National Science Foundation Workshop
December 9-12, 2004
Stanford University, Stanford CA

Report from the Verkhoyansk-Kolyma Breakout Group

Group Members:

Introduction

The Kolyma-Verkhoyansk orogen is one of the most spectacular examples known of Mesozoic continental assembly by accretion of micro-continental blocks and magmatic arcs. The entire landmass (and continental shelves) located east of the Verkhoyansk fold-and-thrust belt was added to the Siberian craton since the Late Jurassic (Parfenov and Natal’in, 1977). This area is also remarkable for the extensive and varied magmatism which accompanied the accretionary process and for its complex structural patterns including a major oroclinal loop at its core. In addition, a portion of the North America-Eurasia plate boundary must cross this region, yet its exact location is not known making this region one of the last gaps in our understanding of the present-day plate framework. Our knowledge of the make-up of this immense region took a major leap forward as a result of a multinational effort that compiled a terrane map for the Northern circum-Pacific in the 1990’s (Nokleberg et al., 1994). Several modern tectonic models for the area have been published by participants in that project (Parfenov, 2001; Nokleberg et al., 1998; Sengor and Natal’in, 1996), but many unresolved tectonic problems were also identified (see Verkhoyansk-Kolyma presentation and Research Targets below).

A unique opportunity: The 2DV Geophysical Transect

The Russian Ministry of Natural Resources (RMNR), in collaboration with the North East Interdisciplinary Science Research Institute of the Russian Academy of Sciences (NEISRI) is acquiring a geophysical profile, including deep crustal seismic data which, once completed, will extend from the coast of the Sea of Okhotsk to the Arctic coast. The total length of this line will be 2400 km. The southern 830 km of the transect have already been acquired and processed. Work on the northern parts will continue until 2007. This major Russian geophysical effort could go a long way towards to elucidating the crustal scale architecture and geodynamic evolution of the orogen, particularly if it serves a the catalyst for the acquisition of new state-of-the-art geological, geochemical, isotopic and neotectonic data aimed at solving the major outstanding scientific questions.

Action Proposed

The combined Verkhoyansk-Kolyma and Magmatism Break out groups joined forces devised a plan of action to take advantage of the on-going geophysical work. The strategy will be propose to the NSF Continental Dynamics that they fund a major project with a two-pronged approach to the problem:

The acquisition of an ancilliary deep crustal seismic line in collaboration with RMNR, NEISRI (Magadan) and the Siberian Branch Russian Academy of Sciences ( Yakutsk). This profile would be oriented approximately east-west from the Kolyma-Omolon accreted terranes to the Siberian Craton. This proposed line which will utilize one of the few existing roads across this orogen is approximately orthogonal to the main structures. It is designed to elucidate:

• the deep structures associated with collision and accretion of a crustal block within this orogen,
• the roots of the main thrust belt where it involves the deepest part of the Siberian margin sedimentary record, and
• the relationship of these deep crustal structures to a major belt of S-type granites intruded coeval with microcontinental collision.

We also propose to carry out targeted multidisciplinary geological and geophysical investigations along key portions of the two ongoing seismic profiles to address specific geodynamic problems and aid the interpretation of the geophysical data. We envision a project similar in scope to the Trans Alaskan Crustal Transect (TACT) ( (e.g. Fuis et al., 1997). Our proposed project, following TACT and the later Bering Strait Transect (Klemperer et al., 2002) would complete our understanding of the geodynamic processes that created our common circum-North Pacific margin and its orogenic belts. Like the previous two transects, the Trans Russian Arctic Crustal Transect (TRACT) will provide new insight into the plate tectonic tug-of-war between Arctic and Pacific plate motions.

Research Targets

Although any number of key tectonic problems and geodynamic issues could be addressed within this vast and geologically varied region, we will focus on a few key outstanding topical questions whose resolution will be of interest to a wide range of geoscientists.

1. Sea of Okhotsk margin

The Okhotsk-Chukotka Volcanic Belt

The 2-DV profile established that a 55 km-thick root of highly reflective lower crust underlies the Okhotsk-Chukotka Volcanic Belt (OCVB) in spite of the lack of evidence for any tectonically-driven crustal shortening in the area during and after its emplacement. This suggests that magmatic underplating was likely the main process involved in creating this crustal welt, and this may be one of the key places world-wide where the problem of deciphering the nature of magmatic additions to the crust (underplating) could be addressed from supracrustal levels to the deep crust and mantle history. This is a topic that can be readily addressed by the completion of ongoing studies on the geochemistry, isotopic compositions and ages of the erupted lavas in the OCVB belt along the 2-DV and proposed geophysical transects and by petrologic modeling of the evolution of the magmas. Geologic mapping establishing location of eruptive centers, age and volume of eruptive products together with geochemistry and isotope geochemistry can be integrated with reflection and velocity data from the lower crust and mantle.

Tertiary Extension

OCVB magmatism in the Magadan region of the 2-DV transect ended with the eruption of basaltic lavas at 74 ± 1.2 Ma (Hourigan and Akinin, 2004). Although most OCVB lavas are flat-lying, along the coast near Magadan a series of normal fault-bound basins and tilted basement blocks have been identified. These half-graben systems continue offshore. This area is also underlain by the thinnest continental crust imaged anywhere along the 2-DV profile (about 32 km thick). Documentation of the offset and the age of these fault systems with low-temperature thermochronologic methods (apatite fission track and U-Th/He dating) would provide critical information on the timing and the process leading to crustal thinning and thus shed light on the controversy of how the Sea of Okhotsk developed, and how this might be linked, geodynamically, to Pacific margin plate motions.

2. Kolyma Region

Proterozoic stratigraphy of the Prikolyma block is remarkably similar to units in southwestern Laurentia (Arizona and New Mexico) (Sears et al., 2004). This correlation had major implications for reconstruction of Proterozic supercontinents and their subsequent evolution. In the Siberia-SW Laurentia reconstruction, Siberia can move from a position adjacent to SW Laurentia to its Permian collision with the Urals by rotating about a pole near Baffin Island along margin-parallel transforms. Northern Siberia could drive the Devonian-Carboniferous orogenies in the Arctic on the way to its Permian collision.. This model is consistent with available paleomagnetic data and can be tested through detrital zircon geochronology and detailed dialed dating of volcanic units. This work would be carried out along the Kolyma river, where outcrops are excellent, in the vicinity of the 2DV transect.

3. South Anyui Suture (SAS)

The SAS separates the terrane collage of the Kolyma-Omolon from Arctic Alaska-Chukotka (AAC) which is believed to be a fragment rifted from North America in the Early Cretaceous. The suture is marked by several complex ophiolites, fragments of arcs, and by intense deformation of the continental margin of AAC. Despite the pivotal role the SAS has played in Arctic plate tectonic models (e.g. Lawver et al., 2002), it is not well understood (Sokolov et al., 2002). The fact that 2-DV will cross this boundary is singularly important as it will reveal its crustal-scale signature. We propose to address the characterization and timing of closure and deformation of the South Anyui Zone by two means: 1) A comparison of the timing of deformation in well exposed sequences on either side of the SAZ and 2) A comparison of the sandstone petrography and detrital zircon geochronology of Jurassic to Cretaceous sediments and their source areas from both sides and within the SAZ.

4. Kolyma-Verkhoyansk Transect

In’yali Debin Zone

The In’yali-Debin zone represents the suture between the Kolyma-Omolon superterrane and the Verkhoyansk margin (area 2 on Fig. 2). Detailed structural analysis, and 40Ar/ 39Ar geochronology are necessary to constrain the kinematics and timing of deformation associated with the final collision, and to correctly interpret the structures imaged by the proposed seismic line. In addition, detailed U-Pb geochrocronology, carried out with the Stanford/USGS SHRIMP-RG, of the isotopic signatures of the S-type collisional granite batholiths, dikes swarms and of xenoliths included in Cretaceous lamprophyre dikes, will elucidate the role of magmatism during collision and will reveal the composition and age of basement which is not exposed on the surface.

Verkhoyansk fold-and-thrust belt

The proposed seismic line will cross the fold and thrust belt long the boundary between the rocks that are part of the North Asia Craton and the Okhotsk terrane, an independent Proterozoic basement block. Collision of the Okhotsk block with the craton has been proposed as the driving mechanism for shortening in the South Verkhoyansk. This event presumably was independent from the collision of the Kolyma-Omolon superterrane and therefore the timing may be different. Structural and thermochronologic studies, using 40Ar/ 39Ar and apatite fission track techniques, will help constrain the timing of deformation in different portions of the orogen and will help arrive at a consistent tectonic model.

References

References

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Hourigan J.K., and Akinin V.V., 2004, Tectonic and chronostratigraphic implications of new 40Ar/39Ar geochronology and geochemistry of the Arman and Maltan-Ola volcanic fields, Okhotsk-Chukotka volcanic belt, northeastern Russia. Geological Society of America Bulletin, V.116. no. 5/6; h.637-654..

Klemperer, S.L., Miller, E.L., Grantz, A., Scholl, D.W., and the Bering-Chukchi working group, 2002, Crustal structure of the Bering and Chukchi shelves: Deep seismic reflection profiles across the North American continent between Alaska and Russia, in, Miller, E.L., Grantz, A. and Klemperer, S.L., Tectonic Evolution of the Bering Shelf-Chukchi Sea-Arctic Margin and Adjacent Landmasses, Geological Society of America Special Paper 360, 387p, Plates and CD, p. 1-24.

Lawver, L.A., Grantz, A. and Gahagan, L.M., 2002, Plate Kinematic evolution of the present Arctic region since the Ordovician, in, Miller, E.L., Grantz, A. and Klemperer, S.L., Tectonic Evolution of the Bering Shelf-Chukchi Sea-Arctic Margin and Adjacent Landmasses, Geological Society of America Special Paper 360, 387p, Plates and CD, p. 333-358.

Miller, E.L., Grantz, A., and Klemperer, S., 2002, eds., Tectonic evolution of the Bering Shelf-Chukchi Sea-Arctic Margin and adjacent landmasses, Geological Society of America Special Paper 360, 387p.

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Nokleberg, W J; and 35 others,1998, Summary terrane, mineral deposit, and metallogenic belt maps of the Russian Far East, Alaska, and the Canadian Cordillera, Open-File Report - U. S. Geological Survey, Report: OF 98-0136, 1 disc, 1998.

Parfenov, L.M., and Natal’in, B.A., 1977, Mesozoic-Cenozoic tectonic evolution of northeastern Asia. Transactions (Doklady) of the USSR Academy of Sciences: Earth Science Sections, v.235, no. 1-6, p.89-91.

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