studying and I’ll explain why below.

From a general perspective, it is important to study the India-Asia collision zone due to the interaction between tectonics and climate and the formation of the Indian monsoon [Molnar et al., 1993], but also because it is a highly populated area (>200 million people in the Hindu Kush Himalaya region) regularly shaken by natural phenomena, such as earthquakes, floods or landslides. For example, the last large earthquake in Nepal, the Gorkha earthquake (Mw 7.8) in April 2015 caused more than 9000 deaths.

From a geophysics point of view, understanding mountain-building processes and the driving forces of plate tectonics has been one of the long-term goals of solid Earth sciences community. The India-Asia collision zone is one of the best examples in which subduction, continental collision and mountain building can be studied in a global plate tectonics perspective. Prior to plate tectonics theory, Argand [1924] and Holmes [1965] thought that the Himalayan Mountains and Tibetan Plateau had been raised due to the northern edge of the Indian craton underthrusting the entire region, causing shortening and thickening of the crust to ∼80 km. This perspective remains widely accepted, but recent ideas suggest that other processes are equally important (more below).

Today, the challenge lies in refining our understanding of the dynamics of India-Asia collision by elucidating the connections between the wealth of observations available and the underlying processes occurring at depth. Decades of study have produced data sets across various disciplines, including: active tectonics, Cenozoic geology, seismicity, global positioning system (GPS) measurements, seismic profiles, tomography, gravity anomalies, mantle-crustal anisotropy, paleomagnetism, geochemistry or magnetotelluric studies. Of these, the GPS data stands out as it clearly shows the distributed deformation across the entire collision zone and suggests that this is a highly dynamic area [Figure 3].

Figure 3 Horizontal GPS velocities of crustal motion around the Tibetan Plateau relative to stable Eurasia from Liang et al. [2013].

Collectively, all these observation data sets stand as a different piece in the puzzle of the India-Asia collision. However, the same data sets can support a number of competing and sometimes mutually exclusive mechanisms for the uplift of the Tibetan Plateau. For example, the mantle lithosphere beneath Tibet has been proposed to be cold, hot, thickened by shortening, or thinned by viscous instability. Other controversies include the degree of mechanical coupling between the crust and deeper lithosphere and the nature of large-scale deformation. It is no surprise then, that several hypotheses emerged over time trying to explain the anomalous rise of the Himalayas and Tibetan Plateau [Figure 4]:

  1. Figure 4 Schematic cartoons of tectonic models proposed to explain the thickening and uplift of the Himalayas and the Tibetan Plateau. (Source: personal institutional web page of A. Ozacar).

    Wholescale underthrusting of the Indian plate below the Asian continent [e.g. Argand, 1924].

  2. The thin-sheet model or distributed homogeneous shortening [e.g. England and McKenzie, 1982].
  3. Homogeneous thickening of a weak, hot Asian crust, involving a large amount of magmatism [e.g. Dewey and Burke, 1973].
  4. Slip-line field model to account for the brittle deformation in and around the Tibetan Plateau and to explain extrusion of SE Tibet away from Indian continent [e.g. Molnar and Tapponnier, 1975]. The same group also proposes a time-dependent model for the growth of Tibetan plateau [e.g. Tapponnier et al., 2001], in which successive intracontinental subduction zones maintain the stepwise growth and rise of the plateau.
  5. Lower crustal flow models for the exhumation of the Himalayan units and lateral spreading of the Tibetan plateau [e.g. Royden et al., 1997, Beaumont et al., 2001].
  6. Delamination or convective removal of the lithospheric mantle that induced isostatic movement, lifting the Tibetan Plateau [e.g. Molnar, 1988].

 

These models were applied either to the Tibetan Plateau or the Himalayan mountain belt and were able to explain the formation of specific tectonic and geological features. However, there is no conclusive answer on which of the hypotheses works best for the entire orogen, and instead, more questions arise:

  • Which forces are at work during continental collision and mountain building?
  • What is the deformation history and evolution of this plate boundary?
  • How was the subduction accommodated in the Neo-Tethys?
  • How does subduction evolve during continental collision?
  • What drives the present-day fast convergence (~4-5 cm/yr) between India and Eurasia?
  • Which forces propagated India northwards between 70-50 million years at anomalously high speeds (up to 16 – 20cm/yr)?
  • How can you form such large elevations over such extended areas?
  • What is the effect of surface processes on uplift?
  • What is the structure at depth beneath the Himalayas and Tibetan plateau?
  • How do the Indian and Eurasia plates deform during collision?
  • How is the deformation accommodated during continental collision?
  • How do mountain belts form and why not all mountain belts look the same?
  • How did the crust beneath Himalaya and Tibet reach double-crustal thickness (normal continental crust is 35-40 km thick, whereas the crust beneath the Himalaya and Tibet is 70-100 km thick)?
  • Which mechanisms help sustain the high topographic amplitudes?
  • Why should an area as broad as the Tibetan Plateau be uplifted so high compared to other mountain belts following collision?
  • Did the Tibetan Plateau and Himalayan mountain belt rise continuously or diachronously?
  • Which the proposed models [Figure 4] can be applied, and where?
  • How do lithospheric heterogeneities and rheology affect the deformation pattern?
  • What is the degree of mechanical coupling between the crust and deeper lithosphere? Is it the “jelly sandwich” model (e.g., Burov and Watts [2006]) or the “creme-brulee” model (e.g., Jackson [2002], see earlier blog post)?
  • Why do the Himalayas have a convex curvature?
  • What about the high deformation of the prominent Himalayan syntaxes (the inflection points of the Himalayan belt): Nanga Parbat in the west and Namche Barwa in the east?
  • What is the effect of the India-Asia collision on climate? Do the Himalayas affect the Indian monsoon or is it the other way around? A chicken-and-egg question.

Seriously, can I even stop asking questions? The question that fascinated me the most during my graduate studies was “Why is the Himalayan-Tibet region so high and broad compared to other mountain belts?”. If we tune our models to Earth parameters, can we build such large elevations in computer simulations? Which factors and forces are at play? Using 3-D numerical models to address this question [Pusok and Kaus, 2015], we were able to obtain distinct topographic modes (different types of mountain belts) [Figure 5] and to show that building topography is an interplay between providing the energy to the system and the ability of that system to store it over longer periods of time. We also suggest that the reason why Himalaya-Tibet is different from the Alps, for example, is because the shape and elevation of mountain ranges can vary depending on the boundary conditions (plate driving forces that control convergence velocity and lithospheric heterogeneities such as the Tarim Basin) and internal factors (rheology), but also on the evolution stage they are in.

To sum up, it is clear that many of the above questions remain unanswered. But I think this is good news, meaning that in the future, exciting new results will shape our understanding of this remarkable region.

Figure 5 3-D Simulation results showing different modes of surface expressions in continental collision models. Modified from Pusok and Kaus [2015].

References:
Argand, E. (1924). La tectonique de l’Asie. Proc. 13th Int. Geol. Cong., 7:171–372.

Beaumont, C., Jamieson, R. A., Nguyen, M. H., and Lee, B. (2001). Himalayan Tectonics Explained by Extrusion of a Low-Viscosity Crustal Channel Coupled to Focused Surface Denudation. Nature, 414:738–742.

Burov, E. B. and Watts, A. B. (2006). The long-term strength of continental lithosphere: “jelly sandwich” or “crème brûlée”? GSA Today, 16(1):4.

Dewey, J. F. and Burke, K. (1973). Tibetan, Variscan, and Precambrian Basement Reactivation: Products of Continental Collision. The Journal of Geology, 81(6):683–692.

England, P. and McKenzie, D. (1982). A Thin Viscous Sheet Model for Continental Deformation. Geophys. J. R. astr. Soc., 70:295–321.

Holmes, A. (1965). Principles of Physical Geology. The Ronald Press Company, New York, second edition.

Jackson, J. (2002). Strength of the Continental Lithosphere: Time to Abandon the Jelly Sandwich? GSA Today, 4–9.

Liang, S., Gan, W., Shen, C., Xiao, G., Liu, J., Chen, W., Ding, X., and Zhou, D. (2013). Three-dimensional velocity field of present-day crustal motion of the Tibetan Plateau derived from GPS measurements. Journal of Geophysical Research: Solid Earth, 118:1–11.

Molnar, P. and Tapponnier, P. (1975). Cenozoic Tectonics of Asia: Effects of a Continental Collision. Science, 189:419–426.

Molnar, P. (1988). A Review of Geophysical Constraints on the Deep Structure of the Tibetan Plateau, the Himalaya and the Karakoram, and their Tectonic Implications. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 326(1589):33–88.

Molnar, P., England, P., and Martinod, J. (1993). Mantle Dynamics, Uplift of the Tibetan Plateau, and the Indian Monsoon. Reviews of Geophysics, 31:357–396.

Moresi, L. (2015). Computational Plate Tectonics and the Geological Record in the Continents. SIAM News, 48:1–6.

Ninomiya, Y. and Bihong Fu, B. (2016). Regional Lithological Mapping Using ASTER-TIR Data: Case Study for the Tibetan Plateau and the Surrounding Area. Geosciences 2016, 6(3), 39; doi:10.3390/geosciences6030039.

Patriat, P. and Achache, J. (1984). India-Eurasia collision chronology has implications for crustal shortening and driving mechanism of plates. Nature, 311:615–621.

Pusok, A. E. and Kaus, B. J. P. (2015). Development of topography in 3-D continental-collision models. Geochemistry, Geophysics, Geosystems, 16(5):1378–1400.

Royden, L. H., Burch el, B. C., King, R., Wang, E., Chen, Z., Shen, F., and Liu, Y. (1997). Surface Deformation and Lower Crustal Flow in Eastern Tibet. Science, 276(5313):788–790.

Tapponnier, P., Zhiqin, X., Roger, F., Meyer, B., Arnaud, N., Wittlinger, G., and Jingsui, Y. (2001). Oblique Stepwise Rise and Growth of the Tibet Plateau. Science, 294(5547):1671–1677.

 

Alaska: a gold rush of along strike variations

November 22, 2017
Alaska: a gold rush of along strike variations
Image Credit: modified by Anne Glerum after NASA Earth Observatory/Joshua Stevens.

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

October 4, 2017
The lost Tethyan seaways: A deep-Earth and deep-time perspective on eastern Tethyan tectonics
Image Credit: modified by Anne Glerum after NASA Earth Observatory/Joshua Stevens.

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)

Animation2_SupplementaryAnimation2_Zahirovic_etal_ESR_agegrid

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.

References

Boyden, J., Müller, R., Gurnis, M., Torsvik, T., Clark, J., Turner, M., Ivey-Law, H., Watson, R., and Cannon, J., 2011, Next-generation plate-tectonic reconstructions using GPlates, in Keller, G., and Baru, C., eds., Geoinformatics: Cyberinfrastructure for the Solid Earth Sciences: Cambridge, UK, Cambridge University Press, p. 95-114.

DiCaprio, L., Gurnis, M., and Müller, R. D., 2009, Long-wavelength tilting of the Australian continent since the Late Cretaceous: Earth and Planetary Science Letters, v. 278, no. 3, p. 175-185.

DiCaprio, L., Gurnis, M., Müller, R. D., and Tan, E., 2011, Mantle dynamics of continentwide Cenozoic subsidence and tilting of Australia: Lithosphere, v. 3, no. 5, p. 311-316.

Gurnis, M., Turner, M., Zahirovic, S., DiCaprio, L., Spasojevic, S., Müller, R., Boyden, J., Seton, M., Manea, V., and Bower, D., 2012, Plate Tectonic Reconstructions with Continuously Closing Plates: Computers & Geosciences, v. 38, no. 1, p. 35-42.

Hafkenscheid, E., Wortel, M., and Spakman, W., 2006, Subduction history of the Tethyan region derived from seismic tomography and tectonic reconstructions: Journal of Geophysical Research-Solid Earth, v. 111, no. B8, p. B08401.

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