Tectonophysics [T]

 CC:Hall E  Wednesday  0800h

Tectonophysics General Contributions I Posters

Presiding:  J Lowman, University of Toronto; S Butler, University of Saskatchewan


Earthquake (1935 Timiskaming M6.2) Triggered Slumps in Lake Kipawa, Western Quebec Seismic Zone, Canada

* Daurio, L (05daurio@utsc.utoronto.ca), University of Toronto at Scarborough Department of Geology, 1265 Military Trail SW435, Scarborough, Ont M1C1A4, Canada
Doughty, M (doughty@utsc.utoronto,ca), University of Toronto at Scarborough Department of Geology, 1265 Military Trail SW435, Scarborough, Ont M1C1A4, Canada
Eyles, N (eyles@utsc.utoronto.ca), University of Toronto at Scarborough Department of Geology, 1265 Military Trail SW435, Scarborough, Ont M1C1A4, Canada

The Western Quebec Seismic Zone (WQSZ) includes the urban areas of Montreal and Canada's national capital Ottawa. It is characterised by frequent large magnitude intracratonic earthquakes (e.g., 1732, M5.8; 1935, M6.2 and 1944, M5.2) centred along the Timiskaming and Ottawa-Bonnechere grabens but seismic risk analysis is challenged by short instrumental records and long recurrence intervals. The M6.2 1935 Timiskaming Earthquake is the largest recorded to date and was felt over some 1.3 million km2 of eastern North America with many aftershocks of magnitude 4 to 5. Its epicentre lies below the western margin of Lake Kipawa in the area where a major crustal boundary (the Grenville Front Tectonic Zone) crosses the Timiskaming Graben. A high-resolution 'chirp' reflection survey of the late glacial and postglacial sediment infill of Lake Kipawa reveals a clear record of earthquake-related ground shaking. Widespread slumps record the down slope failure of the entire late glacial and postglacial stratigraphy indicating that the 1935 temblor was the largest in this area. Systematic mapping of landslides identifies they extend across an area of 600 km2 around the 1935 epicentre. Lakes cover a large area of eastern Canada; a regional-scale survey of lake floors could constrain historic epicentres and postglacial seismic history of the heavily populated WQSZ.


Analogue Modelling of Evaporite Diapirs on Ellef Ringnes Island, Canadian Arctic Archipelago: First Results

* Macauley, J (macauley@geology.utoronto.ca), Department of Geology, University of Toronto, 22 Russell St, Toronto, ON M5S 3B1, Canada
Cruden, A (cruden@geology.utoronto.ca), Department of Geology, University of Toronto, 22 Russell St, Toronto, ON M5S 3B1, Canada
Brent, T (Tom.Brent@NRCan-RNCan.gc.ca), Geological Survey of Canada, 3303 33rd St NW, Calgary, AB T2L 2A7, Canada
Stephenson, R (r.stephenson@abdn.ac.uk), Department of Geology and Petroleum Geology, University of Aberdeen, Meston Building, King's College, Aberdeen, AB24 3EU, United Kingdom

Ellef Ringnes Island is located in the Sverdrup Basin, a pericratonic trough that began rifting in the Carboniferous. The island contains several ovate domes composed of evaporites derived from the Carboniferous Otto Fiord formation. The domes consist of 300-500 m thick, relatively undeformed anhydrite caps underlain by halite cores of unknown thickness with sheared anhydrite margins. The initiation and rate of vertical migration of the diapirs are poorly constrained due to the lack of high quality seismic images, particularly at depths of 8-13 km, at which the evaporite source layer is thought to occur. Buoyancy is argued to provide a fundamental driving force for salt tectonics, however the presence of dense anhydrite caps reduce the effect of buoyancy driven diapirism. Several processes have been suggested for the initiation of salt movement in the Sverdrup Basin, the two most accepted being reactive diapirism due to rift related extension and differential loading from prograding sedimentary sequences. Interpretation of available seismic reflection profiles reveal asymmetric dome geometries with gently arched sedimentary sequences located on their western margins and rim synclines and sediment thickening on their eastern margins. From these observations we propose that basement faults played a significant role in the formation of asymmetric dome structures by acting as planes of weakness that controlled the initiation and direction of salt movement. Preliminary results from scaled 2D analogue models are presented that focus on (1) the emplacement of competent and relatively undeformed blocks of dense anhydrite on top of buoyant salt structures, and (2) the role of basement faults in producing asymmetric dome geometries. The experiments involve forced injection of fluids into a downbuilding overburden of sand as analogues for syn-sedimentary diapiric growth.


The High Obliquity Paradigm

* Sanchez, A (andrea@fisica.edu.uy), Departamento de Astronomia, ICF, Facultad de Ciencias., Igua 4225, Montevideo, Uruguay
Sanchez Bettucci, L (leda@fcien.edu.uy), Area Geofisica-Geotectonica, Instituto de Ciencias Geologicas, Igua 4225, Uruguay

Since the seventy decades, George Williams considers the high Earth's obliquity as the cause for major freezing episodes and other geological relationships. Williams (2008) claimed a high obliquity scenario to explain near equatorial latitude until a rapid decrease of 30º, without a clear cause, ca. 630Ma. During the Neoproterozic, evidences show extreme cooling in the Earth occurs when the continents are shifted at low latitudes. Geological records show evidences related to early freezing Earth (2.4-2.2Ga). Frozen events have astronomical related explanation, where the albedo was the key-factor of the climate. In addition, some authors link the early Proterozoic global freezing with the star-rate formation since the origin of the Earth, during a starburst 2.4-2.0Ga. Also, through numerical analysis was suggested a correlation between the star-rate formation near the Sun, and the occurrence and duration of glaciations over the Earth evolution. From the geological opinion the orogenic events in the Neoproterozoic are associated with important volcanism related with amalgamation processes, which have effects on vertical motions of the crust that produce major effects on climate. These long-term perturbations generate cooling and sea-level fluctuations. The volcanic activity was especially strong (A-type subduction) in Gondwana and Eurasia at Neoproterozoic. At 630Ma occurred a peak in the igneous activity (e.g. Braziliano-Pan African Orogen when the Marinoan glaciation occurs). Regarding the albedo, the eruption of just one volcanic eruption causes climatic change on a scale from months to years. Large eruptions, associated with orogenic processes produce stratospheric volatiles, which difficult the solar radiation achieves the Earth's surface and, consequently, major volcanic activity could drive a cooling effect. From biological viewpoint, the presence of life, as soil-forming microorganisms about 3Ga, accelerates the trapping processes of CO2 in soils. The consequence was the reduction of the atmospheric content of this greenhouse gas, decreasing the temperature. The interaction of astronomical, geological and biological issues could explain a global freezing scenario without extreme hypothesis.


Retrodeformation Anaysis of Quaternary Fault in the Southeastern Part of Korean Peninsula

* Choi, S (sjchoi@kigam.re.kr), Sung-Ja Choi, 30 Gajeong-dong Yusoeng-gu, Daejon, 305-350, Korea, Republic of
Chwae, U (chwae@naver.com) AB: The Suryum fault (F1) with subsidiary branch (F2, 3) varies in strike from N40°E to N14°E and dip < 68-45-25-8°SE toward the surface. New evidence helps redefine the frequency of faulting events and allows calculation of net slip rates. A retrodeformation analysis based on stratigraphic correlation and optical stimulated luminescence (OSL) dates of colluvial deposits reveals four-time faulting events during the latest Pleistocene: reverse →listric normal→reverse→normal. Net slips are in the range 100-130 cm for F1 at a dip angle of 68°-45° and 85 cm for F2 at a dip angle of 45°. Based on OSL ages of 23,000 ± 1,200 and 12,000 ± 2,000 yr, we calculate net slip rates for F1 and F2 ranging from 0.03 to 0.08 mm/yr. A reactivation slip rate on F1 is 0.15~0.4 mm/yr over the last 12,000 ± 2,000-2,800 ± 200 yr. The Suryum reverse faulting might link to Mw ~6-7 earthquakes during the late Pleistocene, based on net slip rates comparable to those of other intraplate regime examples.


Active Neotectonic Structures in Glacial and Postglacial Sediment in Lake Timiskaming, Timiskaming Graben, Ontario/Quebec Canada

Doughty, M (doughty@utsc.utoronto.ca), School of Geography and Earth Sciences McMaster University, General Science Building Room 206 1280 Main Street West, Hamilton, ON L8S 4K1, Canada
* Eyles, N (eyles@utsc.utoronto.ca), Department of Geology University Of Toronto Scarborough, 1265 Military Trail, Toronto, ON M1C 1A4, Canada
Eyles, C (eylesc@mcmaster.ca), School of Geography and Earth Sciences McMaster University, General Science Building Room 206 1280 Main Street West, Hamilton, ON L8S 4K1, Canada

The Timiskaming Graben (TG) is a northwest-trending arm of the Ottawa-Bonnechere Graben and the St. Lawrence Rift System (SLRS) in eastern Canada. Together they form a 600 km long failed rift in the Canadian Shield, extending southward along the border of Ontario and Quebec to the St.Lawrence River Valley onto the Hudson Valley and Lake Champlain in the USA. The Timiskaming Graben preserves faulted outliers of Early Paleozoic limestones and has been reactivated several time during the Phanerozoic most recently during the breakup of Pangea. The 110 kilometre-long, ~100 m deep Lake Timiskaming fills the inner part of the Timiskaming Graben along the border of Ontario and Quebec. It is the postglacial successor to glacial Lake Barlow ponded against the northward-retreating Laurentide Ice Sheet some 9,000 years BP. The sedimentary record of Lake Timiskaming was established by collecting more than 1000 line kilometres of high-resolution 'chirp' seismic profiles, side scan and multibeam survey data between 2003 and 2007. These show that bathymetric relief is the product of ongoing tectonic subsidence where lateglacial Barlow glaciolacustrine and postglacial sediments are extensively deformed by closely-spaced horst and grabens. The greatest subsidence has occurred within a narrow (< 3 km) and deep (up to 209 m) central graben basin. We are able to infer the presence of hitherto unrecognized bounding and relay faults within the graben, and a 20 km long 8 m high fault scarp and sand blows produced by large postglacial earthquakes. The region is one of the most seismically active areas in eastern North America (Western Quebec Seismic Zone) with frequent moderate to large magnitude (> M5) intracratonic earthquakes. Structural activity is ongoing along the Timiskaming Graben and its lateglacial and postglacial sediment record provides the clearest evidence to date of modern intracratonic faulting anywhere in eastern North America.


La Tuna Complex: A possible Neoproterozoic ophiolite

* Peel, E (elenapeel@usp.br), Pós-graduação em Geociencias (Bolsista CAPES) - Instituto de Geociéncias, Universidade de São Paulo., Rua do Lago 562, SP, Brazil., Brazil
Basei, M (baseimas@usp.br), Instituto de Geociencias - Universidade de São Paulo, Rua do Lago 562, SP, Brazil
Sanchez Bettucci, L (leda@fcien.edu.uy), Area Geofisica-Geotectonica, FC-UDELAR, Igua 4225, Uruguay

The Dom Feliciano Belt was developed during West Gondwana amalgamation as a result of collision between the Rio de la Plata and Kalahari cratons. In that context, an association of mafic-ultramafic rocks embedded in mylonites is developed. Based on lithotectonic association it is defined here as La Tuna Ophiolitic Complex. Preliminary petrographic and chemical analysis (mineral assemblages and whole rock chemistry) suggest an ophiolitic nature. This complex is constituted by highly tectonized mafic/ultramafic rocks, including serpentinite, amphibolite and metabasalt. Micaschists and quartzites are also present in the area in contact with the mafic- ultramafic rocks. The serpentinites present accessory magnetite and chromite granules mostly transformed into ferritchromite. Amphibolite occurs as dark green fine-to medium-grained rocks commonly associated with serpentinite. Metabasalt occurs as grayish green aphanitic rock with massive or slightly foliated appearance, generally transformed into chlorite and/or tremolite-actinolite schist. The metamorphism is typically greenschist to lower amphibolite-facies. La Tuna Complex is disrupted by E-W and W-NW shear zones and strike-slip faults. Basement structures dip gently; is strongly deformed and shuffled by low-angle thrusting and subhorizontal shearing. This Complex may represent remnants of Neoproterozoic ocean floor destroyed at some stage during Gondwana amalgamation.


Friction Rheology and Afterslip on the Chelungpu Decollement Inferred From Postseismic Deformation of the 1999 Chi-Chi Earthquake

* Chen, P (peggie1028@hotmail.com), Institute of Seismology, National Chung Cheng University, 168 University Rd., Min- Hsiung Chia-Yi ,621, Taiwan R.O.C.,
Wang, W (seiwhwg@eq.ccu.edu.tw), Department of Earth and Environmental Sciences, National Chung Cheng University, 168 University Rd., Min-Hsiung Chia-Yi ,621, Taiwan R.O.C.,
Hsu, Y (yaru@earth.sinica.edu.tw), Institute of Earth Sciences, Academia Sinica, 128 Academia Road Sec. 2, Nankang Taipei 11529, Taiwan R.O.C.,

We investigate the rheology of the Chelungpu fault and the stress perturbations following the Chi-Chi earthquake to account for the postseismic deformation recorded at 9 permanent GPS reference stations and repeatedly measured at 79 temporary monuments. Kinematic analysis shows that the Chelungpu decollement embedded in midcrust took up most of the afterslip following the Chi-Chi earthquake. We conduct a numerical modeling to demonstrate that the dynamics of the afterslip may most likely result from stress transferred from the mainshock on the decollement that follows a velocity-strengthening friction law. Our model also implies that the distribution of the afterslip is not homogeneous in time or in space; it initiated at the fault patch east of the mainshock then progressively migrated southward.


Multi-Stage Extension And The Mid-Late Miocene Arc-Parallel Extension Event In The Hellenic Ridge

* Marsellos, A E (marsellos@gmail.com), Dept. of Earth & Atmos. Sciences, State University of New York, 1400 Washington Ave., ALBANY, NY 12222, United States
* Marsellos, A E (marsellos@gmail.com), Dept. of Geology, Union College, 807 Union St., Schenectady, NY 12308, United States
Kidd, W S, Dept. of Earth & Atmos. Sciences, State University of New York, 1400 Washington Ave., ALBANY, NY 12222, United States

A dominant localized arc-parallel extension interrupting a regional arc-normal extension accompanied the exhumation of HP-rocks in the southwestern segment of the Hellenic forearc ridge. Along the Cretan- Peloponnese segment, the youngest ZFT cooling ages from the HP-rocks below the PQU-PLK detachment show a consistent correlation with areas of development of arc-parallel (NW-SE) extension lineation. In these areas the samples give ZFT ages of 9.2-10.2 Ma in the southeastern Peloponnese, 9.1-10.4 Ma ages in Kythera, and 13.4-14.6 Ma ages in western Crete. The arc-parallel lineation becomes less prominent and is replaced by an arc-normal lineation farther to the northwest in the Peloponnese, and in western Crete along- arc lineations occur in a few places but are not strongly developed, and the stretching lineation in this area is mostly oblique to or normal to the arc. Zircons giving older ZFT ages occur in samples from areas showing arc-normal stretching lineation. The arc-parallel extension occurred under ductile to ductile-brittle transition conditions. There is also evidence of an older arc-normal extension which occurred entirely under ductile conditions, and the younger arc-parallel structures overprint these earlier ductile structures. ZFT ages plotted versus the associated stretching lineation orientations from the same localities shows that the prominent change of the forearc ridge extension direction occurred between 13-12 Ma in the Peleponnese and Kythera. The earlier ductile structures near the detachment fault mostly show top-to-NE shear displacement in the PQU and PLK units along the southern- Peloponnese-western-Crete ridge. Later ductile to ductile/brittle structures in the southeastern-Peloponnese- Kythera ridge segment show mostly top-SE (arc-parallel) displacement; in westernmost Crete, top-NW (these structures have an arc-parallel component) and top-N occur. The structural record of the PQU exhumation thus shows an earlier ductile detachment of NE-SW extension in the Kythera-Peloponnese area. This same event in Crete is oriented close to a N-S direction because of the arc curvature. Near the end of this ductile extension as the PQU approached the ductile-brittle transition the change to NW-SE orientation of extension on the detachment overprinted the early structures along part of the Peloponnese-Cretan ridge in the Kythera Strait- southeastern Peloponnese. Brittle structures, including significant normal faults that define large-scale features in the inland and submarine topography, cut rocks both above and below the PQU-PLK detachment. Along the Peloponnese- Cretan ridge, these are the controlling structures defining this structural high, and are integral to the development of the present forearc ridge. These latest structures, formed by arc-normal extension, overprint the previous ductile and ductile-brittle structures. We suggest the transition back from arc-parallel to arc- normal extension probably occurred about 11-9 Ma. ago. The young exhumation of the Kythera-SE Peloponnese PQU marks an episode of localized strong stretching of the Hellenic Arc over the subducting oceanic remnant of the African plate; our exhumation ages from the western part restrict this to between about 14 and 9 Ma ago. Increased slab rollback would effectively drive significant expansion and extension in the Aegean forearc. In a larger context, arc-parallel extension is prominent following the Mid Cycladic Lineament from the forearc near Kythera into the backarc (Paros, Ios, Sifnos), also reflecting the expansion and opposite rotations of the western and eastern parts of the Aegean and the Hellenic arc.


Abyssal Hill Deflections at Pacific-Antarctic Ridge Transform Intersections

* Croon, M B (mcroon@ucsd.edu), Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093-0208, United States
Cande, S C (scande@ucsd.edu), Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093-0208, United States
Stock, J M (jstock@gps.caltech.edu), California Institute of Technology, 1200 E California BLvd, Pasadena, CA 91125, United States

Almost complete shipboard multibeam bathymetry coverage at the Menard and Pitman Fracture Zones allowed us to map abyssal hill deviations along their traces. We compared the mapped abyssal hill deflections to a detailed plate motion model for the Pacific-Antarctic Ridge to test how abyssal hill curvature correlates to changes in plate motion direction, which leads to periods of transtension or transpression. To better understand the range of curvatures, we compared our observations with a model for curved cracks [Pollard and Aydin, 1984]. Spreading centers can be considered as giant cracks. The propagation path of a crack under combined loading can be predicted as a function of the stress ratio between the relative stresses required for spreading at the spreading axis (Mode I loading) and stresses resisting sliding along the transform (Mode II loading). We mapped 124 abyssal hill deflections at Menard Fracture Zone and 113 at Pitman Fracture Zone, respectively. The observations show that the amount of curvature can change rapidly over short periods of time. A high abundance of deflected abyssal hills is expected when a significant adjustment in plate motion direction occurs, which puts stress on the transform fault. This is observed, in particular, at the Pitman Fracture Zone, which experienced significant transtension since chron C5y (9.8 Ma) in response to a 17° clockwise rotation of the spreading direction azimuth. In contrast to the abyssal hill tips, which were deflected in response to the changing stress field when approaching a ridge transform intersection, we also mapped several anomalously curved abyssal hill structures. Such anomalous deflections are expected in oceanic crust formed near ridge transform intersections during periods of transpression [Sonder and Pockalny, 1999]. We mapped 19 anomalous abyssal hill deflections at Menard Fracture Zone between chrons C15o and C7 (34.9 to 24.8 Ma) and 15 at Pitman Fracture Zone between chrons C11y and C6B (30.1 to 23.1 Ma), respectively. These anomalously curved abyssal hills are related to block rotation about a vertical axis, in response to the transpression, and typically occurs in seafloor less than 2 Ma old when the oceanic lithosphere is thin. References Pollard, D. D., and A. Aydin (1984), Propagation and Linkage of Oceanic Ridge Segments, Journal of Geophysical Research, 89(Nb12), 17-28. Sonder, L. J., and R. A. Pockalny (1999), Anomalously rotated abyssal hills along active transforms: Distributed deformation of oceanic lithosphere, Geology, 27(11), 1003- 1006.


Abrupt Change in Convergence Rate as a Mechanism to Induce Extension in Highly Coupled Subduction Zones

* Contreras, J (juanc@cicese.mx), CICESE, Geology Department, Km 107 Carretera Tijuana-Ensenada, Ensenada, BC 22864, Mexico

I here present model results that bring into light a new mechanism for continental extension in convergent margins. The model is motivated by observations in the Trans-Mexican volcanic belt, a volcanic arc built above the Middle America subduction zone, that apparently contradict the current understanding of the dynamics of subduction zones. This volcanic arc is dissected along its axis by several arrays of active normal faults with a combined length of 450 km and up to 1.5 km of throw. Previous observations worldwide indicate that continental extension in convergent margins takes place where (i) the upper plate moves away from the trench, and (ii) the subduction zone is only weakly coupled. In western Mexico, neither of these phenomena is observed; North America moves toward the trench and the subduction zone is fully coupled. Moreover, extension is usually observed in the backarc, in Mexico is intra-arc. The model shows that in the case the oceanic slab sinks into the mantle at a steep angle, periods of rapid subduction lead to an increase of suction force under the forearc. This causes the over-riding plate to bend downward building up tensional stress inside the continent, 150-250 km away from the trench, resulting in failure of the associated volcanic arc.


Joints and Mineral Veins in Limestone-Marl Alternations: Arrest and Fracture Frequencies

* Philipp, S L (Sonja.Philipp@geo.uni-goettingen.de), Geoscience Center, University of Goettingen, Goldschmidtstr. 3, D-37077 Goettingen, Germany
Reyer, D (Dorothea.Reyer@geo.uni-goettingen.de), Geoscience Center, University of Goettingen, Goldschmidtstr. 3, D-37077 Goettingen, Germany

Layering is a common feature of many rock masses. In particular, many sedimentary rocks are layered because of depositional changes (stratification), and diagenetic processes. Mechanical layering, where the mechanical properties, particularly the Young's moduli (stiffness), change between layers, may coincide with changes in grain size, mineral content or facies. The mechanical layering of the rock is important because layering commonly results in abrupt changes in local stress fields that may lead to fracture arrest. However, if all the beds in a rock mass have essentially the same Young's modulus and their contacts are welded together (sealed or healed) the beds may function mechanically as a single layer. Here we explore how mechanical layers relate to sedimentary layers in limestone-marl alternations. This we do by investigating the effects of sedimentary layering of the host rock, on the emplacement and geometries of extension fractures such as joints and mineral veins. Detailed field studies were carried out at two localities of well-exposed limestone-marl alternations: (1) the Jurassic Blue Lias at the Glamorgan Coast of South Wales, UK, and (2) the Triassic Muschelkalk in the Kraichgau area, Southwest Germany. In both study areas, calcite veins occur almost exclusively in the cores and damage zones of faults, whereas jointing is pervasive. Fracture arrest, common in mechanically layered host rocks, is primarily controlled by local variations in the stress field, mainly due to three factors: discontinuities (fractures and contacts), changes in host rock mechanical properties, and stress barriers, where the local stress field is unfavorable to fracture propagation. These factors are related in that changes in stiffness and stress barriers are common at contacts between different rock types. A fourth mechanism, namely the material toughness (critical strain energy release rate) of the contact in relation to that of the adjacent layers has been much studied in materials science. Some fractures may also be arrested by slip at contacts. For fractures to propagate, the stress field along their potential pathways must be favorable and essentially homogeneous so that the probability of fracture arrest is minimized. The field observations show that most joints, and many mineral veins, become arrested, primarily at layer contacts. Fractures that are restricted to single layers are referred to as stratabound, whereas for non- stratabound fractures, layering does not affect fracture growth. Different types of fractures react differently to host-rock layering. Fractures formed at great depths are generally less likely to become stratabound, but different fractures also seem to "feel" the rock layers and contacts differently. For example, mineral veins are much more often non-stratabound than joints. In our study areas there is also a clear inverse correlation between layer thicknesses and joint frequencies, particularly for layering on a decimeter-scale. The calcite veins, however, are not related to layer thickness. The results have important implications for the permeability of fluid reservoirs, such as for petroleum, gas, geothermal or ground water. Whereas correlations with layer thicknesses may be used for joint frequencies in the subsurface and thus reservoir permeabilities, it is more difficult to predict how many reservoir fractures are likely to be stratabound. A reservoir where most fractures are stratabound is less likely to develop interconnected fracture systems than a reservoir with non-stratabound fractures. Thus, a reservoir with mostly stratabound fractures may not reach the percolation threshold needed for significant permeability.


Abrupt Change in Zircon Hf Isotopic Compositions at ~420 Ma: Implications for Early Paleozoic Ridge Subduction in the Chinese Altai

* Sun, M (minsun@hku.hk), Department of Earth Sciences, University of Hong Kong, Pokfulam Road, Hong Kong
Yuan, C (yuanchaogig@hotmail.com), Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou, China
Long, X (longxp79@hotmail.com), Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou, China
Cai, K (caikd_117@hotmail.com), Department of Earth Sciences, University of Hong Kong, Pokfulam Road, Hong Kong
Jiang, Y (jydcsu@hotmail.com), Department of Earth Sciences, University of Hong Kong, Pokfulam Road, Hong Kong
Wong, K (kennywongbobo@hotmail.com), Department of Earth Sciences, University of Hong Kong, Pokfulam Road, Hong Kong
Xiao, W (wj-xiao@mail.igcas.ac.cn), Institute of Geology and Geophysics, Chinese Academy of Sciences, Bei Tu Cheng Road, Beijing, China
Zhao, G (gzhao@hku.hk), Department of Earth Sciences, University of Hong Kong, Pokfulam Road, Hong Kong
Wu, F (wufuyuan@mail.igcas.ac.cn), Institute of Geology and Geophysics, Chinese Academy of Sciences, Bei Tu Cheng Road, Beijing, China

Zircon minerals were separated from granitoids, sedimentary rocks, and gneisses from the Chinese Altai. Those with oscillatory zoning and high Th/U ratios are interpreted to have an igneous origin, and were analyzed for their U-Pb and Hf isotopic compositions. These zircons yielded U-Pb ages from 280 to 2800 MaCindicating a long evolutionary history of magmnatic activity in the region. Zircon Hf isotopic compositions show an abrupt change at ~420 Ma, indicating magma sources of both ancient and juvenile materials prior to 420 Ma, but juvenile materials were predominant in the magma sources after 420 Ma. This may imply a large amount of juvenile materials were added to the lithosphere at ~420 Ma and significantly modified the composition of the lithosphere of the Chinese Altai. We use a ridge subduction model to explain such a dramatic change, which can also explain the emplacement of the huge amount of coeval granitic intrusions with depleted isotopic characteristics, the basaltic rocks with complicated chemical compositions, the association of adakite-high Mg andesite-boninite-High Nb basalt, and the high T regional metamorphism. This study was supported by Research Grant Council of Hong Kong (HKU704307P, HKU7040/04P), National Basic Research Program of China (2007CB411308), and the University of Hong Kong.