The Amerasian Basin Puzzle
11 July 2005 - 11 August 2005Project aims
The surface of the Earth is made of rigid plates which have been moving through time and are still in motion today. Both modern and ancient plate motions are responsible for creating the Arctic Ocean as we know it. However the ‘ancient’ Arctic Ocean was significantly smaller than it is today. In fact, prior to about 130 million years (Ma) ago, the Arctic Ocean did not exist!
The physical geography of the Arctic in the past (Arctic palaeogeography) is well constrained to approx. 55 Ma, based on seafloor magnetic anomalies formed during the opening of the Eurasian Basin (for example Jackson and Gunnarsson, 1990). Our knowledge of the Arctic region, derived from magnetic anomalies associated with the opening of the Amerasian Basin at ca. 130 Ma, is poor. Prior to 130 Ma other tools than magnetic anomalies are needed for palaeogeographic reconstructions. Fortunately, tectonic processes such as collision, rifting and subsidence leave distinctive signatures in the geologic record, for example orogenic belts, ophiolites and oceanic basins. Using these signatures our work attempts to unravel the palaeogeographic puzzle resulting from the long and complicated tectonic evolution of the greater circum-Arctic region (see Gee 2005, Pettersson, 2005, Pease, 2003, Pease, 2004, Gee and Pease, 2000, Gee, 1999). This work is also part of other international research efforts addressing the regional tectonic evolution of circum-Arctic palaeogeography: CASP (Circum-Arctic Sediment Provenance) and PLATES & GATES (Plate Tectonics and Polar Gateways).
Left: Polar view of the Arctic Ocean with the Eurasian and Amerasian Basins labelled (after Jackson and Gunnarsson 1990). Chukotka, Wrangel and the New Siberian Islands are thought to have rifted from the Canadian margin during opening of the Amerasian Basin. S – Svalbard; NZ – Novaya Zemlya; FJL – Franz Josef Land; SZ – Severnaya Zemlya; NS – New Siberian Islands; W – Wrangel Island; B – Banks Island; E – Ellesmere Island. Region of figure 1 to the right indicated.
Right: Regional extent of Okhotsk-Chukotka volcano-plutonic belt (OCVB) (after Stone et al. 1992). Locations of basecamps 1 and 2 (BC1, BC2) shown. South Anyui suture,SAS (after Natal’in et al. 1999); NS – New Siberian Islands; W – Wrangel Island. Region of picture 2A indicated.
To unravel Arctic palaeogeography we need to understand the development of the Amerasian Basin (figure 1 left). This also has important implications for global economic resources, since significant proven and potential oil reservoir rocks also occur in Arctic shelf regions, as well as for the global climate system, because cold water flowing from the Arctic and Antarctic Oceans drives global ocean circulation. During the expedition Beringia 2005 our investigations focussed on the Okhotsk-Chukotka volcano-plutonic belt (OCVB) of the Chukotka Peninsula in eastern Russia. The OCVB extends over 3 000 km across Asia (figure 1 right) and its volcanic deposits have been recognized as far away as Alaska (Bergman et al., 1995)! It was generated during the collision between oceanic and continental crust in a geologic setting similar to the modern Andean margin of South America, possibly as the result of the opening of the Amerasian Basin. Consequently this vast and largely unexplored region is important for understanding the development of the Amerasian Basin.
Fieldseason 2005
After a flight from Stockholm via Thule (Greenland) and Fairbanks (Alaska), courtesy of a Swedish military Hercules, the expedition cleared customs in Provideniya, Russia. Our international group of experts consisted of American, Russian, and Swedish participants, as well as Swedish Polar Research Secretariat personnel (picture 1). We worked from two base camps, one at Novoye Chaplino and the other near Kolyuchin Bay. The expedition experienced generally fine weather at both base camps, consequently we achieved most of our scientific objectives.
On 11th July we arrived at base camp 1 in Novoye Chaplino (BC1 on figure 1 left and picture 2A). Novoye Chaplino is a fishing village and staying there was luxurious: a house with electricity and hot water, plus the village boasted a general store! We worked in the surrounding region using a 4WD vehicle or an aluminium boat with a small outboard engine.
A. Simplified geologic map of Novoye Chaplino area superimposed on satellite image. Okhotsk-Chukotka volcano-plutonic belt (OCVB) and older basement (pre-OCVB).
B. Pepperite (BC1). Dark, fragmented sediment scattered within a volcanic matrix (pencil for scale).
On 1st August, we relocated via helicopter to our second basecamp along the Kal’kheurerveem River, near the southern end of Kolyuchin Bay (BC2 on figure 1 right). We spent the remainder of our field season working from this location, with plenty of time to do a thorough job. Here we experienced typical camp life with tents, sleeping bags, small gas burners for cooking, cold water baths, etc. During a few days of rain we saw the small creek next to our camp turn into an impassable torrent! We supplemented our diet with fresh mushrooms (boletus) and humpback salmon. Without vehicular support our work at this location required the collection of geological data and samples by hiking about 15–20 km/day. As the terrain was hilly, the hiking was strenuous.
After about 3 weeks of fieldwork, we re-joined the other Beringia 2005 participants on the Swedish icebreaker Oden. It was one of the highlights of the expedition for us, with a dramatic ship-board landing of the huge Russian MI-8 helicopter, followed by long-anticipated saunas!
Preliminary results
Basecamp 1. At BC1 our research focused on exposures around Tkachen Bay and the Chaplino Peninsula (figure 1 right and picture 2A). Outcrop exposure is generally good, enabling us to establish relative age and deformation relationships. A large component of our work in the coming months will be determining the absolute ages of these units (using radiometric dating techniques on single crystals) in order to constrain the tectonic evolution of the region. This work will be performed at NORDSIM, the Nordic ion-microprobe facility in Stockholm.
At BC1 rocks of the Okhotsk-Chukotka volcano-plutonic belt (OCVB) are widespread. The OCVB comprises intrusive rocks informally known as the Rumilet granites and their inferred extrusive equivalents, thick ignimbrite ash flows. New data from samples just east of our study area provide an age of about 85 Ma for Rumilet plutonism (J. Wright, unpublished data). The inferred correlative volcanic rocks in the southwestern part of the OCVB have similar ages (85–75 Ma; Hourigan and Akinin 2004), indicating that OCVB developed over a relatively short time interval. The OCVB clearly post-dates the opening of the Amerasian Basin by about 45 Ma.
Relatively flat-lying volcanic rocks of the OCVB sit unconformably on steeply dipping, metamorphosed and deformed basement. This ‘older’ basement includes volcanic, sedimentary, and crystalline rocks of unknown age. These are intruded by plutons informally known as the Provideniya granodiorite. The volcanic rocks are composed predominantly of greenschist facies air-fall tuffs and welded tuffs, probably from a coastal setting as water-lain debris flows, isoclinal ‘wet-sediment’ folding, and pepperite are common within the sequence (picture 2B). The pepperite is an important unit because it contains hematized granitic clasts at its base which can be used to determine the age of the older basement in this region. This older basement also contains both mafic and felsic igneous material metamorphosed at amphibolite facies. We do not know the time of metamorphism or tilting of these rocks, consequently determining the age of the heamatized granitic material and the crystalline basement is important for constraining the tectonic evolution of these units.
A. Magma mingling within the Providenia granodiorite (BC1). Darker granodioritic material (note rounded edges) mixing with lighter granitic material (centimeter scale).
B. Pillow basalt (BC2). Solid line approximates pillow outline (pencil for scale).
At several localities we saw magma mixing/mingling textures within Provideniya granodiorite (picture 3A), evidence of a hybrid genesis. Provideniya granodiorite is also hornblende-bearing. Both hybrid and amphibole-bearing magmas are common features of Andean-type collision zones. New data from rocks just east of our study area indicate an age of about 120 Ma for Provideniya plutonism (J. Wright, unpublished data). It is possible that subduction-related, Andean-type magmatism generated at 120 Ma was the result of plate motion reorganization due to the opening of the Amerasian Basin at about 130 Ma. It remains to be shown whether or not this collisional setting was continuous from 120–85 Ma. Age and geochemical analyses will help to test this hypothesis.
Basecamp 2. Existing geologic maps of the region around BC2 were contradictory, so we were not quite sure what the region had in store for us! Our research focused along the river, subsidiary drainages, and the craggy mountain tops, where exposures of fresh rock were best and it was soon apparent that the more recent (unpublished) map was correct with respect to the rocks present.
The region is characterized by a large volume of gabbro associated with minor basalt, pillow basalt (picture 3B) and terrigenous sediment (sandstone and shale). The rocks are mapped as the Velmay Terrane (Sokolov, 1992), an association of chert, basalt, and clastic sediment inferred to be about 225 Ma in age on the basis of fossils (Monotis, Holobia etc). However, we were unable to locate the cited fossil locality. South of the river, the mafic rocks and the sediments both record a sub-horizontal foliation and kink bands. We are not yet sure of the significance of this deformation, but it may be associated with a tectonic boundary to the south. The Velmay Terrane is intruded by igneous rocks of the OCVB and is therefore older than the OCVB.
The association of gabbro and pillowlava among clastic sediments suggests Mezosoic-age rifting, but what was rifting? We collected samples of both the mafic rocks and the OCVB in order to constrain the time of intrusion and the associated deformation here, as well as to determine the absolute age of rifting. We collected samples of sandstone from the Velmay Terrane for provenance analysis to determine what was rifting. We also collected samples of the gabbro for geochemistry in order to determine the tectonic environment in which it formed. All of this analytical work is now underway.
Concluding remarks
Our research requires patience, as we generally target a single location per year. Circum-Arctic projects cover a very large area and it takes many years to get to each important location where another piece of the puzzle can be obtained. The years 2007–2008 have been established as the International Polar Years (IPY). The IPY is designed to increase awareness of the importance of the Earth’s polar regions (Arctic and Antarctic), which are significant not only for societal resources (petroleum, fishing, shipping lanes, etc.), but also for the planet’s biodiversity, environmental monitoring, etc. The circum-Arctic region will continue to be a focus of our research during the IPY, when we hope to investigate the ‘other side’ of the Amerasian Basin in Arctic Canada.