GP42A-01 10:20h
Tectonic Rotation of Northeastern Tibetan Plateau: Refined age and Geodynamic Implications
To understand the timing and distribution of deformation during the Indo-Asia collision since ca. 55Ma, tectonic vertical-axis rotations of crustal fragments can be measured using paleomagnetism. Recent work reveals systematic $\sim$ $25\deg$ clockwise rotations in the Xining-Lanzhou region of the northeastern Tibetan Plateau. These results indicate that important deformation propagated far north of the collision zone early after the initial Indo-Asia collision. When compiled with the existing regional paleomagnetic dataset, they support the occurrence of dextral shear on the eastern margin of the Tibetan plateau. However, the age of this major rotation event is only broadly constrained by previous magnetostratigraphic and palynologic results. To obtain precise age and duration of the rotation, high-resolution magnetostratigraphic sampling has been performed on 2 sections spanning inferred Upper Eocene to Lower Miocene strata in the vicinity of the city of Xining. In addition, 30 paleomagnetic sites (8 samples each) have been collected in red mudstone throughout the sections. Ongoing paleomagnetic treatment reveals straightforward characteristic remanent magnetization (ChRM) held by magnetite. Statistical analysis of ChRM directions of normal and reverse polarities indicates 21.0 $\pm$ $7.3\deg$ clockwise rotation in the lower part of the sections while the upper part remains to be analyzed. Expected correlation to the GPTS will provide precise dating of the rotation event. This critical information will enable putting the rotation into possible geodynamic contexts and select between potential mechanisms responsible for the rotation.
http://www.geo.uu.nl/Research/Paleomagnetism/
GP42A-02 10:35h
Paleomagnetism Indicates Mid-Miocene Clockwise Rotation of the NE Tibetan Plateau
The India-Eurasia collision and subsequent continued convergence during the past 55 million years have produced the Himalayas and the Tibetan Plateau. However, the kinematics as to whether the Plateau deformed as a series of rigid blocks or as a viscous continuum are still being debated. Paleomagnetic studies are useful in determining how the India-Asia convergence is being accommodated. Paleomagnetic declinations of studies in and around the Tibetan Plateau reveal important features such as bending of the Himalayan arc, Cenozoic clockwise rotation of the Indochina block, and rotations at the eastern and western margins of the Tibetan Plateau, but the northeastern Tibetan Plateau remained until now rather unknown territory in terms of its kinematics with respect to surrounding areas. To help understand the deformation history of the NE Tibetan Plateau, we have carried out a magnetostratigraphic study in the typical intramontane molasse basin of Guide in the NE Tibetan Plateau, which is bounded by the sinistral Qilian and Kunlun faults to the north and south, and by the dextral Wenquan and Haiyan faults to the southwest and northeast. The declinations observed in the sedimentary strata in the Guide Basin reveal a clockwise rotation of ca. 30 degrees (+/- 17.5 degrees) during the Middle Miocene, implying that the strike-slip faults were particularly active at times during the interval of 17.5 to 11.5 Ma. This and other paleomagnetic and geologic evidence suggest that the northeastern part of the Tibetan Plateau, north of latitude 30N and east of longitude 95E, has rotated clockwise on average about 21 degrees (+/- 8.5 degrees) during the Middle Miocene. It is noteworthy that the SE and NE Tibetan Plateau areas have rotated clockwise during the Miocene, whereas the southern part of the Plateau has rotated counterclockwise some time after the Mid-Eocene. These results are consistent with the predictions of the rotation patterns in the kinematic model of Avouac and Tapponnier (1993).
GP42A-03 10:50h
Active Transtensional Tectonics Due to Differentially Rotating Upper Crustal Blocks East of the Eastern Himalayan syntaxis, Yunnan Province, China.
Seismological (Holt et. al. 1996), geodetic (King et. al. 1996, Chen et. al. 2000) and geological (Wang et. al. 1995, Wang and Burchfiel 2002) studies have shown that upper crustal material north and east of the eastern Himalayan syntaxis rotates clockwise about the syntaxis, with the Xianshuihe fault accommodating most of this motion. Within the zone of rotating material, however, deformation is not completely homogenous, and numerous differentially rotating small crustal fragments are recognised. We combine seismic (CSB and Harvard CMT catalogues), geodetic (CSB and MIT-Chengdu networks), remote sensing, compilation of existing regional maps and our own detailed field mapping to characterise the active tectonics of a clockwise rotating crustal block between Zhongdian and Dali. The northeastern boundary is well-defined by the northwest striking left-lateral Zhongdian and Daju faults. The eastern boundary, on the other hand, is made up of a 80 km wide zone characterised by north-south trending extensional basins linked by NNE trending left-lateral faults. Geological mapping suggests that strain is accommodated by three major transtensional fault systems: the Jianchuan-Lijiang, Heqing and Chenghai fault systems. Geodetic data indicates that this zone accommodates 10 +/- 1.4 mm/year of E-W extension, but strain may be (presently) preferentially partitioned along the easternmost (Chenghai) fault. Not all geodetic velocities are consistent with geological observations. In particular, rotation and concomitant transtension are somehow transferred across the Red River-Tongdian faults to Nan Tinghe fault with no apparent accommodating structures. Rotation and extension is surmised to be related to the northward propagation of the syntaxis.
http://web.mit.edu/~chrissg/Public/Dali/tectonics.html
GP42A-04 11:05h
Tectonic evolution of fault-bounded continental blocks: Comparison of paleomagnetic and GPS data in the Corinth and Megara basins (Greece)
We report on new paleomagnetic and anisotropy of magnetic susceptibility (AMS) data from Plio-Pleistocene sedimentary units from Corinth and Megara basins (Peloponnesus, Greece). Paleomagnetic results show that Megara basin has undergone vertical axis CW rotation since the Pliocene, while Corinth has rotated CCW during the same period of time. These results indicate that the overall deformation in central Greece has been achieved by complex interactions of mostly rigid, rotating, fault bounded crustal blocks. The comparison of paleomagnetic results and existing GPS data shows that the boundaries of the rigid blocks in central Greece have changed over time, with faulting migrating into the hanging walls, sometimes changing in orientation. The Megara basin belonged to the Beotia-Locris block in the past but has now been incorporated into the Peloponnesus block, possibly because the faulting in the Gulf of Corinth has propagated both north and east. Paleomagnetic and GPS data from Megara and Corinth basins have significant implications for the deformation style of the continental lithosphere. In areas of distributed deformation the continental lithosphere behaves instantaneously like a small number of rigid blocks with well-defined boundaries. This means that these boundaries could be detected with only few years of observations with GPS. However, on a larger time interval the block boundaries change with time as the active fault moves. Paleomagnetic studies distinguishing differential rotational domains provide a useful tool to map how block boundaries change with time.
GP42A-05 11:20h
Linking Ridge Subduction to Paleomagnetic Rotations in the Andes
Paleomagnetic data from 104 sites of upper Oligocene to Pliocene rocks from the coastal and Western Cordillera regions of central and northern Peru record a coherent pattern of counterclockwise rotations generated in the last 10 Ma. Based on the time-space relationship of deformation, magmatism and Nazca-South America plate convergence, the pattern of rotations can best be explained by a punctual and widespread tectonic event linked to subduction of the Nazca Ridge. If correct, slab coupling and concomitant mountain building are most pronounced at the beginning stages of ridge subduction, when down dip motion is inhibited. The system likely then evolves to a more normal subduction regime, probably when it becomes more energetically favorable to renew significant down dip motion rather than transfer convergent motion into building topography through tectonic shortening. As the ridge continues to subduct, significant changes in ridge-trench convergence angle, or possibly major changes in ridge topography, can increase or decrease coupling, and thus modulate compressive deformation. Ridge collisions may have lasting effects on the continental margin by weakening the crust to a level where deformation can proceed with lower levels of differential stress. This may for instance explain why the Subandean Zone (in this model initiated in times of increased coupling between the slab and overriding continental plate) remains tectonically active. It is possible that a succession of buoyant ridge subduction events played an important role in producing the paleomagnetic rotations recorded along the entire Andean chain. In this way, the modern Andes would be composed of distinct parts, with the Juan Fernandez ridge being partly responsible for shaping the Bolivian Orocline at circa 25 Ma, the Nazca ridge for the Peruvian Andes north of the Abancay deflection (15.5 S) until about 7 S at circa 8 to 4 Ma, and the Carnegie ridge for the recent deformation seen in the Northern Andes.
GP42A-06 11:35h
Oroclinal bending in the Bolivian Andes: New evidence for post 10 Ma rotations linked to shortening gradients in the fold-and-thrust belt
The pronounced bend in the Andean mountain chain is, perhaps, the best-known example of an orocline. More than 25 years of palaeomagnetic investigations have revealed vertical-axis rotations of crustal blocks synchronous with the development of the mountain chain. The predominant pattern is of clockwise rotations to the south of the bend and anticlockwise rotations to the north. There is, however, still no consensus as to the magnitude and timing of these rotations, or what the driving mechanism is, with some models arguing that they represent only local vertical-axis block rotations, whereas others suggest that they represent large scale bending of the mountain chain. Similarly some authors suggest that bending occurred progressively throughout the Cenozoic, whereas other suggest that the rotations occured in discrete phases. We present new results from a palaeomagnetic study comprising two extensive regions (grouped together as localities) one either side of the oroclinal axis, in the Bolivian Eastern Cordillera. Palaeomagnetic analyses of samples from 31 sites in the 13-2 Ma Los Frailes ignimbrite complex, south of the oroclinal axis, yield a characteristic magnetization with a mean direction (D=$007\deg$; I=$-43\deg$; k=25; a95=$7\deg$; N=19 flows) that has been rotated by 9 $\pm$ $8\deg$ clockwise (at the 95% confidence level) since ca. 7 Ma with respect to South American reference poles. This is one of the youngest statistically significant rotations derived for the Central Andes. Samples from 12 sites in the 13-5 Ma Morococala and Eucalytpus volcanic complexes, to the north of the oroclinal axis, yield a mean direction (D=$356\deg$; I=$-34\deg$; k=9; a95=$19\deg$; N=9 flows) that suggests an anticlockwise rotation of $2\deg$, although this is not statistically significant. We combine our results from the southern limb of the orocline with all previous palaeomagnetic studies of Miocene to recent rotations for the Eastern Cordillera and Sub-Andes, and compare these rotations with the shortening gradients along the fold-and-thrust belt of the Sub-Andes (where the majority of post 15 Ma Andean crustal shortening has occurred). Through a joint inversion, we show that the observed rotations are consistent with the shortening gradients in the Sub-Andes, and therefore demonstrate that the Bolivian orocline has become more pronounced in the last c. 10 Ma.
GP42A-07 11:50h
Paleomagnetic Definition of Crustal Segmentation, Quaternary Block Rotations and Limits on Earthquake Magnitudes in Northwestern Metropolitan Los Angeles
Paleomagnetic studies of the Pliocene-Quaternary Saugus Formation, in the San Fernando Valley and east Ventura Basin, show that the crust is segmented into small domains, 10-20 km in linear dimension, identified by rotation of reverse-fault blocks. Two domains, southwest and adjacent to the San Gabriel fault, are rotated clockwise: 1) The Magic Mountain domain, 30 +/- 5 degrees, and 2) the Merrick syncline domain, 34 +/- 6 degrees. The Magic Mountain domain has rotated since 1 Ma. Both rotated sections occur in hangingwalls of active reverse faults: the Santa Susana and San Fernando faults, respectively. Two additional domains are unrotated: 1) The Van Norman Lake domain, directly south of the Santa Susana fault, and 2) the Soledad Canyon domain in the San Gabriel block immediately across the San Gabriel fault from Magic Mountain, suggesting that the San Gabriel fault might be a domain boundary. Plio-Pleistocene fragmentation and clockwise rotations continue at present, based on geodetic data, and represent crustal response to diffuse, oblique dextral shearing within the San Andreas fault system. The horizontal dimensions of the blocks are similar to the thickness of the seismogenic layer. The maximum magnitude of an earthquake based on this size of blocks is Mw = 6.7, comparable to the 1971 San Fernando and 1994 Northridge earthquakes and consistent with paleoseismic trenching and surface ruptures of the 1971 earthquake. The paleomagnetic results suggest that the blocks have retained their configuration for the past \~ 0.8 million years. It is unlikely that multiple blocks in the study area combined to trigger much larger shocks during this period, in contrast to adjacent regions where events with magnitudes greater than 7 have been postulated based on paleoseismic excavations.
GP42A-08 12:05h
Kinematic Implications of New Paleomagnetic Data From the Northern Walker Lane, Western Nevada: Counterintuitive Anticlockwise Vertical-Axis Rotation in an Incipient Dextral Shear Zone
The Walker Lane/eastern California shear zone is a major dextral fault system that splays from the San Andreas fault in southern California and shunts 20-25% of the Pacific-North America transform motion east of the Sierra Nevada block. In NW Nevada and NE California, the north end of the system has developed in the past 3-6 Ma and is propagating northwestward in the wake of the retreating Cascade arc. This rapidly evolving region, known as the northern Walker Lane (NWL), is one of the youngest parts of the Pacific-North America transform margin and therefore affords an opportunity to analyze the incipient development of a major strike-slip fault system. The NWL consists of kinematically linked NW-striking, left-stepping right-lateral faults, N-striking normal faults, and subordinate ENE-striking sinistral faults. Major dextral faults terminate in arrays of northerly striking normal faults in the western Great Basin. The en echelon left-stepping pattern of dextral faults is a curious geometry. Although small left steps on individual faults are associated with local shortening, the broad left steps between major dextral faults accommodate little, if any, shortening and are therefore unlike typical restraining bends. One possible model is that the left-stepping dextral faults are primary Riedel shears developing above a dextral shear zone at depth. The ENE-striking sinistral faults may be secondary, conjugate Riedel shears. Paleomagnetic data from the 25.1 Ma Nine Hill Tuff indicate slight anticlockwise rotation of blocks between the overlapping, NW-striking dextral faults. Our work has established a new reference direction (D=341, I=55, a95=4, 11 sites; 83 samples) for Nine Hill Tuff in the presumably unrotated Sierra Nevada. Data from the Nine Hill Tuff in the NWL suggest about 15 deg of anticlockwise rotation of the Virginia Mts, Dogskin Mt, and Seven Lakes Mt (D=326, I=52, a95=7, 12 sites) but negligible rotation of the Fort Sage Mts (D=342, I=55, a95=13, 5 sites). Slight anticlockwise rotation is also supported by paleomagnetic data from the 28.6 Ma tuff of Campbell Creek (D=191, I= -40 [2 sites] compared to reference direction of D=205, I= -43, a95=3 [10 sites in Sierra Nevada]) and the predominance of WNW-striking strata in these fault blocks, which contrasts with a more northerly striking regional norm. However, data from the Nine Hill Tuff do indicate 30-60 deg of clockwise rotation (D=16, I=47, a95=10, 6 sites) in narrow 2 km wide bands along major dextral faults in the NWL. In the transtensional setting of the NWL, the slight anticlockwise rotation may reflect coeval E-W to WNW regional extension and NW-directed dextral shear. In this model, extension induces a domino-like, map-view collapse and slight anticlockwise rotation of fault blocks between the left-stepping dextral faults. The anticlockwise rotation is opposite to the clockwise rotation typically found in dextral shear zones. Anticlockwise rotation may ultimately rotate Riedel shears toward the main shear zone at depth, thus facilitating eventual development of a through-going, upper-crustal strike-slip fault. Ironically, as the system matures and a through-going fault develops, the predominant sense of vertical-axis rotation may reverse and become compatible with the dextral sense of shear. Such complex kinematics may characterize incipient strike-slip fault systems in both transtensional and transpressional settings.