GP43C-1482
No Significant Rotation of the Moesian platform and Rhodope (Bulgaria) since the early Oligocene: implications for the Aegean and Balkan tectonic history
Here, we present new paleomagnetic data from Oligocene (~30-35 Ma) and lower Miocene (~20 Ma) volcanic rocks exposed on the Moesian platform and Rhodope mountains of Bulgaria. A total of 467 samples were analysed from 58 lava sites obtained from 6 volcanic centers. Each lava yields high k-values, typically well above 100, and we interprete the results of a lava site as representing a spot reading of the earth's magnetic field. Rock magnetic tests showed that the ChRM is carried by titano-magnetite. A total of 56 successful lava sites was averaged, resulting in an average direction of D/I = 9.8°/60.5°, and a k/á95 of 23.4/4.0. The k-value can well be explained by secular variation of the earth's magnetic field and we consider this direction reliable. The expected direction for 30 Ma for Eurasia according to Besse & Courtillot (2002) is D: 6.8±4.8° and I: 57.4±3.6°, leading to a non-significant rotation of 3.0±7.6° and a non-significant flattening of –3.1±4.3°. This finding has important implications for the tectonic evolution of the Aegean-Balkan-Carpathian orogenic system. Between 13 and 8 Ma, a 600 km long block in the west-Aegean region between northern Albania and the southern Peloponnesos underwent a 50° clockwise rotation. Classically, this rotating block has been suggested to rotate around a pole in northern Albania. However, the absence of significant rotation in the Moesian platform and Bulgarian Rhodope is at odds with this interpretation: the rotation difference would in this case be accommodated by more than 300 km extension. Even though extension is active in the central and northern Aegean region in this time, it is certainly less than 100 km. In this presentation we will postulate a scenario aiming to solve this space problem, including the relocation of the rotation pole in western Greece, and the accommodation of west-Aegean rotation in the Dinarid-Carpathian system, which can form ground for further analysis of the Neogene horizontal motions around the Moesian platform http://www.geologist.nl
GP43C-1483
An Extended Paleozoic Apparent Polar Wander Path for Baltica: new Permo-Carboniferous Paleopoles From the Donbas Region (Ukraine)
An improved Paleozoic apparent polar wander (APW) path for Baltica is presented here on the basis of six new paleopoles that have been determined from samples collected in the Donbas region in the Dniepr-Donets basin in south-eastern Ukraine. Constructing APW paths allows improving paleogeographic reconstructions that reach further back in time than 200 Ma, where the use of oceanic isochrons and hotspot track has limited applicability. The absence of penetrative regional deformation and the subparallel trending bedding attitudes across the basin suggest that our sites did not suffer from local rotations and their results are interpreted as representative for Baltica. The data presented here improve the paleogeographic reconstruction of Baltica within the collage of the supercontinent Pangea. The six new paleopoles cover a time span from earliest Carboniferous (~356 Ma) to early Permian (~295 Ma). In our reconstruction, Baltica was located at a constant latitude of ~5°N during a major part of the Carboniferous, while at ~310 Ma it started to move gradually northward, reaching a paleolatitude of ~13°N at 295 Ma. From ~355 Ma to 295 Ma Baltica experienced a net ~20° clockwise rotation. Our new data differ with the APW path from Torsvik et al. (submitted) in the time span from ~320-300 Ma, wherein they propose a northward movement from more southerly latitudes. From 300 Ma onwards, our path fits the reference path from Torsvik et al. A possible Permian remagnetization of our sites is not likely, considering the rotational differences in the various time spans, and rockmagnetic analyses that have been performed. We also discuss the usage of the TK03 model (Tauxe and Kent (2004), Geoph. Mon. 145, pp 101-116) that allows for the correction of inclination error caused by compaction during burial, which is insignificant for most sites. This suggest that the NRM has been acquired after compaction.
GP43C-1484
No vertical axis rotations during Neogene transpressional orogeny in the NE Gobi Altai: coinciding Mongolian and Eurasian early Cretaceous apparent polar wander paths
Here we test the role of vertical axis rotations during transpressional mountain building. To this end, we carried out a paleomagnetic study in the NE Gobi Altai of southern Mongolia, sampling widely exposed lower Cretaceous lavas allowing comparison of rotation histories of the Ih Bogd, Baga Bogd and Artz Bogd restraining bends at the eastern termination of the Bogd strike-slip zone. We provide new 40Ar/39Ar ages to show that the stratigraphy of mafic lavas and fluvio-lacustrine sediments on the southern flanks of Mt Ih Bogd and Mt Baga Bogd have ages between ~125 and ~122 Ma, and a mafic sill that intrudes the sequence has an age of 118.2 ± 0.8 Ma. The lavas are older than previously dated lavas south of Artz Bogd, with ages of 119-115 Ma. Paleomagnetic results from the 119-115 Ma lavas south of Artz Bogd show a significant steeper inclination than both results from 125-122 Ma lavas of Baga Bogd and Ih Bogd, as well as from newly sampled and previously published younger lavas and necks of the 107-92 Ma Tsost Magmatic Field and Shovon and Khurmen Uul basalts. We explain this result by insufficient averaging of secular variation and small errors induced by overcorrection of bedding tilt. We show that individual lavas in the SE Artz Bogd locality represent individual spot readings of the Earth's magnetic field and integrate all results obtained from lower Cretaceous lavas in the Gobi Altai. We present a pole, or rather, an apparent polar wander path without significant plate motion from ~125-95 Ma, with n=126, ë=80.8, ö=158.4, ê=25.3, A95=2.5, paleo-latitude = 48.2 with a scatter Së=16.7 (Sl=15.3, Su=17.8) and a regionally consistent direction for the Gobi Altai of D/I = 11.1/65.9, ÄD/ÄI = 3.8/1.9. This is one of the best-determined paleopoles/APWP's for Asia. There is no significant deviation of the 125-95 Ma pole position of the Gobi Altai from the reference positions of Eurasia. Formation of the Ih Bogd, Baga Bogd and Artz Bogd restraining bends was thus not associated with vertical axis rotations larger than our error margin of ~10°. From this we conclude that the Bogd strike-slip zone is a weak fault zone, in which shear is localised, comparable to the large strike-slip systems further south in Tibet, such as the Altyn Tagh fault http://www.geologist.nl
GP43C-1485
Paleomagnetism of the Early Cretaceous island arc complex from the north Sakhalin Island: implications to the Cretaceous paleogeography of the Pacific
Several paleomagnetic studies of Late Cretaceous island arc complexes from the Northwest Pacific demonstrated that the corresponding island arcs had originally been far away from the Eurasian margin and docked to it in the Cenozoic. Hence the Late Cretaceous paleogeography of the Northwest Pacific was very different from the modern one and resembled what we now see in Polynesia. In contrast, the Early Cretaceous subduction-related volcanism is confined to the Okhotsk-Chukotka volcanic belt along the NE coast of the Sea of Okhotsk; both geological and paleomagnetic data firmly indicate that the belt did not move with respect to Eurasia. Thus a drastic difference in paleogeography of the Northwest Pacific in the Early and Late Cretaceous epochs appears to be indicated, and the existence of remote intra-oceanic island arcs becomes of crucial importance. We studied basalt flows of island arc affinity and basalt cobbles from intra-formational conglomerate at the northernmost tip of the Sakhalin Island (54.3°N, 142.2°E); according to radiolaria from silicic intercalations, the complex is of Valanginian-Albian age. Upon stepwise thermal cleaning, a well-defined dual-polarity characteristic magnetization was isolated from all 14 sites studied. This remanence shows about five-fold decrease in dispersion upon tilt correction, thus rendering the fold test positive. The reversal and conglomerate tests are positive too, and the dual-polarity magnetization is most likely to be primary. Its tilt-corrected mean direction (D = 279.3°; I = 54.2°; k=35, a95 = 6.3°, N = 14) corresponds to a paleolatitude of 35° +/- 6°, which is by more than 20° lower than what is expected for the Eurasian plate. In our presentation, we will discuss different implications of these data on paleogeography of the Northwest Pacific in Early Cretaceous time.
GP43C-1486
Preliminary ages and paleomagnetic data on Neoproterozoic Kurgan Fm. from the Lesser Karatau Range in South Kazakhstan: tectonic implications
The Ural-Mongol fold belt stretches from the North Urals to Kazakhstan and Tien Shan to Altai and Mongolia to the Pacific. This belt comprises many microcontinents with Precambrian crust and numerous island-arc domains separated by ophiolitic sutures. Many scientists acknowledge a very important role of the UMB in formation of Eurasia, and numerous controversial models of its evolution have been proposed. The history of the belt is poorly understood, especially its Neoproterozoic and Early Paleozoic stages. This is largely due to nearly complete lack of reliable age determinations and paleomagnetic data from microcontinents. With this shortcoming in mind, we undertook a geochronological and paleomagnetic study of the Kurgan formation, which is the youngest member of the Neoproterozoic succession in the Lesser Karatau microcontinent in South Kazakhstan. This formation consists of weakly metamorphosed sedimentary rocks, mostly redbeds and silicic tuffs, which are deformed into simple folds without penetrative deformation; with erosional and some angular unconformity, these sediments are overlain by dolomites of Nemakit-Daldynian age and Cambrian to Ordovician limestones. Numerous zircon grains from upper and lower tuffaceous units yielded concordant U-Pb ages of 766.4 ± 3.6 Ma and 831 ± 7.5 Ma respectively. Out of 33 sites sampled, 20 are fully demagnetized up to date. Apart from a low-temperature component aligned along the present-day field, two more components are successfully isolated from most samples. An intermediate-temperature component of ubiquitously reverse polarity is of postfolding origin and was likely to have been acquired in the late Paleozoic. A high-temperature component shows two nearly antipodal groups of directions and statistically significant maximum in grouping upon full unfolding; hence this remanence is of pre-Paleozoic age and likely to be primary. In our presentation, we will discuss different implications of this data on Neoproterozoic paleogeography and evolution of Eurasia.
GP43C-1487
Paleomagnetic and Geochronologic Data from Central Asia: Inferences for Early Paleozoic Tectonic Evolution and Timing of Worldwide Glacial Events
The Neoproterozoic to early Paleozoic Ural-Mongol belt that runs through Central Asia is crucial for determining the enigmatic amalgamation of microcontinents that make up the Eurasian subcontinent. Two unique models have been proposed for the evolution of Ural-Mongol belt. One involves a complex assemblage of cratonic blocks that have collided and rifted apart during diachronous opening and closing of Neoproterozoic to Devonian aged ocean basins. The opposing model of Sengor and Natal"in proposes a long-standing volcanic arc system that connected Central Asian blocks with the Baltica continent. The Aktau-Mointy and Dzabkhan microcontinents in Kazakhstan and Central Mongolia make up the central section of the Ural-Mongol belt, and both contain glacial sequences characteristic of the hypothesized snowball earth event. These worldwide glaciations are currently under considerable debate, and paleomagnetic data from these microcontients are a useful contribution to the snowball controversy. We have sampled volcanic and sedimentary sequences in Central Mongolia, Kazakhstan and Kyrgyzstan for paleomagnetic and geochronologic study. U-Pb data, 13C curves and abundant fossil records place age constraints on sequences that contain glacial deposits of the hypothesized snowball earth events. Carbonates in the Zavkhan Basin in Mongolia are likely remagnetized, but fossil evidence within the sequence suggests a readjusted age control on two glacial events that were previously labeled as Sturtian and Marinoan. U-Pb ages from both Kazakhstan and Mongolian volcanic sequences imply a similar evolution history of the areas as part of the Ural-Mongol fold belt, and these ages paired with paleomagnetic and 13C records have important tectonic implications. We will present these data in order to place better constraints on the Precambrian to early Paleozoic tectonic evolution of Central Asia and the timing of glacial events recorded in the area.
GP43C-1488
Late Paleozoic History of the Kazakhstan orocline
Kazakhstan, located in the center of the Eurasian continent, is made of younger crust sandwiched between the old cratons of Siberia, Baltica and Tarim. A prominent geologic feature of Kazakhstan consists of two concentric horse-shoe shaped volcanic arcs, which are Devonian and late Paleozoic in age and with the youngest arc on the inside. Recent paleomagnetic studies in the region show that in the Middle Devonian the volcanic arc, which is now curved, was nearly straight and NW-SE trending. This arc marked the NE margin of the block that some have called Kazakhstania. The southern arm of the curved structure shows no significant changes in its relative orientation since the Middle Devonian, while its northern arm (in present day coordinates) underwent ~ 180 degrees of rotation. To constrain the timing of the rotation we conducted a paleomagnetic study of the Early Permian (295-290 Ma) subduction related volcanics from the central limb of the orocline. Declinations of the primary as well as secondary (Triassic) components isolated in studied rocks agree well with the Baltica reference directions indicating that the bending was essentially over by the earliest Permian. An initial bending of the orocline was probably caused by dextral shear motion of Siberia that affected the northern (Chingiz Range) end of Kazakhstania, while the collision of Tarim with Kazakhstania's southern corner (Tien Shan) created a back-stop. Continued subduction under the northern and southern limbs with an estimated subduction velocity of less than 1 cm/yr eventually led to closure of the intervening Balkhash-Ili ocean and tightening of the orocline. By the early Permian the Balhash-Ili represented a narrow oceanic basin with two subduction zones of opposite polarity, likely similar to the modern Molucca Sea subduction system.
GP43C-1489
Timing and distribution of tectonic rotations in the northeastern Tibetan plateau
We report paleomagnetic data from the northeastern margin of the Tibetan plateau to understand the timing and distribution of deformation (i.e. vertical-axis rotations) during the Indo-Asia collision. Paleomagnetic results throughout the Xining Basin Paleogene and Neogene strata, recently dated using magnetostratigraphy between 52 and 17 Ma, show that some 25° clockwise rotation with respect to the stable Eurasian continent, occurred at ca. 41 Ma. In view of the regional compilation of existing paleomagnetic from the northeastern Tibetan plateau, these results suggest that this region experienced two phases of clockwise rotation distinct in time and place. (1) During a mid-Eocene phase, clockwise rotations took place in the regional Paleocene-Miocene basin system including rotation in the Xining Basin at ca. 41 Ma, thus establishing the existence of widespread deformation at this time. (2) During a mid-Miocene phase, (within 17 to 11 Ma), clockwise rotations are restricted to the Mio-Quaternary basin system, implying that the Laji Shan thrust belt, separating the two basin systems, was active during this time interval. These results are consistent with low temperature thermochronology constraints on tectonics of the Laji Shan indicating exhumation starting at ca. 15 Ma (See Dupont-Nivet et al., this meeting, Session T20). http://www.geo.uu.nl/~forth/people/Guillaume
GP43C-1490
Magnetostratigraphy of the hominin-bearing Hadar Formation (Ledi-Geraru, Ethiopia), and regional evidence for environmental change ca. 3.2 Ma
To date and characterize depositional environments of the hominin-bearing Hadar Formation, magnetostratigraphy was applied to Pliocene lacustrine sediments from the eastern part of the Hadar Basin (Ledi-Geraru research area). Characteristic geochemistry and lithologic features of three tuffaceous horizons at the bottom, middle and top of the stratigraphy are recognized as the Sidi Hakoma Tuff (SHT), Triple Tuff-4 (TT-4) and the Kada Hadar Tuff (KHT), respectively, previously dated by 40Ar/39Ar in other part of the basin. Demagnetizations and rock magnetic analysis of paleomagnetic samples collected at regular 50 cm intervals on a total stratigraphic thickness of 230 meters between the SHT and the KHT enable us to isolate paleomagnetic directions from a primary detrital remanent magnetization mostly carried by (titano-)magnetites basaltic in origin. These results indicate two paleomagnetic reversals bracketing a reversed polarity interval identified as the Mammoth event (chron 2An.2r). The average paleomagnetic direction, consistent with existing paleomagnetic data, indicates a post-3 Ma counterclockwise vertical-axis tectonic rotation (5-10 degrees) of the Hadar Basin and pervasive shallowing of paleomagnetic inclination (5-10 degrees) related to sedimentation and compaction. Ages of tuffaceous layers and of paleomagnetic reversals show excellent consistency between 40Ar/39Ar dating and the most up to date astronomically tuned polarity time scale. Linear interpolations indicate constant sediment accumulation rates (~90 cm/kyr) throughout the section, except for the uppermost part which shows a threefold increase between the top of the Mammoth 2An.2r chron and the KHT. Along with existing chronostratigraphic results from the Hadar Basin, we show that the steady-state deposition, taking place in an eastward tilting basin since ~3.4 Ma, was regionally disrupted ca. 3.2 Ma by a relatively short-lived but significant change in environmental conditions. This disruption may have resulted in evolutionary changes previously documented in Australopithecus afarensis. http://www.geo.uu.nl/~forth/people/Guillaume
GP43C-1491
Paleomagnetic Evidence for the Tectono-Stratigraphic Evolution of the Mt.Galili Area / MER / Ethiopia
The Mt.Galili area (N 9,77°, E 040,55°) is the focus of current anthropological studies on early hominid evolution *[4]. The Mount Galili Formation (MGF)*[5] is subdivided into seven Members, each representing a sedimentary cycle, sustaining temporally interruptions by volcanic activity. Our paleomagnetic investigation concentrates on ascertaining primary magnetisation vectors (PMV) of volcanic layers embodied within the MGF, applying alternating field and thermal magnetic cleaning methods. Magnetite and ti-magnetite are the main carriers of the NRM (basalt, ignimbrite). Hematite shares in amounts up to 20%, Goethite occasionally participates up to 30% in magnitude of the NRM components. Two events of magnetic polarity reversals terminate a magnetic inverse period designating the lot of the MGF unit. The amount of the PMV's rotation in respect to an expected paleodirection*[1] of 183°/-13° are in the range of 1°-52° . The resulting mean PMV data provide implications on (A) rift-related block rotation / tilting in relation to the stable African crust since the Pliocene and they also support (B) stratigraphic age determinations of the MGF: A: Rotational movements cluster in 4 sectors: (a) The SE sector demonstrates almost unchanged orientation of the PMVs whereas (b) it's continuation to the NW sector suffered clockwise rotational tilting (up to 12° rot / 19° tilt). (c) The SW sector (Mt.Galili area s.str.) in contrast embrace a unique counterclockwise rotation component of 7°-17°, but the modulus of vector inclination, concerning individual rift blocks, is variable in either direction. (d) The NE sector (Satkawini) sustained the major counterclockwise rotation (41°°-52° rot / 3°- 17° tilt) We consider the Mt.Galili area being the place where trans-tensional tectonics were active during the late Miocene to create the lateral off-set of magmatic segments marking the centre of the MER. This tectonics are considered to belong to a arcuate accommodation zone in respect to the migration of the Afar triple junction since~4my *[6]. Another impact of this tectonic environment concerns the generation of weakened zones within the thinned crust induced by crosscutting structural items, subsequently allowing the supply of supercritical crustal melts and finally resulting in explosive eruptions. An extensive dispersed ignimbrite layer (Dhidinley Mb.) yielded an absolute age of ~4,1 my( sanidine, Ar/Ar)*[5]. B: Our stratigraphic age determination of the MGF is focused on magnetic reversal events documented in magmatic layers upon and below the mentioned Dhidinley ignimbrite selfsame demonstrating reverse polarity of its PMV. The uppermost Caashacado Mb. of the MGF exhibits normal polarity contained in gray ignimbrite layers at the top of the pile. We consider this reversal event providing a max. age of 3,58 my (Gauss/Gilbert crossover)*[2]. On the other hand the lowermost lava flow containing normal polarity was excavated by rotational rift block tilting. We consider this reversal event providing a min. age of 4,18 my (Gilbert/Cochiti crossover)*[2]. The mentioned age classifications are consistent with biochronological valuations *[3, 5]. References cited: *[1]: Besse, J. & Courtillot, V. (2002): J.Geophys.Res.107/B11/2300 *[2]: Cande, S.C., & Kent D.V.,(1995): J. Geophys. Res., 100, 6,093-6,095. *[3]: Kullmer O.et al (in rev.): Palaios. *[4]: Macchiarelli R.et al (2004): Coll. Antropol. 28 Suppl 2:65-76. *[5]: Urbanek C. et al (2005): Joannea Geologie und Paläontologie 6: 29-43. *[6]: Wolfenden E.,et al(2004): EPSL 224: 213-228.
GP43C-1492
Chronology of the Final Marine Regression in the Eastern Ebro Basin: Late Eocene to Early Oligocene Tectonosedimentary Evolution. (NE Spain)
The Ebro Basin is a triangular shaped foreland basin surrounded by three alpine ranges: the Pyrenees to the north, the Iberian Range to the SW and the Catalan Coastal Range to the SE. During the early Paleocene started the development of the Ebro Basin by flexural subsidence related to the growth of its margins as a consequence of the continental collision of Iberia and Europe. Connection of the Ebro Basin with the open sea was maintained until late Eocene, when ongoing convergence along the Pyrenean margin lead to the final closure of its western connection with the Atlantic Ocean. Since then, a long endorheic period of uninterrupted continental sedimentation leads to the accumulation of a thick sequence composed by alluvial and lacustrine facies. In foreland basins, tectonics plays a fundamental role in the sedimentation, by generating relief in the margins and accommodation space in the basin. Therefore, it is generally assumed that the main sedimentary breaks have a tectonic origin. The tectonic control on the sedimentation has been successfully established along the margins of the Ebro Basin through the study of the geometries of the syntectonic sediments. However, away from the margins, in distal alluvial and lacustrine environments some other factors related to the climate can also exert control on the sedimentation. In order to interpret the sedimentary record of the Ebro Basin in terms of tectonosedimentary and paleoclimatic evolution, we have sampled two magnetostratigraphic sections (1000 m and 600 m, thick) on Eocene-Oligocene continental sequences. Samples were collected at 2-5 m stratigraphic intervals. Stepwise thermal demagnetisation of the NRM of up to 2 samples per site has yielded a local magnetic polarity stratigraphy. Unambiguous correlation with the geomagnetic polarity time scale was feasible based on the presence of late Eocene to early Oligocene mammal fossil localities and previous magnetostratigraphic studies spanning the complete Oligocene stratigraphic record. Conclusions of this study provide reliable age constraints for the marine-continental transition and helps understanding the Late Eocene to Early Oligocene tectonosedimentary evolution of the eastern Ebro Basin.
GP43C-1493
Anisotropy of magnetic susceptibility and quantification of the internal deformation by means of deformed echinoids (Upper Cretaceous, Southern Pyrenees)
The AMS technique has been used as a very sensitive technique to determine the orientation of the strain ellipsoid in rocks. In this work it is presented a study by means of AMS of a NE-SW anticline in the western part of Central Pyrenees (Sierra de Alaiz thrust sheet). The obtained results show deformed sedimentary magnetic fabrics, where the magnetic foliation is parallel to the layering. The stereographic plotting of the K3 axes shows three groups of data, that can be related with the northern limb, the anticline hinge zone and the southern limb. When the magnetic results are bedding corrected the grouping of the K3 axes happens, and the magnetic lineation shows a horizontal NE-SW trend. These results allow us to establish a pre-folding acquisition magnetic fabric. The studied outcrops, in the northern limb, show deformed fossils, mainly echinoids. These fossils show an apparent main flattening plane parallel to the layering. In this outcrop 14 echinoids were orientated and collected. In the laboratory these elements were restored to the field situation and photographed. The photograph was made normal to the layering, the echinoid section was adjusted to an ellipse, and the results were plotted. The plotting of the ratio between long vs. short axes and the orientation of the long axes (Rf/phy method), show a maximum of axes ratio that represent the symmetrical plane of the obtained results. These results show a vertical plane that contains the more elongated section and show an orientation of N055 (D plane). From these results, a photograph in the pole of the D plane was taken. The same methodology described before was applied, the obtained results show an orientation of the higher ratio of 45º respect to the horizontal. This value is the same of the dipping of the level, then the assumption of the more flattening plane was parallel to the layering has been confirmed. The results show a stretching direction that is contained in the layering and it has an orientation of N055, 45. The magnetic lineation obtained in this site is parallel to this direction, then there are a parallelism between the magnetic lineation axis and the more stretching direction obtained by means of the deformed echinoids. If we apply the same conclusions obtained in the regional AMS study, we can infer that the deformation of the echinoids took place during a pre-folding stage. If we know that the flattening plane is parallel to the layering, and also we know the orientation of the long axis (X strain axis), the study in two different sections show that the intermedium axis (Y-axis) is also parallel to the layering. We have two sections that contain the three axes of the deformation, because off it is possible to prove that the flattening plane is parallel to the layering. With these results we can apply the Lisle method to quantify the internal deformation. If we assume that not volume change has happened, there are a shortening (in z axes) of 15.78 per cent and a stretching in the x direction of the 15.11 per cent. These results show that a very important shortening happened before folding, while the restored sections show shortenings of 6 to 8 per cent, the shortening related with pre-folding processes is even higher than this. In this case a tectonic shortening in these materials can be expected during pre-folding and also pre-detachment moments, while when the thrust sheet starts to move to the south all of the internal deformation goes to the detachment level, folding and faulting the upper materials in a passive manner.
GP43C-1494
Kinematic Evolution of the Western Pyrenees Thrust Front From Paleomagnetic Analysis on its Foreland Basin.
The Pyrenees is a collisional orogen formed during the Alpine orogeny. Its southwestern frontal thrust was originated as a result of the Cenozoic inversion of preexisting extensional faults. The emplacement of the frontal thrust in the Western Pyrenees generated a foreland basin, which locally accumulated more than 4,500 meters of Tertiary sediments. The kinematic evolution of the Western Pyrenees thrust front is poorly constrained due to the scarcity of reliable age constraints within the Tertiary sediments. However, the good exposure conditions of syntectonic continental deposits in its foreland basin makes it an excellent scenario to carry out paleomagnetic and structural studies in order to unravel the kinematic history, geometry and evolution of the thrust front. A magnetostratigraphic composite section along the continental basin infill was sampled covering up to 3,000 m of succession. Correlation of the local magnetostratigraphy with the GPTS was helped by a new mammal fossil locality found in continental sediments and attributed to the Agenian local biozone Y (MN2D). The cronostratigraphy of the tectosedimentary units, ranging from lower Oligocene (Cr12r) to lower Miocene, provides further constraints on the timing of two main tectosedimentary events recorded as major unconformities within the basin infill. From this study, sedimentation rates have been also obtained. The analysis of several paleomagnetic sites revealed that no vertical axes rotations occurred in the Tertiary sediments regardless superimposed folding with oblique axes could be observed, and the proximity of adjacent structures as the Estella diapir and the Pamplona fault. Finally, the analysis of the anisotropy of magnetic susceptibility together with collected sedimentary data suggests that magnetic fabrics record both, a depositional and tectonic fabric.
GP43C-1495
Clockwise Rotation and Implications for Northward Drift of the Western Transverse Ranges from Paleomagnetism of the Piuma Member, Sespe Formation, near Malibu, California
New paleomagnetic results from mid-Tertiary sedimentary beds in the Santa Monica Mountains of southern California reinforce the evidence for large-scale rotation of the western Transverse Ranges, originally postulated from observations of basement-structure trends and supported by paleodeclination data from Eocene and younger rocks. Previously published paleomagnetic data indicate that post-Oligocene rotation amounts to 70°-110° clockwise, affecting the Channel Islands, Santa Monica Mountains, and Santa Ynez Mountains. The Sespe Formation near Malibu consists of a lower member dominated by nonmarine sandstone and conglomerate and an upper section, the Piuma Member, which consists of gray-red sandstone and mudstone interbedded with minor tuff and limestone beds. The Piuma Member has a paleomagnetic pole at 36.7°N, 326.7°E (A95min=5.0°, A95max=9.6°), resulting from thermal demagnetization of 34 oriented cores covering beds from Oligocene to Early Miocene age. The data are consistent with significant clockwise rotation (68°±7°) of the region relative to stable North America. Rotation of the western Transverse Ranges is generally viewed as a consequence of Pacific-North American plate interactions after 28 Ma, when east-west subduction gave way to northwest transform motion in southern California. Inclinations from the Piuma study lead to a paleolatitude anomaly of 11°±7°, or a mean northward drift that exceeds generally accepted San Andreas fault displacement by a factor of three. However, analysis of magnetization-direction distributions from this study combined with J. Liddicoat's work at other Sespe Formation sites (total N=131) reveals that sedimentary inclination error, rather than northward drift, may be the primary contributor to the anomaly. Application of the Tauxe and Kent method, which tests the elliptical shape of a given field-direction dataset against a global geomagnetic field model (TK03.GAD), requires mean inclination flattening of approximately 15° to predict the 1.34:1 azimuthal elongation of the Sespe magnetization directions. Compaction may explain the inclination flattening in these sedimentary rocks, but the process does not adequately explain lower-than-expected inclinations found in previous studies of Miocene volcanic rocks of the western Transverse Ranges.
GP43C-1496
Characterization of orogenic remagnetizations within various fold geometries in Carboniferous carbonates from thin skinned fold and thrust belts, SW Alberta and NW Montana
Paleomagnetic and rock magnetic analysis was conducted on a complex fault propagation fold train in Kananaskis Country, Alberta to compliment an ongoing study of orogenic remagnetiztions in the thin-skinned, fold and thrust belt (NW Montana and SW Alberta). The complex structure is composed of an asymmetrical anticline to the west and chevron syncline to the east, with both folds plunging ~15° to the south. The fold train contains a magnetization with two stable ancient components. The characteristic remanent magnetization (ChRM) with northerly declinations and steep down inclinations is removed between ~350°C and the maximum unblocking temperature of 540°C. Tilt tests on the preliminary data reveal that the ChRM is early syntilting in the anticline and syntilting in the syncline. These results from this fold train are similar to a previous study in the Sawtooths (NW MT) which reported that fault propagation folds have a syntilting ChRM whereas fault bend folds contain a pretilting ChRM. An intermediate temperature reversed component is unblocked by 340°C and is late syntilting to post-tilting. Preliminary high-field rock magnetic data from folds in Montana and Alberta show that saturation is reached before 0.3T and the majority of the samples have wasp-waisted hysteresis loops. On a log plot of Mrs/Ms versus Hcr/Hc, the data has a power law distribution that is similar to trends reported by other authors. Interestingly though, samples from a fault bend fold have higher Mrs/Ms ratios than those measured in fault propagation folds, suggesting that strain induced by the various folding styles may influence the rock magnetic properties. Additional studies are underway to test these preliminary results and determine if the differences in the hysteresis ratios are significant. Petrographic analysis shows magnetite replacing pyrite in some samples suggesting an authigenic origin for the ChRM. The intensity of the ChRM as well as the strongest rock magnetic signal is most common in dark gray carbonates that are hydrocarbon reservoirs in the subsurface, suggesting the possibility that the origin of the ChRM may be related to hydrocarbon migration.
GP43C-1497
Rapid, large-scale Neogene rotations and remagnetizations recorded by sediments on the Hikurangi Margin, New Zealand
New paleomagnetic results from the Hikurangi margin, New Zealand, demonstrate that late diagenetic growth of the iron sulfide greigite has occurred at up to 65% of sampling localities. When these remagnetizations are accounted for, coherent vertical axis rotation of the entire Hikurangi margin over the last 7-10 Ma can be inferred south of the Raukumara Peninsula, at a much faster rate (8-14°/Myr) than the presently observed rate of 3- 4°/Myr, which is only likely to characterize the tectonic regime established since 1-2 Ma. These new results are consistent with both long- and short-term deformation on the Hikurangi margin being driven by realignment of the subducting Pacific plate, with collision of the Hikurangi Plateau in the Late Miocene potentially being key to both the initiation of tectonic rotations and the widespread remagnetization of Neogene sediments. Boundaries between the rotating and non-rotating ends of the Hikurangi margin can be linked to long-term discontinuities in intra-plate coupling resulting from structural changes along the plate boundary zone. However, accommodating faster, more coherent rotation of the Hikurangi margin in Neogene reconstructions of the New Zealand plate boundary region, particularly in the Late Miocene, remains a challenge.
GP43C-1498
A refined statistical approach to the paleomagnetic strike test: applications to the Wyoming salient, Sevier fold-thrust belt
Paleomagnetic studies are increasingly being done on curved orogens from around the world. These studies interpret kinematics of curvature in different ways, ranging from mostly primary to largely secondary. Discrepancies in interpretations, sometimes for the same orogen, partly reflect limited sampling, different assumptions, and incomplete statistical analyses. We present a refined statistical approach to strike-tests, in which large-scale structural trends are correlated with restored paleomagnetic trends, and apply this approach to new paleomagnetic and structural data for 164 sites from the Triassic Ankareh Formation, sampled around the Wyoming salient of the Sevier fold-thrust belt. 92 sites preserve a primary Triassic magnetization, and 32 sites preserve a secondary Early Cretaceous component. A weighted least-squares method is used to calculate best fit slopes (which can vary from 0 for primary curvature to 1 for all secondary curvature), confidence intervals, and goodness of fit. Weighting factors are calculated from combined uncertainties in site mean paleomagnetic vectors (typically ~±5° at 1σ), local rotations recorded by variations in mesoscopic structure orientations (~±10°), and structural trend (~±3°). Analysis of Triassic data for the entire salient indicates that 77% (±9% at 2Σ) of present-day curvature is secondary; analyses of individual thrust sheets reveal similar patterns. Residuals have approximately normal distributions for most sites; sites from overturned fold limbs and transfer zones have larger residuals and are not included. Slopes calculated using only uncertainties in paleomagnetic vectors have narrower confidence intervals, but poor goodness of fit, indicating that local rotations contribute to uncertainty. Strike tests are also used to analyze LPS directions estimated from mesoscopic structures and 3-D strain analysis of deformed reduction spots. Best-fit slopes for the entire salient and individual thrust sheets indicate that LPS directions are statistically orthogonal to structural trends. Combined with paleomagnetic data, this indicates that LPS directions had a 23% (±10%) component of primary curvature and can not be directly used to quantify secondary rotation. Confidence intervals were also evaluated using bootstrap statistics and Monte Carlo simulations, which gave similar results. Our approach of systematic paleomagnetic and structural sampling, combined with appropriate weighting to determine confidence intervals and goodness of fit, closely constrains acceptable curvature models for the Wyoming salient.
GP43C-1499
Revisiting the Paleomagnetism of the Ghost Rocks Formation of the Kodiak Islands, Alaska
The Paleocene Ghost Rocks Formation is the second youngest unit in the accretionary complex that comprise the Kodiak Islands, AK, part of the Chugach terrane, and consist of a sequence of turbidites interbedded with pillow lavas. Most workers agree that the anomalous near trench magmatism is a result of the passage of a trench- ridge-trench (TRT) along the margin of North America. However, it remains controversial as to the location where this interaction took place relative to the ancient North American margin. Plumley et al. (1983) conducted a paleomagnetic study on the pillow lavas of the Ghost Rocks Formation and concluded that they had formed at a latitude of 40° N. Discordant locality mean directions at the two localities of the study, Alitak and Kiliuda Bays and a complex two-stage structural correction used at Alitak Bay have cast doubt on the strength of this conclusion. This collaborative structural and paleomagnetic study seeks to resolve the location where the Ghost Rocks Formation formed using more detailed sampling and structural analysis. A total of 176 sites were sampled in 2006 and 2007, from four localities in the Kodiak Islands. This abstract will focus on samples collected from the Jap Bay locality, where sedimentary rocks were sampled. Preliminary results of sites taken at Jap Bay have two components of magnetization. The second-removed component has dual polarity and an improvement in clustering is observed upon tilt correction. These results are consistent with preliminary result of samples collected at two other sites of this study, Alitak and Kiliuda bays, suggesting the magnetic character recorded in the Ghost Rocks Formation is primary in origin. The inclinations are shallow relative to those expected for North America and at this time support models that place the Chugach terrane >1000km south of their present location.
GP43C-1500
Folding at two different scales of the Paradox anticline in the Ordovician Cool Creek Formation, Arbuckle Group, Slick Hills, southwestern Oklahoma: A paleomagnetic fold test study.
The carbonates in the Cambro-Ordovician Arbuckle Group, part of the southern Oklahoma aulacogen, has been the subject of previous paleomagnetic studies with a focus primarily on their origin of the magnetizations. Most previous studies indicate late Paleozoic magnetizations that reside in hematite. However, Elmore et al. (1988) conducted a paleomagnetic study of the Arbuckle Group carbonates from the Slick Hills area utilizing six sites from a north-plunging tightly folded Paradox anticline. Alternating field and thermal demagnetization results from their study indicated a post-tilting remanence that resides primarily in magnetite. Also, based on the difference between the observed and expected remanence directions, they suggested a possible 30° block rotation. As a continuation of their work, this paleomagnetic study was conducted to corroborate the observed 30° rotations utilizing more sites from the Paradox anticline and the use of a more sensitive 2G Cryogenic magnetometer. In addition, the major focus of this paleomagnetic study is to examine the relationship between the timing of remanence acquisition with respect to the primary (F1) and the secondary (F2) folds of the Paradox anticline. To this extent, oriented samples of carbonates have been collected from the Ordovician Cool Creek Formation of the Paradox anticline from the Slick Hills area from both the F1 and the F2 folds. Low temperature demagnetization protocols have been carried out on these samples to remove the effects of multidomain magnetite grains thereby isolating better the characteristic remanence components. Post-low temperature cleaning, the thermal step-demagnetization procedure isolates primarily two components: 1.) a low-temperature steep downward viscous remanent magnetization; and, 2.) a high-temperature characteristic remanent magnetization component, residing primarily in magnetite, with shallow remanence directions scattered towards the east-south-east to south-east. Fold test results indicate a post-tilting remanence for the F1 major folding of the Paradox anticline similar to that observed by Elmore et al. (1988). However, an interesting new observation is the paleomagnetic fold test results from the F2 fold that indicates a syntilting remanent magnetization. In addition, the south-easterly scattered shallow remanence directions from these Ordovician Cool Creek carbonates substantiates a possible 30° rotation that is likely attributed to the left-lateral wrench faulting observed in this area.