GP54A-01 INVITED
Palaeomagnetism and Orogenies
Palaeomagnetism has a wide range of application potential and has proved extremely important in palaeogeographic reconstructions and to elucidate aspects of local and regional tectonics. Application of palaeomagnetism to local-regional tectonic studies requires high quality palaeomagnetic reference data but these are often missing or associated with large uncertainties - high quality reference apparent polar wander paths are therefore urgently required for many continents. A complete plate kinematic analysis of for example continent-continent collisions is made difficult since longitude is essentially unconstrained from palaeomagnetic data and modelling old orogens such as the Caledonian Orogeny in Silurian times relies on additional geologic data. Similar modelling problems apply to the somewhat younger Variscan Orogeny and Pangea formation in the Late Carboniferous. In modelling Permian and younger collisions we use 'zero longitudinal average motion of Africa' as the best possible approximation and we show examples of palaeomagnetic modelling of young orogens such as the Himalayas and how these models compare with hotspot and mantle plate motion frames.
GP54A-02 INVITED
Toward a Revised Middle Jurassic Reconstruction of the Eurasian Landmass
New Permian and Middle Jurassic paleomagnetic data from the Tarim Basin pose a paradox. Their declinations are identical to Upper Carboniferous to Neogene rocks collected from the same section, and their inclinations are similar to today's. When assuming that lower than expected inclinations in continental sediments arise from inclination shallowing effects, then the paleolatitudes of all Upper Carboniferous to Present rocks from Tarim are indistinguishable. After accounting for local vertical axis block rotations occurring in the last 20 million years, these observations suggest that Tarim experienced no apparent polar wander since the Carboniferous. Our Middle Jurassic pole positions Tarim 23.6±8.4° farther south than that predicted from the coeval reference pole for Eurasia; however, no geological argument exists to support the closure of a large ocean basin between Tarim and Siberia since the Middle Jurassic. We conclude that Tarim has experienced little or no apparent polar wander since the Carboniferous. Moreover, we propose a new Middle Mesozoic reconstruction of Eurasia using the new Middle Jurassic pole from Tarim. Our reconstruction results in a more geologically compatible solution for the eastern Asian blocks over previous reconstructions.
GP54A-03
The rotation of Iberia during the Aptian and the opening of the Bay of Biscay
We present paleomagnetic results of lower Cretaceous limestone and marl sequences from the Organyà Basin (Southern Pyrenees, Spain), 42 sites (810 samples), ranging in age from 125 Ma to 92.5 Ma (lowermost Aptian to Cenomanian). Positive fold tests prove a primary (pre-folding) origin of the NRM. The characteristic remanent magnetization (ChRM) direction of the lowermost Aptian limestones is 309°/55° ± 1.6° (α95) (N = 112). The marl formations from the lower and middle Aptian have a ChRM declination of 334°±1.4° and an inclination of 53°±1.4° (N = 282). Upper Aptian marls & limestones show a declination of 345°±1.3° and inclination of 53°±1.3° (N = 177). The rotation is ~40° anticlockwise with respect to Eurasia and occured during the Aptian (125- 112 Ma). The Albian and Cenomanian sites all have similar directions, on average 356°/48°±2.2° (N = 239) and fit the apparent polar wander path of Eurasia. This indicates that the rotation of Iberia was completed before the Albian. The rotation of Iberia is now firmly constrained to the Aptian and its relation to other regional tectonic events will be discussed, including the evolution of the Bay of Biscay.
GP54A-04 INVITED
What do remagnetizations tell us about regional thermo-tectonic processes in the India-Asia collision zone?
Remagnetization is a common phenomenon throughout the Himalayan Range. Stable secondary remanences mostly reside in pyrrhotite formed and magnetized during last metamorphism. Many sites have samples with both normal and reverse polarities and single specimens even reveal antipodal directions during thermal demagnetization. Therefore the time span of remanence blocking was obviously sufficiently long to average on paleosecular variation. K-Ar data on illites provide a time constraint for remanence acquisition; ages ranging from Eocene/Oligocene to Oligocene/Miocene in the western and central Himalaya, respectively. The pyrrhotite remanences record net displacements since remanence acquisition. It has been demonstrated that they can contribute to decipher the regional thermo-tectonic orogenic evolution. Block rotations about vertical axes show that oroclinal bending and rotational shortening between India and Asia are the dominating processes in the western and central part of the Himalayan arc. Less data are available from the eastern part of the collision zone. First paleomagnetic results indicate that in this region large-scale material flow around the East Himalaya Syntaxis becomes significant. The transition between the western and eastern realm seems to be located in western to central Nepal. It is interesting that the general pattern of paleomagnetic block rotations match with present-day rotations derived from GPS velocities suggesting that recent crustal movements are representative also for longer-term geological processes. This is particularly important for models of the evolution of the Tibetan Plateau in which decoupling of horizontal movements of the upper crust and mantle, enabled by a soft partially molten middle crust, plays a major role. The age and nature of the secondary pyrrhotite remanences is crucial for interpretations. Their origin is either thermo-remanent related to cooling or thermo-chemical related to the pro-grade or peak metamorphic phase. Coinciding results for remanence directions of formations with peak temperatures higher and lower than the Curie temperature of pyrrhotite, respectively, suggest that remanence acquisition was close to cooling in either case. West of ca. 89°E pyrrhotite remanences clearly postdate main Himalayan folding (F1) evidencing that F1 phases stopped before metamorphic cooling. East of the Yadong Graben scatter of remanence directions may indicate that significant small-scale deformation continued after cooling in this region. In a number of areas along the Himalayan arc in situ directions show distinct small circle distributions which document large-scale tilting processes after cooling or synchronous to cooling. Application of small circle methods is essential for analysis and interpretation. Associated tilt axes predominantly show an E-W trend parallel to the main Himalayan structures; however, some are oriented closer to N-S. It is therefore likely that these late-orogenic tiltings are related to the widespread formation of the Miocene North-Himalayan gneiss domes. In the context of a partially molten middle crust beneath the Tibetan Plateau the domes could be an expression of channel flow towards frontal regions of high erosion. Paleomagnetism is a potential tool to "map" doming structures in the crust also in regions without visible indications at surface.
GP54A-05
Paleomagnetic Results From Cretaceous Arc Terranes in Central Hispaniola: Implications for the Paleogeography of the Caribbean Plate
Paleolatitude constraints on Cretaceous arc components are critical for testing kinematic models of the origin and evolution of the Caribbean plate. Rotational controls on lithospheric structures that comprise, or are adjacent to, the North American- Caribbean plate boundary zone reveal the complex tectonic mechanisms that have accommodated North American and Caribbean plate interactions. The general consensus among previous paleomagnetic work supports northward transport and anti-clockwise rotation for Greater Antilles arc terranes, yet the extent of displacement remains poorly constrained. We report new paleomagnetic results from the Lower Cretaceous Los Ranchos Formation and Upper Cretaceous Tireo Group arc volcanic and volcaniclastic rocks of the eastern and central cordillera of Hispaniola. A stable ChRM component carried by magnetite (unblocking temperatures 550-580° C) or hematite (650-675° C) was isolated in 6 sites. For the Tireo, 4 sites give a mean direction of D=320.7°, I=19.1°, k=98.3, α95=9.3° and yield a positive fold test. The site mean inclination implies a paleolatitude of 9.8° +5.3° / -4.9°. Comparison of the observed Tireo paleomagnetic pole against the 80 Ma North American reference pole indicates anti-clockwise vertical-axis rotation of 22.9° ± 9.1°, and northward transport of 10.9° ± 8.8°, with respect to the North American plate. The 2 remaining sites from the older Los Ranchos Formation significantly differ from the Tireo Group results, although insufficient sites for the Los Ranchos preclude the calculation of a mean direction. However, shallow inclinations and westerly declinations of the existing Los Ranchos data suggest near equatorial placement and additional anti-clockwise vertical-axis rotation.
GP54A-06
Early Jurassic paleopoles from the Hartford continental rift basin (eastern North America): Was an abrupt change in polar wander associated with the Central Atlantic Magmatic Province?
The recent recognition of what may be the largest igneous province on Earth, the ~200 Ma Central Atlantic magmatic province (CAMP), with its close temporal proximity to major biotic turnover at the Triassic/Jurassic boundary, adds impetus for seeking confirmation of possibly related geodynamic phenomena. For example, CAMP emplacement seems to coincide temporally with an abrupt change in North American apparent polar wander at the so-called J1 cusp, which has been suggested to reflect a major plate reorganization or an episode of true polar wander. However, early Jurassic paleopoles from the Moenave and Wingate Formations from the Colorado Plateau that virtually define the J1 cusp have few reliable counterparts from elsewhere in North America. The thick section of cyclical Lower Jurassic continental sediments with interbedded CAMP lava flows in the Hartford basin of Connecticut and Massachusetts provides an opportunity to test the reality of the J1 cusp. We collected about 400 oriented samples distributed over 80 outcrop sites that represent a ~2500 meter-thick composite section of the Shuttle Meadow and East Berlin sedimentary formations, which are interbedded with CAMP lava units, and the lower Portland Formation, which consists of cyclical lacustrine to fluvial sediments of Early Jurassic age that conformably overlie the CAMP extrusive zone in the Hartford basin. Normal and reverse polarity ChRM directions define a coherent magnetostratigraphy and are supported by a reversal test and a positive fold test. The distribution of ChRM direction from the sediments is flattened and the mean is significantly shallower than from the coeval CAMP lavas. E/I analysis of the Hartford sedimentary ChRM data produces a result consistent with the geomagnetic field model at a mean flattening factor of 0.54; the corrected mean direction is steeper and not significantly different from the mean inclination of the Newark and Hartford CAMP volcanic units.
GP54A-07
Reconstructing the kinematics of thrust sheet rotation in the Wyoming Salient: a paleomagnetic study of Triassic redbeds
Understanding the kinematics and mechanics of orogenic curvature are long standing questions within the structural geology and tectonics community. At the root of these questions is when and how orogens acquire curvature relative to protracted deformation histories. In an effort to describe the kinematic history of the Wyoming salient of the Sevier fold-thrust belt a collaborative effort is underway to determine the three main displacement field components (translation, rotation and strain). Paleomagnetic data are reported here from 164 sites collected from redbeds of the Triassic Ankareh Fm. from throughout the Wyoming salient. Sampled redbeds carry three distinct remanent magnetizations distinguished by field tests and magnetic behavior: 1) a recent viscous magnetization that is usually removed by 250° C, but is sometimes stable through typical hematite unblocking temperatures, 2) a Cretaceous chemical remagnetization (32 sites), and 3) a near-primary Triassic magnetization that is carried by hematite and is stable up to 680° C (92 sites). Site mean directions have a high degree of scatter from the reference Triassic and Cretaceous directions in both site declination and inclination, suggesting significant local rotation and tilt subsequent to magnetization acquisition. Palenspastic structural restoration of individual site means reveals local rotation, significant regional orogenic rotation, and differentiable thrust sheet rotation patterns that are all consistent with overall trends of structures found around the belt. Rotations are more pronounced in the northern and southern sections of the belt, near the intersection of the fold-thrust front with the Teton/Gros Ventre and Uinta Mountain uplifts. Statistical analysis of the rotations within and between individual thrust sheets suggests that between 70 and 80% of present-day curvature of the salient is secondary and must be accounted for in any kinematic model of thrust-belt evolution. Analysis of data for individual thrust sheets suggests an overall slightly greater component of secondary rotation within more interior thrust sheets. When combined with mesoscopic structure, strain, and thrust translation data, these results support a curvature model involving a combination of primary thickness variations in the sedimentary wedge, differential shortening to maintain orogenic wedge taper, and interaction with Laramide foreland uplifts.
GP54A-08
Deformation History of the Paleocene Quottoon Pluton (British Columbia) at 54N: Constraints From Paleomagnetism and Miocene Dike Orientations
New paleomagnetic results from the Paleocene Quottoon pluton (Douglas Channel, Coast Mountains, British Columbia) and the orientations of Miocene dikes that intrude it reveal two episodes of deformation that correlate with regional events. The very-well determined site-mean remanence directions from 15 sites in the tonalite intrusion form two spatially-distinct groups with declinations that are substantially (63 deg and 40 deg) CCW of the appropriate NAM reference direction. The trends of over 100 dikes intruding the Quottoon do not appear to have been dispersed by this differential vertical-axis rotation, strongly suggesting that the deformation (an oroclinal bending known as the Hawkesbury Warp) predated dike intrusion at approximately 20 Ma. The orocline may be the result of southward decreasing amounts of E-W extension related to development of the Queen Charlotte Basin Remanence directions from the Quottoon also show evidence of block tilting manifested as inclinations significantly steeper or shallower than expected. These tilt domains (a few km wide) are smaller in scale than the vertical-axis rotation domains (10s of km wide). Large jumps in site-mean inclinations between tilt-domains occur over distances as short as 0.5 km, suggesting that boundaries between the domains are sharp. Of the 7 domains, 6 are tilted W-side or NW-side up, a pattern that is at odds with alternatives to the "Baja BC" hypothesis that invoke ubiquitous NE-side-up tilting to explain anomalous paleomagnetic results from Cretaceous plutons in the Coast Mountains. Restoring the blocks to their pre-tilt orientations (and applying the same rotations to dikes that intrude them) steepens the average dip of the dikes by 4 deg to 85 deg, suggesting that block-tilting was post-mid-Miocene. Furthermore, tilt magnitudes along a transect perpendicular to the length of the orogen mimic a smooth, V- shaped trend in (U-Th)/He-in-apatite ages, with the location of maximum tilting corresponding to the youngest age (2.8 Ma). This correlation suggests that post-4 Ma erosionally-driven exhumation may have caused the tilting.