The Mapenduma Anticline: “Wood-chipper” erosion along a tropical thick- skin mountain belt

by Philip S. Prince, Virginia Division of Geology and Mineral Resources

The island of New Guinea offers outstanding examples of just about any type of type of tectonic process you might want to see, and the Mapenduma Anticline (MA from here on out!) of West Papua, the Indonesian-administered northwest half of the island of New Guinea, is no exception. The MA hosts hosts the highest peak on New Guinea, Puncak Jaya (or Carstensz Pyramid), which rises to ~4,800 m from a near sea-level foreland basin, making it the highest island peak on Earth (more on this “island” definition at the end). Puncak Jaya hosts small glaciers at its summit, which is interesting because of New Guinea’s equatorial latitude. The Grasberg Mine, the world’s largest gold mine and second largest copper mine, is developed into rocks of the MA at 4100 m above sea level. It’s easily located in the image below by looking for the HUGE spray of white sediment on the foreland plain at lower right, and just following the river to the mountaintops.

mapenduma blog1
The Mapenduma Anticline stretches for ~300 km along the western front of New Guinea’s central mountain range. For a sense of scale, it’s about 25 km from the gray mountaintops to the left of “Anticline” to the outer edge of rugged, dark green topography to the right of “Anticline.”

All of these features reflect the tectonic development of the MA and its interplay with extremely intense but relatively localized tropical erosion. I use the term “wood-chipper” in the title because erosion of the MA thrust sheet has been intensely focused at its leading edge, somewhat like pushing a tree limb into a chipper. Rocks at the leading edge of the thrust sheet are rapidly removed by erosion driven by meters of annual rainfall, but the back limb of the thrust sheet remains relatively un-eroded and preserves sedimentary limestone cover rocks that were never buried particularly deeply.  Cloos et al. (2005) and Hill et al. (2004) provide good discussions of this process. The yet-to-be-eroded cover limestones are clearly visible in Google Earth–they are the gray area along the crest of the mountain range in the image below.

map model comparison

map summary pic 2
Structure of the Mapenduma Anticline is interpretive, but most papers agree on a general model supported by exposures along the road to the Grasberg Mine. A generalized cross section through the top image would likely show features presented in the model below. It’s a fairly simple geometry–just a huge, thick, basement-involved thrust sheet whose leading edge is lost to erosion. Minor deformation of sediment cover and syn-orogenic sediment occurs in the footwall of the main thrust fault.

This video puts the process in motion. Note how lots of material is removed from the leading edge of the anticline, just above the basal thrust, and not much is removed further to the left of the model to preserve cover rocks at the highest topography. This is the focused, “wood-chipper” concept. The model is only intended to provide an illustration of the basic ideas involved in MA evolution. Thicknesses are not perfectly scaled, and no existing normal fault in the basement rock is used to nucleate the MA basal thrust. Instead, a weak base layer (white microbeads) was used in a small area at the base of the model to localized development of the basal thrust.

The MA is a young feature–most studies of the area fit its development into the last 8 million years or so, meaning its basal thrust accommodated a few kilometers of movement every million years and erosion along the mountain front was equally efficient over comparable time scales (again, check out Cloos et al., 2005). Lots of rock is missing from the leading edge of the MA–a general conceptual projection of the missing material might look like the image below, which most likely underestimates the length of thrust sheet lost. Note also that the location of the deformation front with respect to the main basal fault suggests that the eroded material was removed as quickly as it was uplifted. If the thrust sheet had advanced across the footwall prior to being eroded, the deformed area would be more broad.

map projection final
A conceptual projection of what already got eaten by the erosional wood-chipper. Minimal overturned forelimb is illustrated, after Hill et al. (2004).  Again, it’s about 25 km from the gray mountaintops to the foot of the rugged zone. This projection probably illustrates about 20 km of removed line length, and should probably illustrate even greater erosional loss. Angle of the basal thrust would alter how much displacement is needed to expose rocks seen at the surface today.

Because of the location of the Grasberg Mine, Google Earth imagery is very good along a portion of the MA. It allows the cover limestones at the mountain crest to be viewed in great detail, along with the highly erosive rivers on the mountain front and the deposition of their load on the foreland, particularly where sediment flux is amplified by the mine (which is not good…).

map limestone
Bedding is clearly visible in Miocene cover limestones in the vicinity of Puncak Jaya, along the 4.5+ km spine of the central mountain range. YouTube has some Puncak Jaya climbing videos, and some show first-hand images of these rocks in good detail. These are also worth a look in Google Earth, as some structure and lots of oriented fracturing/jointing are visible.
map incision
An extremely steep river cascading down the deeply eroded portion of the MA. Scale is hard to appreciate, but the river drops over 400 m across the series of falls at the center of the image. A low-flow condition is shown here; a zoomed-in view shows a widely scoured gorge that clearly carries monstrous flows across this steep gradient.
map deposition
Rivers with exceptional transport capacity on the steep gradient of the mountain front deposit their loads on the flat foreland basin surface. The huge braid plain here results from bad practices at Grasberg, which has had numerous issues with environmental malpractice and a  poor relationship with some of the local population.

The MA is considered a “thick-skin” feature because it rides on a thrust fault rooted deep in continental crustal, or basement, rocks, represented by the thick zone of blue and gray material deep in the sand model.

basement and cover
Gray-and-blue material represents deeper sedimentary (and possibly metasedimentary) rocks and undelying continental crust. Here in the model, erosion has  breached into the basement just above the basal fault system. 

In contrast, “thin-skin” orogenic systems involve thrust faulting rooted in shallower sedimentary rocks above the basement. The basal fault of the MA probably descends 10’s of kilometers below the surface, rooting in a ductile shear zone in the continental crust at considerable depth. The flat bottom of the sand model  actually prevents an accurate representation of the fault trajectory, as it should continue to descend to greater and greater depth towards the left side of the model. There are, of course, plenty of thick-skin orogens on Earth, with the Laramide-style ranges of the American Rockies being outstanding and well-exposed examples. MA is interesting because the abruptness of its topographic front and localized, extreme syn-tectonic erosion permit a cross-sectional view of a big slab of crustal rock, with some nice cover rock still on top of it.

thick blog final

thin final
The thick-skin style (top) is defined by basement-involved thrusting all the way to the deformation front. Thin-skin tectonics (bottom) involve faulting restricted to shallower sedimentary units (purple and white). Basement thrust sheets exist in thin skin systems, but they are more distant from the deformation front and have ramped onto sedimentary glide planes. This image shows a thin-skin model superimposed onto the MA model frame.

In my opinion, a better way to define the thick-skin nature of settings like the MA is to say that basement rock is involved in thrusting nearly all the way to the deformation front, the leading edge of folding and/or faulting in the orogen. Basement thrust sheets are certainly present in thin-skin systems like the Appalachians or Bolivian sub-Andes, but they have ridden onto shallower sedimentary glide surfaces and remain several thrust sheets away from the deformation front.

map def front

andes def front
A comparison of the Mapenduma Anticline front with the sub-Andes of Salta Provnice, Argentina. The sedimentary fold belt in the sub-Andes is very broad, while disturbance of cover rocks and syn-orogenic sediment occurs over just a few kilometers in Mapenduma, where basement-invovled faulting effectively occurs at the structural front.

Different pre-compression conditions seem are thought to favor these different styles of thrust belt development (Pfiffner (2006) is a good read), and crustal architecture has been invoked to explain the contrasts between the thick-skin MA system and the more thin-skin Papua Fold Belt in Papua New Guinea to the southeast (well-illustrated  in Figs. 9-13 here:  http://searg.rhul.ac.uk/pubs/hill_hall_2003%20New%20Guinea%20margin%20evolution.pdf) .

The MA has developed against strong, rigid Australian continental crust, while the Papua Fold Belt has developed above thinned, broken continental crust that had accumulated a greater thickness of sedimentary fill. These contrasts are actually visible in Google Earth with bathymetry.

map tectonic setting

This image also qualifies the “highest island peak on Earth” description of Puncak Jaya. At low Pleistocene sea levels, New Guinea was not an island. The Arafura Sea separating it from Australia is only 10’s of meters deep, and would have been dry land at lowstands over 100 meters below modern sea level, so “island” is a very relative term over even short periods of geologic history. Even so, the MA is a really cool feature that nicely demonstrates a variety of intersecting processes.

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