GP51A-0732
Back-arc extension and contraction leading to Neogene rotations in the eastern Mediterranean?
The Aegean, western Anatolian and Carpathian regions are well known for major Neogene block rotations. In
this contribution we show new paleomagnetic results from western Anatolia and Bulgaria, which complete the
record of timing and distribution of major block rotations, and their relation to (back-arc) extension and
compression. A new paleomagnetic reference direction from the Moesian platform and the Rhodope in
Bulgaria show that this region has been firmly attached to Eurasia since the Eocene. This new direction
provides a reference direction with which the rotation results from western Greece and the Carpathians can
be compared to resolve the rotation differences, and their tectonic accommodations within this complex
tectonic province. We conclude that the west-Aegean region rotated ~40° clockwise with respect
to the Moesian platform since 15 Ma, and infer that this rotation difference was in part accommodated in the
Aegean extensional back-arc, and in part by extrusion tectonics (and possibly renewed contraction) in the
Balkanides of Albania and Serbia, leading to the motion of the Tisza Block around the northwestern edge of
the Moesian platform, into the Carpathian back-arc. In the southwestern Anatolian region, at the eastern
edge of the Aegean extensional back-arc, we show that counterclockwise rotation of the Bey Daðlarý
platform also occurred since ~15 Ma. New paleomagnetic results also show absence of rotation with
respect to Eurasia in northwestern Turkey. This rotation difference and inferred rates of extension are in line
with the exhumation of the Central Menderes Massif between 15 and 5 Ma, as well as renewed contraction in
the Isparta Angle. In both the western and eastern Aegean region the rotation pole is situated within the
rotating domain, leading to contraction and/or extrusion west of the western rotation pole, and east of the
eastern. The central segment is thus extending, and the rates and direction of extension are in line with the
well-established extensional history of the Aegean back-arc. The rates of northward motion of the
contractional segments correspond to the rate of northward motion of Africa in the middle to late Miocene.
Aegean back-arc extension started, however, well before 15 Ma, and the onset of back-arc extension hence
did not lead to these rotations. Prior to 15 Ma, we tentatively suggest that the Aegean back-arc was bounded
by discrete transform faults that may be linked with slab edges accommodating slab retreat. The renewed
contraction outside the Aegean back-arc, with rates comparable to African motion suggests that these
regions experienced back-arc compression, suggesting a renewed connection between the underriding and
overriding plates since 15 Ma. We will discuss the implications for the Aegean geodynamic changes in the
middle Miocene.
http://www.geologist.nl
GP51A-0733
A simplified Red Bed Inclination Correction: Case Study from the Permian Esterel Group of France.
Magnetic anisotropy-based inclinations corrections have been performed in the paleomagnetic laboratory at Lehigh University, on both hematite and magnetite-bearing sedimentary rocks. Results of these corrections indicate a latitudinal variation of inclination shallowing with the formations initially located at mid latitudes suffering from more shallowing than those initially closer to the equator, consistent with the tan (Im)= f * tan (If) relationship observed by King (1955) for inclination shallowing, where Im is the measured inclination and If is the field inclination during deposition. Shallowing of the paleomagnetic vectors can be expressed in terms of the flattening factor f, that relates tan (Im) to tan (If). Anisotropy- derived hematite f factors from the Maritime Provinces of Canada and Northwest China were combined with f factors derived from corrections that use models of geomagnetic field secular variation (the EI technique of Tauxe and Kent, 2004) on red bed Formations from North America, Greenland and Europe. The dataset was used to derive a probability density function for f. The mean f value will allow a simplified inclination correction for hematite-bearing red bed formations that are suspected to be affected by inclination shallowing. This approach was tested by correcting the Permian Esterel Group red beds from France: using the distribution mean f factor of 0.64 (±0.11, ±1 standard deviation), the corrected red bed paleopole becomes statistically indistinguishable from the paleopole measured for the Esterel Group volcanic rocks that have not suffered from inclination shallowing. f data was also compiled for magnetite-bearing sedimentary rocks from the Perforada Formation and the Valle Group from Baja California, Mexico, the Pigeon Point Formation of Central California, the Ladd and the Point Loma Formations from Southern California, the Nanaimo Group of British Columbia and the Deer Lake Group of Newfoundland that have been corrected for inclination shallowing, yielding a most probable f factor of 0.67 (±0.06). Based on our results, the maximum amounts of shallowing that can be expected for sedimentary rocks is 12.4° for hematite-bearing rocks, and 11.8° for magnetite-bearing rocks. These values are statistically indistinguishable. Therefore, we combined the datasets and have obtained an f factor of 0.66 (±0.1) that can be used for either hematite or magnetite-bearing sedimentary rocks. A major implication of this result is that a rock's NRM, either acquired by chemical processes soon after deposition or by depositional processes that accurately record the ambient magnetic field, may be susceptible to similar amounts of inclination shallowing, most likely caused by burial compaction.
GP51A-0734
Constrain on the Kinematic Evolution of the Central Apennines (Italy) Using Paleomagnetic and Geological Data
The Apennines describes two major first-order arcuate features: the Northern Apennines Arc and the Southern Apennines-Calabrian Arc. We compare and discuss these major arcs, which are characterized by several differences, mainly related to paleogeographic domains, stratigraphic successions, structural setting and geodynamic evolution. The two arcs join in the Central Apennines, which are bounded by two main ca. N- S structural features: the Olevano-Antrodoco and the Sangro-Volturno lines. We reconstruct a kinematic model related to the Tortonian-Quaternary evolution of the Central Apennines thrust system, by evidencing the differences in sign and magnitude of vertical-axis rotation introduced in the Northern and Southern Apennines during the Neogene orogenesis. In fact, the Northern Apennines Arc is characterized by counterclockwise (CCW) rotations in the northward sector, changing to clockwise (CW) toward the Central Apennines, and by amount of shortening up to approximately 15-20 km. Instead, minor salients in the Central Apennines show strong CW and CCW rotations confined near the apex and corresponding to maximum shortening of ca. 15 km (e.g., the Gran Sasso salient). Finally, the Southern Apennines-Calabrian Arc is characterized by CCW rotation in the Southern Apennines, changing to strong CW rotations in Sicily; the amount of shortening increase south-westward up to 80 km (e.g., the Matese- Frosolone salient). The different kind of paleomagnetic rotations documented in curved structure belonging to the Northern and Southern Apennines may be ascribed to both different driving mechanisms and paleogeographic-structural heritage. We thus propose to use the different style of paleomagnetic rotations in arcuate shapes as a tool to discriminate if structures located in the Central Apennines geologically belong to the Northern Apennines Arc or to the Southern Apennines-Calabrian Arc. Furthermore, the Sangro-Volturno line should not be interpreted as a major tectonic separation between the two major Arcs, but represent the oblique ramp of the Plio-Quaternary frontal thrust of the Central Apennines.
GP51A-0735
Paleomagnetism of Jurassic and Cretaceous rocks bounding the Santa Marta massif - NW corner of Colombia, South America
The Santa Marta massif (SMM) is a complex terrain located in the NW margin of South America, bounded by the left-lateral Santa Marta fault to the west and the right-lateral Oca fault to the north. The SMM is cored by Precambrian metamorphic and Jurassic intrusive rocks, whereas along the SE flank crop out Jurassic volcanic rocks overlying unconformably by Limestones of Cretaceous age. Paleomagnetic analysis of 30 sites in the Jurassic and Cretaceous units in the SE region uncovered two principal magnetic components. The component "a", isolated in low coercivity and temperatures, has declinations to the north and moderate positive inclinations representing the actual field direction (n=11, D=347.6 I=23 K=30.77, a95=8.4). The component "c", with high coercivity and temperatures, has two orientations. After two-step tilt corrections, the first has northward declination and positive, low inclination (n=9, D=12, I=3, K=18.99, a95=12.1); this direction was uncovered in Cretaceous and some Jurassic rocks near to the Santa Marta fault, and we consider it as a Cretaceous component. The second direction was uncovered only in Jurassic rocks and has NNE declinations with negative-low inclinations (n=9, D=11.3 I=-14.3 K=12.36, a95=15.2); this direction represents a Jurassic component. Jurassic and Cretaceous directions isolated in areas faraway of the Santa Marta Fault suggest slight clockwise vertical-axes rotation. The Jurassic component suggests northward translation of the SMM from Paleolatitude -7.3, to near the magnetic equador in the Cretaceous, and to northern latitudes in the Cenozoic.
GP51A-0736
Block Rotations And Translations in the Isthmus of Panama
In order to determine rotations and paleolatitudinal movements of several tectonic terrains in Panama, a total of 23 paleomagnetic sites of Upper Cretaceous to Pleistocene tuffs, lavas and limestones were sampled in five areas of the Panama Isthmus. In the Colon area, the left-lateral Rio Gatun fault places Upper Cretaceous and Miocene rocks to the north and Eocene-Oligocene strata to the south. Mean directions of two sites in the northern block, after tilt correction, are westerly (Upper Cretaceous, D=275.4 I=-20.8 k=20.46 a95=10.9; Miocene, D=264.6 I=5.3 k=292.35 a95=3.5), whereas the mean direction of a site in the southern block is pointing North (Oligocene, D=346.3 I=14.3 k=18.43 a95=14.4). Comparison between Oligocene and Miocene directions documents a counterclockwise rotation of the northern block of the Rio Gatun fault (81.7º ± 13.3º) with respect to the southern block. Cretaceous and Cenozoic sites record the northward path of the trailing edge of the Caribbean plate, from 10.8ºS for Late Cretaceous to 6.6ºN for Oligocene. Components isolated in the El Valle volcano and Canal areas for Middle Miocene to Pleistocene rocks, indicate normal and reverse directions similar to the present direction of the magnetic field, therefore indicating no major latitudinal displacement of the Panama Isthmus since Oligocene.
GP51A-0737
Paleomagnetism of the Todos Santos Formation in the Maya Block, Chiapas, Mexico: Preliminary Results
Preliminary results of a paleomagnetic study on jurassic volcanic rocks (U-Pb 188.8 +/- 3.2Ma) locally interbedded with red beds assigned to the Todos Santos Formation, sampled in the Homoclinal Tectonic Province of the Neogene Fold Belt, Chiapas-Mexico, reveal multi component magnetizations acquired during pre- and post- folding of these rocks. The samples responded well to thermal demagnetization, but not so to AF demagnetization, suggesting that a high coercivity mineral phase like hematite is the main remanence carrier. The post-folding B-component direction of Dec=174.3 Inc=-30.6 (k=46; alpha95=13.6; N=4) represents a recent Tertiary? overprint; while the pre-folding C-component direction of Dec=329.9 Inc=7.8 (k=12.5; alpha95=16.3; N=8) is in agreement with a previously reported small data set for the Todos Santos Formation. When compared to the North American reference direction (Jurassic Kayenta Formation) the observed direction indicates a counterclockwise rotation of 35.9 +/- 16.6 degrees, and moderate north to south latitudinal displacement. If a reference pole from NE North America is used, the amount of counterclockwise rotation and latitudinal displacement are both slightly reduced. If the assumption that Jurassic strata in Chiapas reflect displacement of the Maya Block, then these data are consistent with reconstructions of the Maya Block in the Gulf of Mexico region. Other sites sampled in Jurassic strata suggest that in addition to the interpreted regional rotation, local (vertical-axis) rotations may have affected the region in more recent times.
GP51A-0738
Deformation Analysis of Curved Folds in the Southern Upper Magdalena Valley. Colombia
In the Upper Magdalena Valley of Colombia there are folds with changes in the strike of their axes (e.g La Hocha Anticline), the origin of which was studied by integrating field, paleomagnetic and magnetic anisotropy data. Two structural domains were defined: the northern is a symmetrical fold and has a strike N10W, whereas the southern is an asymmetrical fold and has a N30E strike. The principal fault is the San Jacinto Fault, which is a reverse fault with slikensides indicating also a horizontal component of displacement. In the hanging wall of the San Jacinto Fault Mesozoic rocks crops out forming an anticline, whereas in the footwall Cenozoic rocks with vertical and locally overturned beds are exposed. Four magnetic components were isolated, being the low coercivity/temperature component similar to the present field. A characteristic component isolated in Cretaceous and Cenozoic rocks is a pre-fold but post-Oligocene component (n=5, D=0.8, I=2.4, K=62.33, a95=9.8). Two characteristic components were isolated in Jurassic rocks. The directions of the first are similar between domains (north domain: n=10 D=20.7 I=35.6 K=13 a95=12.5; south domain: n=6 D=12 I=28.9 K=16 a95=9.5), suggesting no relationship between strike changes and declination variation. The other component has negative inclination (n=2 D=13.8 I=-27.1 K=61.42 a95=11.8) indicating a mean paleolatitude of 14.4S in Jurassic time. Paleomagnetic data suggest that the curved fold is controlled by the geometry of the paleobasin and support the northward translation of accreted terranes along the Western margin of South America during the Jurassic. Anisotropy of magnetic susceptibility analysis in Jurassic and Cretaceous rocks document three magnetic fabrics: (1) Undeformed fabrics are located in the back limb of La Hocha Anticline; (2) tectonics fabrics are associated to reverse fault planes in the north and south domains and indicated a relationship between the anisotropy ellipsoid with the local stress field; and (3) magnetic lineation associated to joins with EW strike in the back limb of the northern domain. Keywords: Curved folds; La Hocha Anticline; Deformation; Paleomagnetic;
GP51A-0739
Magnetostratigraphy of the Western Borrego Badlands, Anza-Borrego Desert, California – Implications for Tectonics and Litho- and Biostratigraphic Control
Over 200 meters of non-marine fluvial-floodplain sediments and volcanic ashes are exposed in the Borrego Badlands as part of the northwest margin of the Borrego-San Filipe Basin in the Anza-Borrego Desert, Califorinia. Our stratigraphic and paleomagnetic investigations provide data regarding the age and character of the sediments and tectonics of the region. The intrabasin sediments in Mammoth Cove and Rainbow Wash are especially favorable for paleomagnetic and biostratigraphic analysis due to their fine-grained lithology, abundant vertebrate fossils (Borrego Local Fauna) of Irvingtonian Land Mammal Age, and a continuous syndepositional record of alluvial fan, lacustrine, and playa-margin deposition in response to wrench tectonics associated with the seismogenic San Jacinto Fault Zone. The mean paleomagnetic directions following thermal demagnetization to 600°C are Incl. = 44.9°, Decl. = 11.6°, alpha-95 = 10.4°, k = 13.5, n = 16 for samples from the Brunhes Normal Chron, and Incl. = -46.5°, Decl. = 201.3°, alpha-95 = 8.6°, k = 18.1, n = 17 for samples from the Matuyama Reverse Chron. Those directions are reasonable when compared to the directions reported for older deposits in the Vallecito-Fish Creek Basin in the eastern part of the Anza-Borrego Desert (Johnson et al., 1983); those directions are Incl. = 41.6°, Decl. = 30.6°, alpha-95 = 7.4°, k = 13.1 for samples that record normal polarity, and Incl. = -35.1°, Decl. = 219.4°, alpha-95 = 7.4°, k = 9.6 for samples that record reverse polarity. By contrast, sediments deposited during the lower Matuyama Reverse Chron and upper Gauss Normal Chron where we sampled in Rainbow Wash are rotated clockwise by a greater amount – almost 90°. That rotation patterns the nearly 90° of clockwise rotation of volcanic rocks in the Western Transverse Ranges (Luyendyk, 1991) and non-marine Sespe Fm. (Liddicoat, 1990; 2001) north of the Los Angeles Basin that began about 15 million years ago (Luyendyk, 1991).
GP51A-0740
Paleomagnetic Data Bearing on the Evolution of the Walker Lane Belt Transfer Zone From mid-Miocene to Present: an Investigation of the Inferred Southern and Eastern Boundaries.
The Walker Lane Belt (WLB) transfer zone, which initiated in the mid-Miocene, presently links the Eastern California Shear Zone (ECSZ) in the south to the Central Nevada Seismic Belt (CNSB) and WLB to the east and north, respectively. This transfer zone is part of a diffuse intracontinental deformation zone that accommodates some 25 percent of the current motion between the North American and Pacific plates. The boundary of the transfer system is clear on the northern and western margins but the extent of the system to the south and east is only inferred. The extent of deformation and development of the WLB transfer zone since the mid-Miocene is being examined by a paleomagnetic study of 125 sites that includes Miocene to mid-Pliocene volcanic and shallow intrusive rocks near the inferred southern and eastern boundaries. Results from 39 sites inside and along the southern boundary (i.e. Goldfield Hills, Montezuma Range, Clayton Ridge) show about 30° of clockwise rotation (D = 028.3°, I = 57.8°, α95 = 3.9°, discordant from the expected Neogene direction of D = 358°, I = 55°). The area where 13 of these 39 sites are located (i.e. northern Amargosa Range, eastern Slate Ridge) was previously thought to lie outside of the inferred boundary, yet it also shows about 30° of clockwise rotation (D = 031.2°, I = 52.4°, α95 = 6.7°). Areas along the eastern boundary (i.e. southern San Antonio Range) of the transfer zone are still under investigation; data obtained to date are not internally consistent. Overall, the available paleomagnetic data suggest that the southern extent of the WLB transfer zone was larger than previously expected during the mid-Miocene to mid-Pliocene, and based on previous paleomagnetic, structural, and geodetic studies of the area, support a transition from more diffuse to localized deformation (forming the Mina Deflection) at about 3 Ma.
GP51A-0741 INVITED
Paleomagnetism of the Miocene Columbia River Basalt Group in Oregon and Washington from the Pacific Coast to the Columbia Plateau: Magnetostratigraphy, Vertical-Axis Rotations, Paleosecular Variation, and Remagnetization
Identification of individual flows within the Columbia River Basalt Group (CRBG) has mostly relied on minor differences in geochemistry, but magnetic polarity has also proved useful in differentiating flows and establishing a temporal framework. Within the thick, rapidly erupted Grande Ronde Basalt four major polarity chrons (R1 to N2) have been identified. Because cooling times of CRBG flows are brief compared to rates of paleosecular variation (PSV), within-flow paleomagnetic directions are expected to be constant across the extensive east-west reaches of these flows. Vertical-axis rotations in OR and WA, driven by northward-oblique subduction of the Juan de Fuca plate, thus can be measured by comparing directions for western sampling localities to directions for the same flow units on the relatively stable Columbia Plateau. Clockwise rotations calculated for outcrop locations within the Coast Range (CR) block are uniformly about 30° (N=102 sites). East of the northwest-trending en échelon Mt. Angel-Gales Creek, Portland Hills, and northern unnamed fault zones, as well as north of the CR block's northern boundary (~Columbia River), clockwise rotations abruptly drop to about 15° (N=39 sites), with offsets in these bounding fault zones corresponding to the Portland and Willamette pull-apart basins. The general agreement of vertical- axis rotation rates estimated from CRBG magnetizations with those determined from modern GPS velocities indicates a relatively steady rate over the last 10 to 15 Myr. Unusual directions due to PSV, field excursions, or polarity transitions could provide useful stratigraphic markers. Individual flow directions, however, have not been routinely used to identify flows. One reason this has been difficult is that remagnetization is prevalent, particularly in the Coast Ranges, coupled with earlier demagnetization techniques that did not completely remove overprint components. Except for the Ginkgo and Pomona flows of the Wanapum and Saddle Mountains Basalts, reference Plateau directions for the CRBG are poorly known. Moreover, field and drill- core relations indicate that flows with different chemistries were erupted at the same time. Renewed sampling, therefore, has been undertaken eastward from the Portland area into the Columbia River Gorge and out onto the Plateau. Resampling of the Patrick Grade section (23 flows) in southeastern WA has shown that overprint magnetizations were not successfully removed in many flows at this locality in an earlier study [1]. This brings into question blanket demagnetization studies of the CRBG as well as polarity measurements routinely made in the field with hand-held fluxgate magnetometers. [1] Choiniere and Swanson, 1979, Am. J. Sci., 279, p. 755
GP51A-0742
Late Paleogene Volcanic Rocks from the Qiangtang Terrane (Central Tibet) Record Shallow Paleomagnetic Inclinations
Here we present the first Cenozoic volcanic paleomagnetic results from Central Tibet. Three well-dated, approximately 40-35 Ma volcanic localities within the Qiangtang terrane record a mean tilt-corrected inclination of 43.0±7.9°, which is significantly shallower than the 54.1±2.7° predicted for Central Tibet by late Paleogene Eurasian reference poles (Besse and Courtillot, JGR 2002). Rock magnetic data and mixed polarities suggest the characteristic remanent directions are primary. The mean inclination of coeval sedimentary rocks from Qiangtang and Songpan-Ganzi, 37.7±4.8°, is only 5-7° shallower than our volcanic result. Therefore, we suggest that most of the inclination anomaly observed in late Paleogene and early Neogene rocks here and throughout Central Asia has a tectonic explanation. Geologic constraints from Northern Tibet and Alxa can accommodate at least 350 km of shortening within this region since Eocene time, which accounts for only 4° of the shallowing. Although the remaining inclination discrepancy between paleomagnetic directions observed in our volcanic rocks and those predicted from the reference poles could be due to additional, unaccounted shortening, it more likely means that the Cenozoic Eurasian reference poles do not adequately represent the Siberian part of the Eurasian landmass. Our volcanic paleomagnetic data are consistent with the interpretation of Hankard et al. (JGR 2007) based on shallower than expected inclinations from Mongolian volcanic rocks that the Siberian craton was located farther south during the early Cenozoic and has since moved northward due to the Indo-Asian collision (Cogne et al., 1999).
GP51A-0743
Was there a Greater Asia before India-Asia collision?
Previous paleomagnetic studies in central Asia have suggested some 1500 km of intracontinental shortening occurred north of the India-Asia suture. This suggests the existence of a "Greater Asia" that was about one and half times the size of the present Tibetan plateau and the same extent as Greater India. However, recent studies indicate that the paleomagnetic directions carried by continental red beds may have a significant bias toward shallow inclinations. If this is true for the red beds in Tibet, it could lead to an overestimate for the southern extension of Greater Asia and hence an older India-Asia collision age. To test the accuracy of paleomagnetic directions, we have collected samples from Late Cretaceous red beds and lava flows and Eocene tuffs from the Linzhou basin, Lhasa block. Our laboratory work successfully isolated characteristic remanent directions from the red beds and volcanic samples. Preliminary analysis of systematic demagnetization data appears that the paleomagnetic inclinations carried by the red beds are significantly shallower than those from the lava flows. Therefore, "Greater Asia" could be much smaller than previously thought. We will report detailed paleomagnetic results and elongation/inclination analysis for the red beds directions, and discuss their implications for India-Asia collision.
GP51A-0744
On the Mechanism of the Acquisition of Stable Secondary Pyrrhotite Magnetic Remanences and Implications for Thermo-Tectonic Evolution -The Example of the High Himalayan Crystalline of Solu Khumbu (Nepal)
Combining magnetic techniques, petrographic analyses and detailed field observations high-grade metamorphic gneisses and leucogranites of the Higher Himalayan Crystalline (HHC) have been examined in view of their suitability for paleomagnetic investigations. Sampling has been accomplished on a south-north transect at ca. 87°E along the Dudh Kosi from a few km south of Lukla (close to the Main Central Thrust) to Dugla in the north ( Everest region). Detailed field mapping and thin-section analysis show that the sampled transect underwent at least three metamorphic events (M1, M2 and M3). M1 and M2 represent pro-grade metamorphism (amphibolite to granulite facies, and granulite facies, respectively) overprinted by M3 retrograde metamorphism during exhumation and cooling (green schist facies). Hence, the sampled transect is divided in a (1) south segment which underwent M1 and M3, and a (2) north segment characterized by M1, M2 and M3. Regardless the complexity and variability in the remanence behavior between sampling localities and within specimens, a dominant characteristic magnetic remanence component has been isolated, depending on its unblocking temperature range: Major unblocking, between 250-350°C, is attributed to pyrrhotite (ChRMpyr). The presence of the latter has also been demonstrated by thermal demagnetization of SIRM. IRM semi-quantitative analysis (data acquisition and inversion) strongly support the dominance of pyrrhotite. Stable and well grouping ChRMpyr directions occur widely in the paragneisses and migmatitic ortho- and paragneisses as well as in calc gneisses of the HHC. Grain sizes of pyrrhotite are obviously in a sufficiently hard coercive range to enable stable remanence recording below the Curie temperature (~ 325°C) in a temperature interval where brittle deformation had commenced. A thermoremanence is likely acquired and its age will approximately correspond to the age of last metamorphic cooling event (i.e., Miocene). Normal and reverse polarities are observed, partly co-existing within single specimens, supporting the thermoremanent origin of the ChRMpyr. First microscopic observations, stepwise thermal demagnetization and SIRM behavior during thermomagnetic runs by MPMS, all indicate that pyrrhotite occurs within a grain size range up to 30-40 μm, and hence, support our assumption that pyrrhotite occurs as hosted inclusions (nano- to micrometer scale) in the silicate minerals of the gneisses (e.g. plagioclase and garnet). Consequently, the structure and grain size of the host minerals will define the remanence behavior of the pyrrhotite. The thermoremanent origin of the ChRMpyr allows the determination of remanence- acquisition ages (~ age of last peak metamorphic cooling event), and quantification of late-orogenic block rotations and long-wavelength folding/tilting in the zone of frontal collision between India and Eurasia.
GP51A-0745
Paleomagnetic Study of the Devonian Reef Complexes of the Canning Basin, Western Australia
The reef systems in the Canning Basin, Western Australia perhaps are the best exposed and least deformed examples of ancient reef systems known in the world. The recently commenced multi-disciplinary research project in the Devonian reef complex of the Canning Basin is a broad investigation of the depositional history of a carbonate platform using paleomagnetic, stable isotope geochemistry (inorganic and organic), sedimentology, and biostratigraphy. By focusing on the world-class exposures in the Canning Basin, this project seeks to provide a global stratigraphic reference frame for key intervals in life history such as the Frasnian-Fammenian mass extinction event, as well as providing a useful analogue for resource models of other carbonate reef systems elsewhere in the world. This reference frame will consist of a high resolution magnetostratigraphic profile to supplement the presently-sparse Global Polarity Timescale (GPTS) for the Devonian, as well as a chemostratigraphic profile (chiefly carbon isotopes) to identify possible shifts in the global carbon budget associated with biotic crises and/or climate change. Additional goals include identification of the conditions leading up to, and possible causes of the mass extinction event, and testing for a possible mid-Paleozoic episode of True Polar Wander. We report here on a paleomagnetic study of two magnetostratigraphic sections in the Canning Basin to address the goals mentioned above. Paleomagnetic samples have been drilled on the Late Frasnian limestones in the north of the Windjana Gorge National Park, with a total number of 400 core samples. So far preliminary paleomagnetic analysis on pilot samples reveals two characteristic remanent components. One component has a blocking temperatures less than 400 degree Celsius, probably a component of secondary overprint; another component has a blocking temperature around 580 and around 680 degree Celsius, indicating the presence of magnetite and hematite, respectively. The latter component is believed to carry the primary remanence.
GP51A-0746
Paleomagnetism of Harutagawa formation in the Hohi Volcanic Zone in northeastern part of Kyushu Island, Japan
The Beppu-Shimabara graben in the Hohi Volcanic Zone (HVZ) in northeastern part of Kyushu Island, Japan is thought to be a volcano-tectonic depression. Volcanic stratigraphy and age studies of the area have unraveled the late Pliocene structural formation history of HVZ (e.g. Kamata, 1994, Kido, 2007). The age and sedimentation rate of lacustrine deposits in HVZ is one of the keys for interpreting the temporal relation between the formation of Beppu-Shimabara graben and the huge pyroclastic flows appeared in the area. We study the magnetostratigraphy of the Harutagawa formation, which is one of those lacustrine deposits. The formation is dominated by conglomerates and mad stones to siltstones, in the lower and upper parts, respectively, but bares many tuff layers all over the formation. Fission track ages of two tuff layers, one is from lower part and the other is from upper part of Harutagawa formation, are determined as 3.86 ± 0.77Ma and 3.6 ± 0.2Ma, respectively (Kido, 2007). Samples for paleomagnetic analyses have been collected at 28 sites in several continuous outcrop of the Harutagawa formation. The sites are set to be spaced equally in the stratigraphy. Samples were collected by a portable electric motor drill. A few pilot specimens from each site are subjected to progressive thermal and alternating field (AF) demagnetization. However, AF demagnetization is not effective. All remaining specimens are, therefore, submitted to the progressive thermal demagnetization. The samples have mean magnetic intensity of 1.7 × 10- 4A/m and 7.7 × 10-5A/m before and after demagnetization, respectively. Samples from 6 sites have no stable component or are thought to be completely remagnetized by the present magnetic field. As the result, 22 sites are determined their polarities; 8 were reversed and 14 were normal. The normal polarity sites were correlative to Sidufjall, Nunivak and Cochiti subchrons in the Gilbert reversed polarity chron. This correlation indicates that the Harutagawa formation deposited in less than 0.7m.y. It gives a sedimentation rate of 130m/my, which is rather slow for formation of those faces.
GP51A-0747
Position of Dharwar Craton During Paleo-Neoproterozoic Period: Paleomagnetic Data of Cross-Cutting Dyke Swarms From Harohalli Area, Karnataka, India
One hundred oriented hand samples were collected during 2006 and 2008, from Paleo-Neoproterozoic diabase dykes from Harohalli area (Karnataka) belonging to the Archaean Dharwar Craton of Indian subcontinent. The area consists of several dyke swarms of different ages with cross-cutting relationships. Of them, the most conspicuous is the ~2.37 Ga (Halls et al., 2007; French et al., 2004) E-W trending quartz dolerite swarm. Among the samples collected, some are from fresh outcrops exposed in quarries and some from more weathered road-side outcrops or small field exposures. In few cases it is difficult to differentiate between the small exposures and the boulders in the field. Besides, very high Q-values obtained from petrophysical determinations for one of the sites indicate that the site is struck by lightning. Paleomagnetic and rock magnetic investigations carried out in the Laboratory for Solid Earth Geophysics of the University of Helsinki, including alternating field and thermal treatments for the samples, reveal the presence of two to three remanence components. The characteristic remanent magnetization component in the East-West trending quartz dolerite dykes is upward directed magnetization, which is strikingly similar to that reported by Halls et al. (2007) and Dawson and Hargraves (1994). Some sites reveal antipodal directions thus indicating field reversals. The characteristic component places India at high (southern?) paleolatitudes and the Dharwar Craton can be juxtaposed at 2.37 Ga with the Yilgarn Craton of Australia, where nearly coeval dykes (the Widgiemooltha swarm, 2.4 Ga) yield similar dyke trends in the reconstruction. This result may point to a long lived mantle plume under joint Dharwar-Yilgarn cratons. However, the origin of the other remanence components is less well understood. Further, a recent field study (2008) reveals that the N-S trending olivine bearing dyke swarm is in fact the oldest one in the area since it appears to be cut by the E-W swarm. In this paper, we present paleomagnetic data of cross cutting dykes of Harohalli area including baked contact tests, coupled with rock magnetic and petrophysical data in order to better define the APWP of the Dharwar Craton. In particular, we will seek evidences of possible block rotations or tiltings within the Dharwar craton in the light of feldspar clouding and paleomagnetic declination and inclination data. We also try to define the position of Dharwar craton in various proposed supercontinent assemblies during the Paleo-Neoproterozoic times.
GP51A-0748
Geochronology and Paleomagnetism of Mafic Igneous Rocks in the Olenek Uplift, Northern Siberia: Implications for Mesoproterozoic Paleogeography
We present a new, reliably dated Mesoproterozoic paleopole for Siberia, based on a combined geochronological and paleomagnetic study of mafic rocks within the Mesoproterozoic Sololi Group of the Olenek Uplift in northern Siberia. Ion microprobe (SHRIMP) U-Pb analysis of zircon and baddeleyite yields crystallisation ages of 2036 ± 11 Ma for a basement granite and 1473 ± 24 Ma for a large dolerite sill within the Kyutingde Formation. The baddeleyite result indicates that the lower Sololi Group is significantly older than was suggested by previous K-Ar results. Paleomagnetic analysis of the dolerite sill and related mafic intrusive rocks yields a paleopole at 33.6° N, 253.1° E, A95 = 10.4°. A positive baked-contact test between the Kyutingde sill and sedimentary country rocks shows that the magnetisation is primary. Comparison of this paleopole with coeval results for Laurentia provides a revised reconstruction between Siberia and Laurentia, and implies that these two continents were parts of a single Mesoproterozoic supercontinent since at least 1473 Ma. We argue that Siberia, Laurentia, and Baltica belonged to the same supercontinent between 1473 Ma and mid-Neoproterozoic time.
GP51A-0749
STRATIGRAPHIC AND PALEOMAGNETIC COMPARISONS OF MESOPROTEROZOIC STRATA AND SILLS FROM THE BELT BASIN, NW MONTANA, USA, AND NW ANABAR SHIELD, RUSSIA: TESTING A PRECAMBRIAN PLATE RECONSTRUCTION
Mesoproterozoic sedimentary strata and mafic sills overlie Archean and Paleoproterozoic basement rocks with profound unconformity in NW Montana and along the NW margin of the Anabar Shield in northern Siberia. The two localities plot adjacent to one another on a Precambrian plate reconstruction proposed by Sears and Price (2003) that places the NE margin of the Siberian craton against the SW margin of the North American craton. The plate reconstruction predicts that these strata occupied contiguous parts of an intracratonic basin prior to late Neoproterozoic breakup of Rodinia. Here we show that the Mesoproterozoic stratigraphic sequences, sedimentary structures, and lithologies of the NW Anabar margin closely match the Neihart, Chamberlain, and Newland formations of the Little Belt Mountains of Montana. They may predate opening of the Belt Supergroup rift basin at ca. 1500 Ma, when a major mafic magmatic episode occurred in both regions. Preliminary paleomagnetic data from the Siberian section will be compared with the Laurentian APWP to evaluate the reconstruction.