GP44A-01
Reappraisal of Displacement of Northern Cordilleran Terranes Since the Triassic and Jurassic
Triassic-Jurassic paleomagnetic data from the Cordillera of Canada had initially indicated displacements of about 1000 km from the south. Later, as a result of revisions in the APW path for North America, it was contended that there had been no such displacements. The Cordilleran data are from Wrangellia (Nicolai/Karmutsen (225 Ma) and Bonanza (195 Ma) formations) and Stikinia (Hazelton (195-180 Ma) and Stuhinni (~210 Ma) formations). These are two largest exotic superterranes in the Cordillera. Data all are from massive lavas (mainly) or diabase sills whose bedding attitudes are well controlled and whose magnetizations are believed to record accurately the field direction. On the other hand, several of the APW paths determined from North America that were sometimes used for reference are dominated by results from sedimentary rocks, which recently published work using directional distributions and anisotropy measurements has demonstrated are commonly affected by inclination flattening, for example, Late Triassic and Early Jurassic sedimentary rocks in rift basins of eastern North America. When corrected for inclination flattening, sedimentary data agree with inclinations from coeval igneous rocks when these data are available, for example, 200 Ma CAMP volcanics, but don't support the J1 cusp, previously a cornerstone of many APW paths for North America. In order to avoid possible shallow bias in latitude determinations, we make a special effort to avoid errors arising from inclination-flattening in sedimentary rocks by (1) using only data from sedimentary rocks that have been corrected for inclination-flattening, and by rejecting all other sedimentary data, and (2) incorporating igneous data from all other major continents. Relative to present geography the global APW path relative to North America begins in Mongolia in the Early Triassic, trends north-northwest towards the estuary of the Ob River by the end of the Early Jurassic, where it lingers before rapidly moving to the vicinity of Nunivak Island by the end of the Jurassic (and then boomerangs to the Cretaceous standstill position in the Chukchi Sea). Cordilleran magnetizations always have inclinations that are shallower than expected from the reference path. There is therefore no ambiguity about the sense of displacements - they are always from the south relative to cratonic North America. However, within these structurally disrupted terranes, only individual poles and not their paths can be constructed and there are uncertainties in relating individual results to the global polarity time-scale, and so we cannot say, from the paleomagnetic evidence alone, whether the terranes were in the southern or the northern hemisphere during the early Mesozoic. Assuming the closest (northern hemisphere) position, all the Cordilleran terrane strata nevertheless give displacements of about 1000 km or more. The Nicolai of Alaska yields a displacement of ~2500 km which reflects the strong northward motion of SE Alaska in the Tertiary. We believe this settles the debate: Triassic and Jurassic rocks of Wrangellia and Stikinia all have been displaced significantly from the south. Notable also are the strong anticlockwise rotations of 40°-60° from the Nicolai (Triassic), Bonanza and Hazelton (Jurassic), which have been observed also by Enkin throughout the stratigraphy of the Skeena fold- belt of central Stikinia. The Karmutsen (Triassic) of Vancouver Island shows a huge 150° anticlockwise rotation, perhaps a composite of several deformation phases.
GP44A-02
Paleomagnetism of the Paleocene Ghost Rocks, Kodiak Islands, Alaska: Implications for Paleocene Pacific-Basin/North America Plate Configurations
Paleocene volcanic and sedimentary rocks of the Ghost Rocks, part of the Chugach accretionary complex, may provide important constraints for possible paleogeographic models of plate configurations along the NW margin of North America. The Ghost Rocks, along with other Late Cretaceous to E. Eocene units of southern Alaska, contain anomalous near-trench igneous rocks which decrease in age from NW to SE along this margin and record relative motion of a trench-ridge-trench triple junction with respect to the margin. Thick sections of similar-aged volcanic rocks also exist in coastal Washington and Oregon. Two models have been proposed to explain these anomalous volcanic rocks. Moore et al. (1983) ascribes the progression of ages of the TRT rocks to migration of the Chugach terrane past a Kula-Farallon-NA boundary located off of present- day Oregon. Haeussler et al. (2003) call for the existence of two TRT triple junctions, one located off the present-day Oregon margin, the other of which migrated from NW (Sanak) to SE (Baranof) between Paleocene and Eocene time, which requires an intervening oceanic plate (the Resurrection Plate). Our work has included extensive sampling of both volcanic and sedimentary units of the Ghost Rocks, and detailed structural studies. The geological work indicates that much of the Ghost Rocks, particularly the large section containing volcanics at Alitak Bay, is composed of intact blocks bordered by fault zones, with steep upright to locally overturned bedding. The paleomagnetic results have revealed that the majority of the sedimentary rocks have poorly-behaved magnetizations carried by authigenic sulphide minerals. The majority of the igneous rocks (primarily beautiful exposures of basaltic-andesite pillows) have well-defined magnetizations with unblocking temperatures ranging from 450 to 550 C. The sites from Alitak Bay were corrected for vertical-axis rotations by rotating site-specific bedding strikes to agree with an average regional strike of 250 degrees. Incremental rotations applied to the site-mean directions (a paleomagnetic rotation test) indicate that best clustering occurs at the optimal rotation; thus the Alitak rocks were magnetized prior to rotation. Because the resulting rotation-corrected structure at Alitak Bay is monoclinal, all versions of paleomagnetic fold tests on the Alitak Bay rocks are inconclusive. Volcanics from Kiliuda Bay, using data from Plumley et al 1983 and this study, also have well-defined magnetizations. A regional fold test using the combined rotation-corrected Alitak Bay results and the results from Kiliuda Bay indicates best clustering occurs at 100% untilting, resulting in a combined site mean of D = 162, I = 60, k = 19, a95 = 6, N = 30. While a pre-tilting, and pre-rotation, remagnetization cannot be entirely ruled out, our new data suggest these rocks likely retain their original magnetization. Based on this we conclude that the Ghost Rocks were likely magnetized at a latitude of 41° ±7 N. These data thus support migration of at least a portion of the Chugach terrane, and its TRT-related rocks, from a position off shore present-day Oregon, since Paleocene time.
GP44A-03 INVITED
Paleomagnetic Approaches to Evaluating Extensional Tectonism in the Western North American Cordillera
The mid-Cenozoic cessation of Mesozoic and early Cenozoic crustal shortening processes in the western Cordillera of North America (WCNA) and profound thermal modification of the western North American lithosphere have resulted in a unique natural laboratory for the study of intracontinental extension. In the Basin and Range Province and related extensional terranes (e.g., Rio Grande rift), numerous paleomagnetic investigations, compiled in a GIS-based format for this paper, have contributed to our understanding of deformation processes related to continental lithosphere extension, and these can be basically divided into two, rather general, research questions. One is the overall contribution of vertical axis crustal rotation to lithosphere extension. The other involves the origin and history of crustal deformation, specifically rotation about a sub-horizontal axis (tilting), associated with slip along regionally extensive normal fault systems, many of which are well-exposed at the surface and presently have very shallow orientations in the crust. Answers to these questions provide important contributions to the kinematic history of lithosphere extension. The magnitude and sense of vertical axis rotations of the crust are evaluated using paleomagnetic declination data, assuming inclination has not changed, which are referenced to either expected field directions (absolute determinations) or to data from the same unit at an "unrotated" location (relative determinations). Slip on normal faults may result in significant tilts of both upper and lower plate rocks to the fault or fault system and paleomagnetic data may, under particular circumstances, be used to estimate tilt magnitude. Several paleomagnetic investigations have provided high-quality data sets to provide the answers to specific questions to local areas. From a regional perspective, the available paleomagnetic data show that the pattern of vertical axis rotations related to extension in the WCNA is heterogeneous. Furthermore, no single mechanism can be applied to explain the observed pattern of crustal rotations. Some data from crystalline rocks in the hanging wall and footwall of normal fault systems show that paleomagnetically significant tilting can occur in these rocks; data from regionally extensive detachment systems show that there may be little geometric modification to the lower plate while upper plate rocks have been translated over considerable distance.
GP44A-04
Rates and timing of vertical-axis block rotations across the Sierra Nevada-Walker Lane transition in the Bodie Hills
We use paleomagnetic data from Tertiary volcanic rocks to address the rates and timing of vertical-axis block rotation across the Sierra Nevada-Walker Lane transition in the Bodie Hills, California/Nevada. In zones of continental deformation, block rotations are an important mechanism for permanent stain accommodation, and thus may be crucial to testing geodetic block models and resolving geologic-geodetic slip discrepancies. In our study, data included in the paleomagetic site means are high quality AF demagnetization results (least squared fits that generally include 5-7 points with MAD values less than 1). Thermal demagnetization results match the AF directions, and both thermal demag and rockmag results indicate strong ChRM, mostly carried by single domain magnetite. The site means used to calculate the VGPs all have a95 values less than 10 (mostly 2-5) and include 6-11 sites each. Each site (and thus site mean) has a reasonably well-known structural correction. The VGP scatter values range from 12 to 16 degrees, indicating that they include appropriate secular variation. The mean declinations and 95 percent confidence limits for each VGP timeslice are statistically distinct from one another (71 ± 9, 39 ± 13, and 11 ± 11 degrees). The slope of a linear regression fit to the age versus declination data gives a rate of vertical axis block rotation of approximately 3-4 degrees/Myr. Fitting two separate lines to the age vs. declination data would indicate an increase in the rates of rotation since ~10 Ma. Two possible interpretations of the data are: (1) the rotations began during or before the Middle Miocene, or (2) rates of rotation were high initially (e.g. ~10 Ma) and decelerated until the Pliocene. These data have implications for the (1) timing and spatial extent of distributed strain accumulation related to the initiation of the San Andreas Fault-Eastern California Shear Zone-Walker Lane transform plate boundary, (2) transfer of transform plate boundary deformation into the maturing Walker Lane, and (3) the initiation of transtensional block rotations and bounding fault systems.
GP44A-05
65 km of Post-Cretaceous Offset Across Owens Valley, CA? Constraints Using Paleomagnetism
Recent studies suggest that several pre-Cenozoic piercing points have been offset ~65 km along a cryptic structure sub-parallel to the Owens Valley, CA. These piercing points include: 1) Sr isotope 0.706 isopleth, 2) Permian and Triassic structural features of the White and Inyo Mountains, CA, 3) offset Independence Dike swarm, 4) offset Cretaceous dikes (c.a. 83 Ma) and plutons (c.a. 102 Ma) of the Sierra Nevada and Coso Range, CA. The present study aims to: 1) test the offset hypothesis on the Cretaceous dikes and leucogranites by comparing paleomagnetic results on the proposed offset rocks, and/or 2) if the proposed offset is found to be consistent with paleomagnetic data, to use the remanence results to examine kinematic behavior of the crust during and since offset. Preliminary results indicate that, where sampled at three localities, the c.a. 83 Ma dikes that are proposed to have been offset are of opposite paleomagnetic polarity on either side of the proposed offset. This result, if confirmed by additional sampling, would falsify the hypothesis of contemporaneity and thus original continuity and subsequent offset. On the other hand, if additional sampling demonstrates that dikes are of mixed polarity on either side of the proposed offset, then these data would constrain the age of dike emplacement to around the C34n-C33r boundary at 83.5±0.7 Ma. As expected, the c.a. 102 Ma leucogranites, emplaced during the Cretaceous long normal, both exhibit normal polarity. However, these rocks show a discordance in remanence direction which is likely the result of differential tilt and/or vertical axis rotation of the sampled units. Vertical axis rotation may be likely, as suggested by dike strike variations. A baked contact test on the sampled rocks of the Coso Range confirms that those units retain a primary magnetization.
GP44A-06 INVITED
Vertical axis rotations observed in geodetic and paleomagnetic data
Paleomagnetic and Global Positioning System (GPS) geodetic data can both be used to measure vertical axis rotation rates in crustal rocks. Paleomagnetic data sample the history of finite rotations from some past interval of time but the tectonic setting of the rotations may be unclear. Moreover, recent rotations are difficult, and instantaneous rotation rates impossible, to detect. Modern GPS observations measure the current, decade-scale instantaneous rotation rates and help provide the tectonic setting. We estimate modern-day vertical axis rotations of crustal blocks from several deforming zones by inversion of GPS velocities. The methodology attempts to account for elastic deformation that can be strong in short-term geodetic deformation fields. In general the geodetic estimates of rotation rates compare very well to several- million-year-scale rotation rates derived from paleomagnetic observaions. The most rapid modern rotations appear to occur near subduction zones. In the US Pacific Northwest, even a fine scale feature such as the rapid landward decrease in rotation rate evident in 12-15 Ma Columbia River basalt flows is faithfully recorded in 12-years of GPS data. However, in some cases, notably the California Transverse Ranges, rapid rotations evident in paleomagnetic data do not appear to be continuing today. I will discuss how we extract rotations from geodetic data and how these compare to paleomagnetic results in the western US, New Zealand and other parts of the world.
GP44A-07 INVITED
Paleomagnetism and Magnetostratigraphy of Late Cenozoic Sedimentary Rocks in the Western Salton Trough, CA
Sedimentary rocks in the western Salton Trough record the evolution of Late Cenozoic tectonic processes within this active plate margin zone. Studies of magnetostratigraphy and paleomagnetism from these rocks provide important constraints on the timing and kinematic nature of these processes. The results for several years of study, which integrates new stratigraphic, structural, and geochronologic work, will be summarized here, with emphasis on new findings and opportunities for future work in synorogenic basins such as this. Our work is based on studies of ~250 paleomagnetic sites collected in several stratigraphic sections. In the ~5.5 km thick FCVB, we have determined that the age of initial sedimentation in this basin, which likely marks the onset of regional extension, is ~8.1 Ma. Analysis of basin subsidence, from our combined stratigraphic work, points to three distinct phases of subsidence: a moderate (~0.4 mm/yr) rate from 8.1 to 4.6 Ma, a rapid (~2 mm/yr) rate from 4.6 to 3.1 Ma, and a return to moderate rates of ~0.4 mm/yr from 3.1 to ~1.0 Ma. Magnetostratigraphy of rocks exposed in the Borrego and Ocotillo Badlands north of the FCVB show that these sections range from ~0.6 to ~1.3 Ma in age. A widespread contact that changes laterally from a conformity to an angular unconformity (coinciding with the base of the Jaramillo sub-chron) marks the ~1.1-1.3 Ma initiation of significant dextral slip along the San Jacinto and related faults in this area. In terms of rotation, our work indicates that the most significant rotations (~20° CW) are recorded in the older (> 3 Ma) rocks of the FCVB. The results from rocks < 2.0 Ma in the FCVB, and rocks < 1.3 Ma in the Borrego and Ocotillo Badlands, all indicate minimal rotations (0 to 10°). These results are in contrast to the larger rotations inferred to have occurred after 1.0 Ma by previous work (e.g., Johnson et al, 1983), but our results do agree well with GPS-based models that suggest CW rotation rates of 0.5 to 2°/m.y. for this area. Combining these results, we tentatively conclude that the more rapid subsidence and larger rotations recorded in the older rocks took place during slip on the Western Salton Detachment Fault. Paradoxically, the transition from oblique extension to strike-slip/transpression at ~1 Ma seems to have not produced significant vertical axis rotations. Our ongoing work is focused on using the sedimentologic record in the FCVB to test competing hypotheses for tectonic and climatic influences on basin filling. To this end, we are developing higher-resolution magnetostratigraphic studies (including use of relative paleointensity) in conjunction with cosmogenic-isotopic concentrations of detrital quartz (Oskin et al, this meeting). To better understand the nature and timing of kinematic variations, traditional directional analyses combined with analyses of the shape-distribution of post- Brunhes overprints will be employed to decipher the spatial and temporal pattern of rotations in these rocks.
GP44A-08
New Paleomagnetic Data from the Quaternary Bandelier Tuff: Implications for the Tectonic Evolution of the Rio Grande Rift
The Rio Grande rift (RGR) is comprised of several right-stepping en echelon basins, including the Albuquerque, Española and San Luis basins. These basins are separated by relay zones, which developed as extension directions changed through time. In the Espaņola basin, displacement was transferred from the Cañones fault (mainly Miocene) to the Pajarito fault (Quaternary) and motion along the Embudo fault shifted from dominantly dip slip to dominantly strike slip. In the part of the RGR transected by the Pajarito fault system and the Jemez Lineament, Quaternary ignimbrite sheets (from the Valles Caldera) of the Bandelier Tuff (BT) have been mapped in detail and correlated geochemically. New paleomagnetic data from BT exposures in the Jemez Mountains to the west of the Pajarito Plateau include nine localities (36 sites, 401 samples). We examined spatial variability of vertical-axis rotations in the BT and tested for temporal variations in declination changes. Thermal and alternating field demagnetization resolves a well-grouped characteristic remanent magnetization (ChRM) component characterized by a moderate coercivity, high unblocking temperature and reverse polarity, comparable to previous BT data. Of the 34 accepted site means, 27 have α95 < 5°. Analysis of the geographic distribution of the data reveals no statistically significant rotation either north/south of the Jemez Lineament or east/west of the Pajarito Fault. Similarly, our data do not record any measurable rotations within the 0.36 Ma time frame between the Otowi and Tshirege eruptions that sourced the BT. Our data suggest that the kinematic termination of fault linkage and relay zone development bounding the west side of the Espaņola basin took place before BT volcanism.