Tectonophysics [T]

T31E MCC:3005 Wednesday 0800h

Intraplate Tectonics: North China and Other Regions III

Presiding:Y Zhang, University of Michigan; X Song, University of Illinois at Urbana-Champaign

T31E-01 INVITED 08:00h

Temporal And Spatial Variation In Geochemistry Of Cenozoic Basalts In North China: Implication For The Origin Of Intraplate Volcanism

* Xu, Y (yigangxu@gig.ac.cn) , Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou, 510640 China

In North China, Cenozoic basalts were erupted in association with formations of graben basins and their regional boundary faults. Radiometric dating shows that eruption age is between early Tertiary to Quaternary. While Sr-Nd isotopic ratios and relative abundances of incompatible elements are consistent with a dominant asthenospheric origin, Cenozoic basalts from both sides of the Daxin'anling-Taihang gravity lineament (DTGL) that separates the North China Craton (NCC) into two parts, namely the western and eastern NCC, show contrasting evolution trends. In the western NCC, magmas evolved from xenolith-bearing alkali basalts of late Eocene-Oligocene age to coexisting alkali and tholeiitic basalts of late Miocene-Quaternary age. This change in basalt type is accompanied by a decrease in La/Yb and increase in Yb content. This temporal variation in basalt geochemistry is interpreted as reflecting progressive lithospheric thinning in the western NCC during the Cenozoic. An opposite trend is observed for Cenozoic basalts from the eastern NCC, suggesting lithospheric thickening during this time period. This thickening was probably related to regional thermal decay following peak magmatism in the late Cretaceous-Early Tertiary. Such contrasting lithospheric processes may reflect diachronous extension in the NCC, with extension taking place earlier in the eastern NCC than in western NCC. Recent seismic tomography reveals a flat subducting slab at the transition zone. The west end of this stagnant slab virtually coincides with the location of the DTGL. It is thus possible that Cenozoic volcanism resulted from interaction between two different tectonic regimes. The generation of Cenozoic basalts from the eastern NCC could be related to the back-arc extension owing to late Mesozoic-Paleogene Pacific subduction underneath the Asian continent, whereas subsequent extension in the western NCC may have been induced by the early Tertiary Indian-Eurasian collision. This interpretation also implies that the formation of the DTGL was likely coeval with the late Mesozoic destruction of the Archean lithospheric root beneath the eastern NCC.

T31E-02 08:20h

Geochemistry of basalts from Northeast China

Chen, Y (yangcz@umich.edu) , The University of Michigan, Department of Geological Sciences, Ann Arbor, MI 48109-1063 United States
* Zhang, Y (youxue@umich.edu) , The University of Michigan, Department of Geological Sciences, Ann Arbor, MI 48109-1063 United States

One of the notable areas of intraplate volcanism is northeast China. In an area of about two million square kilometers, there are numerous volcanic fields (about 600 volcanos known, Liu, 1999) with ages from Late Cretaceous (80 Ma) to recent (the most recent eruption was 300 years ago). Individual volcanic centers are usually small and a volcanic field typically consists of several to tens of volcanic centers. In this report, we review petrological and geochemical data in literature, present our new data, examine compositional correlations for this vast volcanic province, and explore possible controls of geochemistry by other parameters such as lithosphere thickness, crustal thickness and heat flow. Preliminary results show that not all data are consistent, probably reflecting analytical uncertainties of various labs. Isotopically, the basalts range from depleted mantle to bulk silicate Earth. Some elemental concentrations are correlated, and the correlation is strong along one linear belt of volcanic fields. There are systematic geochemical differences between alkali basalts from northeast China and intraplate alkali basalts from east Australia or from Hawaii. Basalt chemistry seems to vary systematically along a linear volcanic belt. The variations can be roughly explained by varying the degree and depth of mantle partial melting (such as the presence or absence of garnet), and might be related to lithosphere thickness. More work is in progress to examine basalt chemistry and other parameters and to understand the significance of the correlations and variations.

T31E-03 08:35h

The Late Paleozoic- Early Mesozoic tectonomagmatic activities in the Yanshan belt: implications for the Mesozoic intraplate deformation

* Zhang, S (tozhangshuanhong@163.com) , Institute of Geomechanics, Chinese Academy of Geological Sciences, No.11 Minzudaxue Nan Road, Haidian District, Beijing, 100081 China
Zhao, Y , Institute of Geomechanics, Chinese Academy of Geological Sciences, No.11 Minzudaxue Nan Road, Haidian District, Beijing, 100081 China
Song, B , Beijing SHRIMP center and Institute of Geology, Chinese Academy of Geological Sciences, No.26 Baiwanzhuang Road, Xicheng District, Beijing, 100037 China
Hu, J , Institute of Geomechanics, Chinese Academy of Geological Sciences, No.11 Minzudaxue Nan Road, Haidian District, Beijing, 100081 China
Xu, G , Institute of Geomechanics, Chinese Academy of Geological Sciences, No.11 Minzudaxue Nan Road, Haidian District, Beijing, 100081 China
Wu, H , Geological Information Center, Ministry of Land and Resource, Yanjiao, Hebei Province, Sanhe, 065201 China
Liu, J , Institute of Geomechanics, Chinese Academy of Geological Sciences, No.11 Minzudaxue Nan Road, Haidian District, Beijing, 100081 China
Pei, J , Institute of Geomechanics, Chinese Academy of Geological Sciences, No.11 Minzudaxue Nan Road, Haidian District, Beijing, 100081 China

Reactivation of pre-existing structures or thermal weakening zones associated with magmatism is very important for the localization of intraplate deformations (e. g., Holdsworth et al, 1997; D'Lemos et al, 1997). The Yanshan tectonic belt, which is located at the northern margin of the North China block (NCB), is a typical intraplate fold-and-thrust belt in Mesozoic (Davis et al, 1998; 2001). Although much work has been done during last a few decades, the Late Paleozoic-Early Mesozoic magmatic and deformation history of the Yanshan tectonic belt is still poorly understood. Recently, several variably deformed Late Paleozoic-Early Mesozoic granitoid plutons such as Longhua quartz diorite (311 -A2 Ma), Daguangding quartz diorite (327 -A7 Ma), Boluonuo quartz diorite (303 -A4 Ma), Guanglingshan granite (253 -A4 Ma) etc. in the Yanshan belt were determined from the so-called Archean-Paleoproterozoic crystalline basement by our SHRIMP U-Pb zircon dating, which indicate that the strong magmatisms accompany with deformation occurred during Late Paleozoic to Early Mesozoic. Preliminary geochemical analysis results show that the formation of these plutons is a result of the southward subduction of the Paleo-Asian oceanic plate beneath the NCB and the collision between the Paleo-Mongolian composite terrane and the NCB. Structural analyses of the Late Paleozoic plutons and the country rocks revealed several E-W trending mylonite zones formed during Late Paleozoic-Early Mesozoic, and the inherences between the former mylonite zones and the Mesozoic regional fault zones such as the Damiao fault, the Shangyi-Chicheng fault, and the Fengning-Longhua fault, are very obvious. Because some of the mylonite zones and deformed plutons are unconformably overlain by Jurassic volcanic or sedimentary rocks, and the eldest strata overlain upon the unconformity are Lower Jurassic Nadaling Formation and Xiahuayuan Formation, they had been deformed and exhumed to surface before Early Jurassic. 40Ar-39Ar dating and deformation environment analysis results of the mylonites indicate a multistage process of deformation from Late Paleozoic to Early Mesozoic (Cui et al, 2000; Hu, et al, 2003; Zhang, 2004). These multistage deformation zones, with strong Late Paleozoic-Early Mesozoic magmatic activities, formed many pre-Mesozoic crustal heterogeneities and thermally weakened zones, which played a very important role on the initiation of the intraplate deformation in Yanshan area from Early Mesozoic (pre-180 Ma?, Davis et al, 2001). Therefore the Mesozoic Yanshan intraplate fold-and-thrust belt is developed from a Late Paleozoic plate boundary tectonic belt.

http://www.agu.org/yanshan

T31E-04 08:50h

Processes of Active Intraplate Deformation in the Mongolian Altai and Gobi Altai

* Cunningham, D (wdc2@le.ac.uk) , University of Leicester, Department of Geology, Leicester, LE17RH United Kingdom

The Mongolian Altai and Gobi Altai regions are natural laboratories for studying active processes of intracontinental, intraplate transpressional mountain building. This type of orogen has unique structural, geomorphological and topographic characteristics that are generally underappreciated by geologists and geophysicists. Both ranges are dominated by 25 $\pm$5 km-spaced regional strike-slip faults which are linked to oblique-slip thrust and dip-slip thrust faults. These discrete belts of deformation and uplift are separated by non-extending sedimentary basins or geomorphologically mature landscapes. Individual ranges can be characterised as terminal restraining bends, double restraining bends, partial restraining bends, thrust ridges linked to strike-slip faults, triangular block uplifts between conjugate strike-slip faults, and individual thrust ridges. Neither orogen contains a regional thrust wedge, nor bounding foredeep, and structural vergence is inconsistent. Basins between ranges can be classified as ramp basins, confined asymmetric thrust-bound basins, open-sided thrust basins, strike-slip basins, and tectonically inactive remnant lows. Although parts of the Altai and Gobi Altai were previously a Mesozoic rift province, very few examples of fault reactivation and rift inversion have been identified in either region. The geometrical relationship between basement structural grain, rigid Precambrian block boundaries, and SHmax dictates Late Cenozoic fault kinematics and overall mechanisms of uplift. Ongoing transpressional deformation is incompletely slip partitioned as indicated by historical oblique-slip earthquake focal mechanism solutions and oblique-slip fault displacements as revealed by outcrop slickenside data. The geometry of active faulting is best described as regional transpressional duplexing, intermediate between strike-slip and thrust duplexing. Late Cenozoic upper crustal transpressional deformation may be balanced in the lower crust by crustal thickening due to lower crustal inflation, and possibly lower crustal outflow towards the Russian Altai-Sayan region where late Cenozoic plateau uplift is recorded, but unexplained.

T31E-05 09:05h

Tertiary Deformation in the Kashgar Basin, Southern Margin of the Tian Shan, China

* Heermance, R V (richard@crustal.ucsb.edu) , University of California, Santa Barbara, Department of Geological Sciences, 1006 Webb Hall, Santa Barbara, CA 93106 United States
Chen, J (chenjie@eq-igl.ac.cn) , State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Seismological Bureau, P.O. Box 9803, Beijing, 100029 China
Scharer, K (kscharer@darkwing.uoregon.edu) , Department of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403 United States
Burbank, D (burbank@crustal.ucsb.edu) , University of California, Santa Barbara, Department of Geological Sciences, 1006 Webb Hall, Santa Barbara, CA 93106 United States

The northwest corner of the Tarim Platform, near the town of Kashgar, China, is an actively deforming fold-and-thrust belt related to uplift of the Tian Shan. Mapping of 5000 km$^{2}$ in the area north of Kashgar city and west of the Kepintage thrust zone indicates at least 15 km of shortening has occurred since the Miocene. Shortening is accomodated along 5 major structures that extend $>$60 km ~east-west along strike. These structures, from north to south, are the basin-bounding Tuotergongbaizi fault(TF), the Kepintage-Yishilakekalawuer fault (KYTF), the Keketamu syncline-anticline pair (KEK), the Atushi-Talanghe (AT) anticline, and the Kashi (K) anticline. Smaller-displacement imbricate faults and small-wavelength folds occur throughout the region associated with these major structures. Scharer et al (in press) document 7 km of shortening in the AT and K anticlines. Our new values add at least 8 km of shortening accomodated in the TF, KYTF, and KEK structures for a total of $>$15 km in the region. Uplifted and deformed fluvial terraces are observed along the AT, K, and KYTF, indicating that deformation is active and occurring throughout the fold-and thrust belt, not only in the southern foreland. In the north the TF and KYTF are south-vergent faults that juxtapose Paleozoic basement with Tertiary sediments. The KEK anticline is a south vergent fault-propagation-fold, but the AT and K are detachment structures that display both north and south vergence. Detachment horizons are likely gypsum-rich Miocene layers near the base of the ($>$5 km thick) Tertiary section. The TF cuts poorly sorted, debris-flow facies conglomerate in its footwall. The conglomerate changes facies southward, toward the foreland, to well-sorted fluvial deposits. New magnetostratigraphy reveals the the basal age of the conglomerate becomes younger towards the south, from $\sim$5.2 Ma 10 km south of the fault, to $<$2 Ma 20 km south of the fault. These observations imply significant erosional topography along the TF prior to 5.2 Ma, and progradation of a conglomerate wedge into the basin. Our shortening estimates for this area since the latest Miocene imply minimum long-term shortening rates of $\sim$3mm/year, or less than half of the present-day shortening rates (7-10 mm/year) determined by GPS measurements across the region.

T31E-06 09:20h

Slip-distribution and Rupture Pattern of the 14 November 2001, Mw 7.8 Kokoxili Earthquake (China)

* Klinger, Y (klinger@ipgp.jussieu.fr) , Tectonique - IPGP, UMR7578 - BP89 4 place Jussieu, Paris, 75005 France
Michel, R (remi.michel@cea.fr) , LDG/CEA, BP12, Bruyeres le Chatel, 91680 France
Van der Woerd, J (jeromev@eost.u-strasbg.fr) , EOST, 5 rue Descartes, Strasbourg, 67000 France
Xu, X (xiweixu@vip.sina.com) , Institut of Geology, Chinese Earthquake Administration, Beijing, 100029 China
Tapponnier, P (tappon@ipgp.jussieu.fr) , Tectonique - IPGP, UMR7578 - BP89 4 place Jussieu, Paris, 75005 France

The Mw=7.8 Kokoxili earthquake ruptured about 430 km of the Kusai segment of the Kunlun fault (from 90.2E to 94.7E). This event, the largest continental event ever recorded in Asia, follows nearly four years later, the 8 November 1997, Mw=7.6 Manyi earthquake. The surface break follows the geomorphic trace of the fault, which bears evidence for cumulative displacements. Using Ikonos and Quickbird satellite images with nominal resolution of 1m and 60cm, respectively, we mapped in detail the central segment of the rupture and documented the slip-partitioning process that took place during the earthquake rupture. Some new details about the rupture in the epicentral area west of the Tayang Lake are also presented. From field explorations of the rupture we show evidences for normal fault breaks and left-lateral ruptures along the steep faceted southern flank of the BukaDaban Feng, a 40km-long range reaching 6800m asl. Hence this mountain range clearly results from continuous oblique left-lateral faulting along the extensional corridor connecting the epicenter area to the main rupture segment. Due to the remoteness and harsh climatic condition of the earthquake location, an exhaustive survey of the entire fault rupture is difficult. While several teams have been involved in measuring geomorphic offsets due to this earthquake. The amount of data collected, however, remains small compared to the length of the rupture. Also considering the complexity of the rupture, measuring the earthquake dislocation has proved to be difficult in many places and has raised arguments about the shape of the slip-curve and the maximum horizontal offset associated to the Kokoxili earthquake. Using sub-pixel intercorrelation of optical satellite images (satellite Spot, pixel 10m) we produced a slip-curve that encompasses the entire rupture with an independent measurement every kilometer on average. This technique allows us to detect displacement down to 1m. The slip-distribution we obtained shows clearly that the maximum slip is highly localized on few asperities whereas it remains moderate (<4m) along large parts of the rupture. The maximum slip we could measure is 12m, which is much lower than some values previously reported. The large variation of slip along the rupture for a single event bears important consequences and should be seriously considered when long-term rupture behavior of the fault is discussed based on cumulative offset measurements or paleoseismological investigation.

T31E-07 09:35h

Mesozoic and Cenozoic Intracontinental Deformation in Southeastern Mongolia

* Webb, L E (lewebb@syr.edu) , Dept of Earth Sciences, Syracuse University, Syracuse, NY 13244 United States
Johnson, C L , Dept of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112 United States
Minjin, C , School of Geology, Mongolian University of Science and Technology, Ulaanbaatar, 210646 Mongolia
Sersmaa, G , School of Geology, Mongolian University of Science and Technology, Ulaanbaatar, 210646 Mongolia
Affolter, M D , School of Geology, Mongolian University of Science and Technology, Ulaanbaatar, 210646 Mongolia
Manchuk, N , School of Geology, Mongolian University of Science and Technology, Ulaanbaatar, 210646 Mongolia

Structural and basin analysis studies of the NE-trending East Gobi Fault Zone (EGFZ) reveal distinct deformation events postdating Late Paleozoic arc accretion and continental amalgamation. This presentation, a companion abstract to Johnson et al. (2004), focuses on the kinematic signatures of these deformation events and their relationship to the larger Asian tectonic framework and its evolution. Metamorphic tectonites of basement blocks in the EGFZ, previously mapped as Precambrian basement, belong to a Late Triassic sinistral shear zone that comprises a suite of synkinematic intrusions, amphibolite-facies gneisses and greenschist-facies mylonites. The shear zone is dominated by steeply dipping foliations and subhorizontal ENE or WSW-plunging stretching lineations. At higher structural levels, variably metamorphosed Paleozoic sedimentary sequences are caught up in discrete zones of mylonitic deformation. We have documented the shear zone at several localities over a distance of 250 km along strike in the EGFZ. High-angle fabrics associated with the Late Triassic sinistral shear zone were exploited by Early Cretaceous basin-bounding faults that formed in the brittle-ductile to brittle regime. A NW-SE horizontal extension direction indicated by this oldest generation of brittle faults is similar to that implied by stretching lineations at the Yagan-Onch Hayrhan metamorphic core complex located to the SW along the Chinese-Mongolia border. Early Cretaceous rift basins were inverted by mid Cretaceous dextral transpression along mainly WNW-trending faults. All Mesozoic rocks are cut by Cenozoic conjugate strike-slip and thrust faults that accommodate NNW-SSE compression. These youngest faults are associated with gouge zones, cut unconsolidated alluvium, and control the present day topographic expression of basement blocks within the EGFZ. Our data suggest that extrusion tectonics have been active in Asia since the Early Mesozoic, and bear on the viability of tectonic models that postulate linkages between the Altyn Tagh and EGFZ. Though a Cenozoic linkage between the two faults is permissible, major sinistral offset on the EGFZ likely occurred in the Late Triassic.

T31E-08 09:50h

Sedimentary Basin Evolution in the Context of Polyphase Intracontinental Deformation: New Insights From Southeastern Mongolia

* Johnson, C L (cjohnson@mines.utah.edu) , Dept of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112 United States
Webb, L E (lewebb@syr.edu) , Dept of Earth Sciences, Syracuse University, Syracuse, NY 13244
Minjin, C (minjin@must.edu.mn) , School of Geology, Mongolian University of Science and Technology, Ulaanbaatar, 210646 Mongolia
Sersmaa, G (sers@must.edu.mn) , School of Geology, Mongolian University of Science and Technology, Ulaanbaatar, 210646 Mongolia
Affolter, M (qfl247@netscape.net) , Dept of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112 United States
Manchuk, N (minjin@must.edu.mn) , School of Geology, Mongolian University of Science and Technology, Ulaanbaatar, 210646 Mongolia

At least four major intracontinental deformation events have punctuated the geologic evolution of SE Mongolia since Late Paleozoic time. Much of the structural deformation was concentrated along the northeast-trending East Gobi Fault Zone (EGFZ), which we have studied in multiple localities along its 250 km (minimum) length. Our structural and geochronologic studies, described in more detail in a companion abstract by Webb et al. (2004), define a general tectonic framework of Late Triassic sinistral shear, Late Jurassic-Early Cretaceous extension, mid-Cretaceous transpression and basin inversion, and Cenozoic compression and strike-slip faulting. Perhaps the best-known phase of basin development in this area is Late Jurassic-Early Cretaceous rifting which formed multiple NE-SW-trending grabens, as well as a metamorphic core complex. This rifting event, in which most structures of the EGFZ were active as a series of high angle normal faults, postdates motion on a Late Triassic sinistral shear zone. As a result of Early Mesozoic sinistral shear, Paleozoic basins may have been dismembered and offset by some 100s of km prior to late Mesozoic rifting. Similarly, we have new evidence for mid-Cretaceous and Cenozoic transpression along the EGFZ following the rifting event. Postrift reactivation of the EGFZ may have coincided with Cenozoic activity along the Altyn Tagh fault zone, and suggests that motion along these faults may have been linked for at least part of their histories. Multi-phase reactivation of the EGFZ has important implications for evolution of sedimentary basins that formed along it, as well as for other regions subject to intracontinental deformation. Such implications include the importance of inherited shear zones, particularly in areas where basement is extremely heterogeneous, as a controlling factor in the development of polyphase sedimentary basins. This work also emphasizes the need for careful analysis of offset piercing points in order to accommodate accurate paleogeographic reconstructions of the basin.