Site Mean Directions Research Articles

坑井試料の残留磁化測定と地質解釈への意義

Magnetic properties of core samples in stratigraphic test wells are presented in an attempt to apply to comprehensive geological interpretation. Stability parameter of magnetization of igneous basement cores in MITI “Kesennuma-Oki” and “Goto-nada” indicates that rock magnetic property is essential for reliable interpretation of geomagnetic anomaly in sedimentary basins. Sedimentary core samples from MITI “Kesennuma-Oki”, “Soma-Oki”, “Joban-Oki” are described to obtain information on core-orientation. Magnetic cleaning for them shows that drilling-induced secondary magnetization is common in deep borehole samples. After careful rock magnetic cleaning, a site-mean direction of primary remanent magnetization for the Cretaceous sample in MITI “Soma-Oki” has large westerly declination. Together with other paleomagnetic data, the present result suggests a differential tectonic movement and reorganization of sedimentary basins in northeast Japan since the Cretaceous. Thus rock magnetic study of borehole samples contributes to understand the origin of basement, tectonic history, and regional basin evolution.

Paleomagnetic constraints on fault motion in the Hilina Fault System, south flank of Kilauea Volcano, Hawaii

Movement of the south flank of Kilauea Volcano in Hawaii has been associated with catastrophic landslide events. The surface expression of this former movement is the Hilina Fault System with fault scarps as high as 500 m. Paleomagnetic directions for lava flows exposed in the Hilina Fault scarps at Puu Kapukapu and Keana Bihopa on the Hilina Pali are used to determine the average rate of movement along faults (slip surfaces) separating the two sections. This paper reports results from two independent paleomagnetic studies within the Hilina Pali area. Twenty-one paleomagnetic sites (143 cores) were sampled by the Michigan Technological University group from lava flows between the Mo’o Ash and Middle Pohakaa Ash at Keana Bihopa in the footwall block of the 500-m-high Hilina Pali fault scarp. Thirty paleomagnetic sites (152 cores) were collected by the California Institute of Technology group from lava flows between the Mo’o Ash and Middle Pohakaa Ash in the 300-m-high Puu Kapukapu section (the hanging-wall block). A comparison of site-mean directions show that lava flows in the lower part of the Puu Kapukapu section have been tilted more than lava flows in the upper part with respect to the Keana Bihopa section. The systematic steepening of remanent directions downsection at Puu Kapukapu indicates that slippage of this block occurs along listric normal faults. The average amount of backward tilt of the Puu Kapukapu block, based on a comparison of mean directions from the two sections, is 7.8°±7.7°. Using slope stability methods, the average rate of movement of the Puu Kapukapu block since deposition of the Middle Pohakaa Ash is 1.7–2.4 cm/yr, and the average displacement (subsidence) is 680–740 m. Assuming that the average displacement resulted from a series of earthquakes producing subsidence equal to that observed in the 1975 Kalapana earthquake (3 m of subsidence along a 40-km segment of coastline on the south flank of Kilauea Volcano), one Kalapana-size earthquake occurring every 200 years would account for this displacement. Lastly, overall mean directions for the two sections indicate that Puu Kapukapu has rotated counterclockwise with respect to the Hilina Pali by 14.8°±8.5° about a nearby vertical axis. This also suggests that slippage between the two blocks occurs along listric normal faults.

Permian and Mesozoic extensional faulting within the Caledonides of central south Norway

Palaeomagnetic data from fault rocks along major faults in the Laerdal-Gjende Fault System cutting the Caledonian structure in the Jotunheimen area of central south Norway reveals a multi-component remanence pattern. Sample and site-mean directions from the fault rocks obtained by thermal cleaning demonstrate a simple pattern of normal polarity low blocking components and reverse polarity high-blocking directions. The magnetic signature of the Laerdal-Gjende Fault System fault rocks is identical to that observed on breccias on late faults along the west coast of Norway. Based on available palaeomagnetic reference data, we assign ages of mid-late Permian and late Jurassic-early Cretaceous for important phases of faulting and breccia formation along the Laerdal–Gjende Fault System in central south Norway. Structural windows, partly exposing basement along the axis of the Caledonides in southern Norway were exhumed by footwall uplift on major faults in the Laerdal–Gjende Fault System. The consanguinity of fault rock data from the Laerdal–Gjende Fault System and fault rocks in western Norway, and comparison with displacement on the offshore continuation of the Laerdal–Gjende Fault System along the Hardangerfjorden Shear Zone, indicate that the main tectonic events responsible for the development of the North Sea basin also significantly affected the geology of central south Norway.

Precision of the paleomagnetic method: An example from the Quaternary Eifel volcanics (Germany)

The total within-site dispersion of paleomagnetic data results from natural misalignment processes and experimentally produced dispersion. Although some of the sources of dispersion may be determined by experiments, this is not possible for others. The total accuracy of the paleomagnetic method so far has been determined only on historic lava flows. Thirty-seven Quaternary basaltic lava flows from the Eifel, Germany, have been re-sampled using the same outcrops as before. The new site-mean directions do not differ significantly from those of the original study. The angular distance between pairs of site-mean directions may be approximated by a Fisher distribution. Assuming that both studies are affected by the same natural dispersion processes, we obtain as the best value for the experimental dispersion sexp = 4.33°. The total within-site dispersion shows a log-normal distribution with a mean of stot = 5.39°. From these values we calculate that the dispersion due to natural processes in the Eifel volcanic field is snat = 3.21°.

MAGNETIC STRATIGRAPHY OF VESUVIUS PRODUCTS II — MEDIEVAL LAVAS

Abstract Within the framework of an extensive palaeomagnetic survey of Vesuvius products 67 sun-orientated samples from 5 lava flows, presumably emplaced in medieval times, have been collected in Torre del Greco: 3 flows from the lava flow sequence at the Villa Inglese quarry, stratigraphically below pyroclastics of AD 1631, and the remaining 2 from nearby outcrops at Scogli di Prota and Torre Bassano, respectively. The linearity analysis, carried out using very stringent criteria on at least 6 specimens per site subjected to either PAFD or PTD, has allowed us to define clearly the characteristic magnetisation patterns. Their within site mean directions define a SVC path covering a time interval of a few hundred years around AD 1000, as suggested by a comparison with part of the transferred Etna SVC. The time interval of AD 968–AD 1037 previously suggested for the lava flows investigated may be too narrow and whilst the Torre Bassano lava flow can be correlated with one of the Villa Inglese quarry lava flows investigated, the Scogli di Prota lava flow is the youngest of all.

Paleomagnetic constraints on Siluro-Devonian Laurentian margin tectonics from northern Appalachian volcanics

Paleomagnetic analyses of Silurian mafic volcanics from the overstep sequence of sedimentary and volcanic rocks deposited over the Central Mobile Belt of the northern Appalachians provide insight into the mid-Paleozoic tectonic history of the Laurentian margin. Stepwise thermal demagnetization of the subaerial mafic volcanics of the Ludlovian Fivemile Brook Formation of northwestern Maine reveals two ancient high-temperature, dual-polarity remanences. A tilt-corrected mean direction yields an inclination of 41° (σ I = 3.9°, N = 4 sites) and the magnetization is interpreted as a near-primary magnetization acquired well before the end of Early Devonian deformation. An in situ mean direction ( D = 165°, I = 35°, α 95 = 6°, k = 65, N = 10 entries from 8 sites), is interpreted as a secondary overprint with a pole (22°S, 306°E) near the Early Carboniferous segment of the North American apparent polar wander path (APWP). Conglomerate and fold tests of the high-temperature characteristic remanence preserved in Wenlockian subaerial mafic volcanics of the Bryant Point Formation and a red bed of the South Charlo Formation, both of the Chaleur Group of northeastern New Brunswick, constrain paleolatitude and deformational age. The inclination-only fold test peaks at 50% unfolding with a mean inclination of −35° (σ I = 8.5°, n = 148). Synfolding acquisition of magnetization is Wenlockian, based on a negative conglomerate test at the base of the section and a positive conglomerate test at the top of the section. Clockwise streaking of site mean directions away from a predominantly northerly declination is consistent with post-middle Wenlockian dextral shear in the Chaleur Bay region. Comparison of a locus of paleomagnetic pole positions with the North American APWP supports a Silurian age of the magnetization of the Chaleur Group. The age and synfolding nature of this remanence furthermore requires that deformation associated with the Acadian orogeny in New Brunswick began by mid-Silurian times. Moreover, inclinations from these units indicate that, within paleomagnetic resolution, there has been no significant latitudinal displacement with respect to stable North America since the Silurian.

Paleomagnetic evidence for Neogene tectonic rotations in the northern Apennines, Italy

Paleomagnetic analysis was carried out in the northern Apennines on Eocene to Pliocene Epiligurian units. Five Early Miocene and two Middle Miocene sites yielded dual polarity site-mean directions which show signs of clustering after correction for bedding tilt. These likely primary magnetizations, in conjunction with data from the literature, give an overall mean Late Oligocene–Middle Miocene paleomagnetic pole which shows a large and significant counterclockwise rotation of 52° (±≈8°) with respect to the Africa reference paleopoles (or a similar amount of rotation with respect to the coeval Europe reference paleopole). However, this paleopole falls close to the roughly coeval paleopole for Corsica–Sardinia, which is here calculated by averaging data from the literature. Three additional Early Miocene sites from an area west of Parma affected by Pliocene tectonics yielded site-mean directions which pass the fold test and are rotated counterclockwise by a lesser amount than the rest of the Miocene sites. Most of the remaining sites bear paleomagnetic directions acquired after tilting during a recent phase of remagnetization. We suggest that the large-scale rotation observed in the northern Apennines was associated with the motion of the Corsica–Sardinia block within the general context of the Africa–Europe relative motions. A compilation of published data from the central Apennines also shows a differential rotation of the northern relative to the southern Umbria belt which occurred after the motion of Corsica–Sardinia and may have been due to pivoting of the northern Umbria belt against a deep-seated lineament during the non-rotational opening of the Tyrrhenian Sea.

Palaeomagnetism of the ejecta-bearing Bunyeroo Formation, late Neoproterozoic, Adelaide fold belt, and the age of the Acraman impact

A new palaeomagnetic study has been conducted on haematitic shales and siltstones of the late Neoproterozoic Bunyeroo Formation in the Adelaide fold belt (Geosyncline), South Australia, which host an extensive horizon of shock-deformed rock fragments and microtektite-like material of probable impact origin. Thermal step demagnetisation of 116 samples of red shale and siltstone from six sections (sites) revealed a high-temperature component with a bedding-corrected site-mean direction of remanence of D = 56.6°, I = 29.3° (α95 = 10.7°) that gives a pole at 18.1°S, 16.3°E (dp = 6.5°, dm = 11.8°). The high-temperature component provides a positive tectonic-fold test (99% level of confidence). The Bunyeroo high-temperature remanence direction is near the remanence direction (D = 50°, I = 40°) indicated by modelling the subsurface magnetic source of the central high-amplitude anomaly at Acraman, Australia's largest confirmed meteorite impact structure 220–350 km west of the Adelaide fold belt, and also is close to the mean direction (D = 48.3°, I = 54.7°, α95 = 5.2°) determined for surface melt rock from Acraman. Statistical tests show that the virtual geomagnetic poles indicated by the directions for the subsurface central magnetic source and surface melt rock at Acraman may be regarded as subsets of the Bunyeroo palaeomagnetic pole position, indicating that the three pole positions are statistically indistinguishable. The results imply that the subsurface magnetic source and surface melt rock acquired their remanence in the ambient geomagnetic field during cooling, after the impact when structural disturbance had ceased, while the Bunyeroo Formation was accumulating. The agreement among the various remanence directions argues strongly that the ejecta horizon in the Bunyeroo Formation was derived from Acraman.The present findings confirm that the Acraman impact occurred in the late Neoproterozoic, about 590 Ma, which is the age of the Bunyeroo Formation provided by RbSr whole-rock shale dating of equivalent and contiguous strata in the Adelaide fold belt region. The Bunyeroo palaeomagnetic data give a palaeolatitude of ∼ 15°, indicating a low palaeolatitude for the Acraman impact and supporting other findings that the Adelaide fold belt occupied low to equatorial palaeolatitudes during the late Neoproterozoic.

Magnetic stratigraphy of Etan products. I-medieval lavas

Mean magnetisation directions from 6 lava flows of questionable historical age have been determined. The results, which are based upon very stringent linearity criteria during demagnetisation, point out erroneous dating and the corresponding lavas can be ascribed to medieval activity of Mount Etna, as already suggested for some of the investigated flows, covering a time interval of some centuries around AD 1000. The site mean direction of AD 1536 historically dated lava flow agrees with a previous study (Tanguy et al., 1985), while the site mean directions of AD 1566 and AD 1169/812 differ significantly from those reported earlier. Neither AD 812 nor AD 1169 dating is consistent with the site mean direction of the related flow being compatible with an emplacement age possibly a few decades before AD 1000. The 3 lava flows historically dated AD 1595 exhibit different within site mean directions indicating that they cannot be referred to coeval activity of the volcano; the site mean direction of one of the flows is compatible with an emplacement occurring very close to AD 1000. The resulting SVC is significantly shifted eastwards with respect to that of previous study (Tanguy et al., 1985). However, its older part nearly overlaps the medieval Vesuvius SVC relocated to Etna. This indicates that the strict linearity analysis carried out during demagnetisation has allowed to clearly define the characteristic magnetisations of the investigated lava flows. The adopted procedure is appropriate for performing successful magnetic stratigraphy surveys in volcanic areas.

Palaeomagnetism of Precambrian rocks from Gabon, Congo craton, Africa

Palaeomagnetic results for 75 orientated hand samples (25 sampling sites) from the Ogooué Series amphibolites (27), Archean basement amphibolites and granulites (14) and Precambrian dolente dykes (34) are presented. Magnetization of many amphibolites showed unstable directions during alternating field (AF) and thermal demagnetization experiments and no directions of magnetization could be isolated. However, a characteristic direction could be isolated in a few amphibolite sites and for the granulites studied. The best group of site mean directions Dm=54°, Im=69° (κ=27, α 95=18°, N=4) yielded a palaeomagnetic pole located at 44° E, 19° N (K=10, A 95=30°). This pole is dose to a palaeomagnetic pole obtained for granulites from the Jequi ` ́ es complex (São Francisco craton) dated by the 40Ar/ 39Ar method as 2.0 Ga, suggesting that both areas belonged to the same tectonic unit at that time. The unmetamorphosed dolerite dykes (5) displayed multi-component behavior, but a common characteristic direction could be isolated for four dykes, Dm=262°, lm=49° (κ=13, α 95=27°, N=4), which yielded a palaeomagnetic pole located at 72° E, 8° N (K=9; A 95=33°). However, this pole is different from those obtained for dyke swarms in Brazil, which are supposedly of comparable age.

A Tertiary age from paleomagnetism for mississippi valley-type zinc-lead mineralization in Upper Silesia, Poland

Ore and host rocks of the Middle Triassic Muschelkalk carbonates were sampled at 35 sites in three operating mines and several quarries in the Cracow-Silesia Mississippi Valley-type district of southern Poland. Palcomagnetic analysis was done using alternating field and thermal step demagnetization and saturation isothermal remanence tests. Specimens from three limestone and two early dolomite sites retain a dual-polarity A remanent magnetization component (D = 43 degrees , I = 51 degrees , k = 40, alpha 95 = 12 degrees ), which yields a Middle to Upper Triassic pole. A resides in single domain to pseudosingle domain magnetite in limestone and is probably primary, and in single domain-pseudosingle domain magnetite and hematite in early dolomite and is probably diagenetic. Two limestone sites carry a Tertiary remagnetization and most early dolomite sites carry a partial Tertiary remagnetization, or hybrid remanence, identified by palcomagnetic fold and breccia tests. Late dolomite and Mississippi Valley-type mineralization from 14 sites retain a dual-polarity C remanence component (D = 3 degrees , I = 66 degrees , k = 34, alpha 95 = 7 degrees ), which defines a Tertiary pole position. Conglomerate and breccia tests confirm that the clasts in late dolomite were remagnetized. The Tertiary age for the Mississippi Valley-type ore deposits is within the post-mid-Jurassic to pre-Miocene window permitted by the geologic evidence for mineralization. The age also supports gravity-driven fluid flow models for ore genesis that are associated with the Alpine orogeny. The dual polarities and elongate distribution of site mean directions suggest a 1 to 20 m.y. duration for the late dolomitization and mineralization event.

Comment On ‘Magnetostratigraphy of the Hettangian Langmoos Section (Adnet, Austria): Evidence For Time-Delayed Phases of Magnetization’ By Y. Gallet, D. Vandamme and L. Krystyn

The recent paper by Gallet, Vandamme & Krystyn (1993) deals with the age of magnetization of Lower Jurassic nodular limestones (including the Adnet Limestone Formation) exposed near the village of Adnet (Austria). The authors conclude that for the red variety of the limestones at Adnet the ‘growth of haematite in an early diagenetic process is questionable’. Three sites of these limestones from the Adnet locality were studied by Channel1 et al. (1992). The three site mean directions are slightly more dispersed after tilt correction (N = 3, kJk, = 2.33) suggesting a post-tilt magnetization, however, the fold test is not significant in a region where the beds dip fairly uniformly to the SW at about 10”. We wish to point out that the results of a conglomerate test for the red variety of the Adnet Limestone at Adnet indicate that the highest blocking temperature haematite magnetization component is, indeed, early diagenetic in origin. A conglomerate test carried out on the blue/gray variety of the Adnet Limestone, indicates that the characteristic magnetization in this variety is post-Mesozoic, and probably Neogene in age. As pointed out by Gallet et al. (1993), the Lower Jurassic nodular limestones exposed near Adnet show colour variations from red to blue/gray, and the colour variation is seen to cut bedding surfaces indicating that either the blue/gray or the red colouration of the limestones is secondary in origin. We have not seen these colour

Paleomagnetism of the Carboniferous Tepuel Group, central Patagonia, Argentina

Samples of the Carboniferous glaciogenic sequences of the Tepuel Group, western central Patagonia, Argentina, were collected from 23 sites in two localities, Sierra de Tepuel (43.7°S, 70.5°W) and Las Salinas (43.2°S, 70.3°W). Most were fine sandstones to siltstones of late Visean to Westphalian age. A characteristic remanent magnetization was determined in most sites by alternating field and/or thermal cleaning. At Sierra de Tepuel all samples were partially overprinted by a secondary magnetization apparently related to Mesozoic magmatic activity in this area. However, site mean directions of the characteristic remanence from both localities passed a tilt test, indicating a pretectonic (pre‐Early Permian?) magnetization. Isothermal remanent magnetization experiments showed magnetite as the main and often the only magnetic carrier, while thin section analyses indicated a detrital origin for this mineral, suggesting a depositional or postdepositional magnetization. The paleomagnetic pole for the Tepuel Group (Namurian‐Westphalian) at 316.1°E, 31.7°S (number of sites N = 16, minor and major semi‐axes of confidence δp = 15.0°, δm = 16.0°), is considered a key pole for the Middle Carboniferous of South America and Gondwana, confirming the apparent polar wander path for the supercontinent for which the pole moved from central to southern Africa in the Early Carboniferous, followed by a rapid motion from southern Africa to Antarctica during the late Westphalian. The agreement of the Tepuel pole with this Gondwanan path confirms that Patagonia was already part of South America by the Middle Carboniferous.

A palaeomagnetic study of the Umvimeela Dyke, Zimbabwe: evidence for a Mesoproterozoic overprint

Palaeomagnetic and magnetic susceptibility results are reported from sixteen new sites (complementing the previous seven) on the Umvimeela Dyke, one of the two major satellite dykes parallelling the Great Dyke of Zimbabwe. Four of the new sites are in the deformed northernmost section of the dyke with a further four just to their south. The magnetisation of the long cratonic section of the Umvimeela Dyke is characterised by a component P, carried principally by magnetite and considered primary, with a mean direction D=219.8°, I=−58.7° with k=116.8 and α95=3.3°. This direction is almost identical to those of the Great Dyke and two other associated intrusions. The angular dispersion of site mean directions for the Umvimeela Dyke is only 6.5°, suggesting a short time of emplacement and cooling. A second component S, carried by magnetite, titanomagnetite and ilmenohaematite, is present at most sites but in bigger proportions as the Limpopo and Zambezi Belts are approached. Considerations such as geographical distribution, magnetic susceptibility and dyke geology suggest S is an overprint. Its direction (D≈195°, I≈10°) is similar to that of the ∼ 1100 Ma Umkondo dolerites and it is suggested that it was introduced chemically at or just before the onset of Pan-African activity in the region. Of the four sites in the deformed section of the dyke one carries the P component, but the other three have been completely remagnetized. At two of these a component Z was detected which may have been introduced ∼ 650 Ma ago. This suggests that this section, and the accompanying Musengezi Complex of the Great Dyke, were deformed in the late Pan-African orogenic phase of the Zambezi Belt.

Tilt and rotation of the footwall of a major normal fault system: Paleomagnetism of the Black Mountains, Death Valley extended terrane, California

Research Article| October 01, 1993 Tilt and rotation of the footwall of a major normal fault system: Paleomagnetism of the Black Mountains, Death Valley extended terrane, California DANIEL K. HOLM; DANIEL K. HOLM 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138 Search for other works by this author on: GSW Google Scholar JOHN W. GEISSMAN; JOHN W. GEISSMAN 2Department of Earth and Planetary Sciences, The University of New Mexico, Albuquerque, New Mexico 87131-1116 Search for other works by this author on: GSW Google Scholar BRIAN WERNICKE BRIAN WERNICKE 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138 Search for other works by this author on: GSW Google Scholar Author and Article Information DANIEL K. HOLM 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138 JOHN W. GEISSMAN 2Department of Earth and Planetary Sciences, The University of New Mexico, Albuquerque, New Mexico 87131-1116 BRIAN WERNICKE 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1993) 105 (10): 1373–1387. https://doi.org/10.1130/0016-7606(1993)105<1373:TAROTF>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation DANIEL K. HOLM, JOHN W. GEISSMAN, BRIAN WERNICKE; Tilt and rotation of the footwall of a major normal fault system: Paleomagnetism of the Black Mountains, Death Valley extended terrane, California. GSA Bulletin 1993;; 105 (10): 1373–1387. doi: https://doi.org/10.1130/0016-7606(1993)105<1373:TAROTF>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Paleomagnetic data have been obtained from Miocene intrusions, including crosscutting mafic and felsic dikes, and Proterozoic crystalline rocks to evaluate deformation during Miocene and younger extension and unroofing of the Black Mountains, Death Valley, California. Synrift intrusions contain a well-defined and, at the site level, well-grouped magnetization, interpreted to be of dual polarity, the in situ direction of which is discordant in declination and inclination with an expected late Cenozoic reference direction. The oldest of these intrusions (11.6 ± 0.2 Ma) gives a magnetite-dominated remanent magnetization of high coercivity and high laboratory unblocking temperatures (about 550 to 580 °C) that is interpreted to predate unroofing from midcrustal depths. In situ site mean directions of this magnetization are directed toward the west and west-northwest with moderate to shallow positive (down) and negative (up) inclinations. The variation in direction of magnetization, particularly inclination, with site locality around the turtleback structures along the western flank of the Black Mountains, is interpreted to result from folding of the intrusion after remanence acquisition.Younger intrusions (≤8.7 Ma) generally give magnetizations with inclinations similar to expected Miocene values. Two populations of in situ site means are identified: one with southwest declination and negative inclination; the other with northward declination and positive inclination.A preferred interpretation for footwall deformation involves, from oldest to youngest: (1) southwest-side down tilting of the entire range block of some 20° to 40° to possibly 70° and, at least locally, folding of the crystalline rocks, on a trend parallel to the Death Valley turtlebacks, between 11.6 and 8.7 Ma; (2) progressive east-to-west footwall unroofing between 8.7 and ca. 6.5 Ma; and (3) clockwise rotation (50° to 80°) of much of the Black Mountains after the core detached from stable terrane to the west. We interpret late rotation of the Black Mountains as oroflexure related to right-lateral shear along the Death Valley fault zone. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Paleomagnetism of the late Paleozoic Slide Mountain terrane, northern and central British Columbia

Paleomagnetic samples were collected from Mid-Pennsylvanian to Lower Permian red argillaceous cherts at two localities of the Slide Mountain terrane: 18 sites from the Sylvester allochthon in northern British Columbia and 11 sites from Sliding Mountain in central British Columbia. A secondary component of natural remanent magnetization in the Sylvester samples yields a paleomagnetic pole that can be brought into coincidence with the Jurassic portion of the North American apparent polar wander path by inferring vertical-axis rotation during obduction of the allochthon. Both localities yield a characteristic component (ChRM) with unblocking temperatures from 650 to 680 °C. After structural correction for bedding tilt, all inclinations of ChRM are negative, consistent with magnetization during a reversed-polarity interval in the northern hemisphere. Site-mean ChRM directions show consistent inclinations but distinct stratigraphic groupings of declinations. Inclination-only statistics indicate that the ChRM passes a tilt test within the Sylvester allochthon and regionally between the two localities. The ChRM was apparently acquired prior to structural imbrication within the Sylvester section and regional differential tilting. We interpret the ChRM to be a primary magnetization acquired at or soon after deposition during the Permo-Carboniferous reversed-polarity superchron. The mean ChRM inclination of −16.7° ± 6.0° from the Sylvester allochthon indicates a paleo-latitude of 8.8° ± 3.4°N, which is corroborated by a paleolatitude of 1.9° ± 1.5°N from the Sliding Mountain locality. When compared with expected Pennsylvanian–Permian paleolatitudes, a net poleward translation of 20.3 ± 3.7° is implied for at least the sampled lithotectonic component of the Sylvester allochthon.

A new Mesozoic apparent polar wander loop for South China: paleomagnetism of Middle Triassic rocks from Guizhou Province

Middle Triassic marine platform carbonates and sandstones in southeastern Guizhou province (South China Block) were sampled for a paleomagnetic study. A total of 91 samples from 14 sites were collected, including sandstone samples from two limbs of a fold. Progressive thermal demagnetization isolated single characteristic component magnetizations in the limestone unit and two components in the sandstones; all components show mixed polarities. The mean direction in the limestone unit is 352/ − 34 ( in-situ) and 1/ − 4 (tilt-corrected) ( α 95 = 12° ). Component 1 in the sandstones isolated within higher temperature ranges exhibits consistent directions in five sites out of six, yielding an overall site mean direction of 36/75, k = 10, α 95 = 45 ( in-situ) and 355/13, k = 178, α 95 = 5.2 (tilt-corrected) and passes a fold test at the 99% confidence level. The directions of component 2 with lower unblocking temperatures in the sandstones are best grouped at 10% unfolding ( k = 278, α 95 = 6 ), giving a mean direction of 351/18 that conforms to the mean directions of the characteristic component in the limestone unit as well as component 1 in the first five sandstone sites. Our interpretation, therefore, is that all the magnetizations in the both limestones and the sandstones were acquired at the time of folding. Our paleomagnetic directions from Guizhou are similar to those of Paleozoic rocks, from which folding associated remagnetizations were also observed, in the vicinity of Nanjing in the northeastern part of the Yangtze Block. The folding associated remagnetizations in both areas thus most likely have the same age, i.e., Late Triassic/Jurassic. We further infer that the southern Guizhou province has not been rotated since Late Triassic/Jurassic times by considering the similarity of the pole positions obtained from remagnetizations in Yunnan, Guizhou and the Nanjing area. Moreover, we suggest a new early Mesozoic apparent polar wander loop for the South China Block, going through the poles of the Triassic/Jurassic remagnetizations. This loop points to rotations of the South China Block as a whole during the late Triassic-Jurassic period.

Pervasive remagnetization of paleozoic rocks acquired at the time of Mesozoic folding in the South China Block

A total of 152 paleomagnetic samples was collected from 23 sites distributed over six Paleozoic formations, with ages ranging between Middle Ordovician and Early Permian, in the Tangshan type section in the Nanjing area of South China. Stepwise thermal demagnetization has successfully resolved two magnetization components from these samples. Component A with northerly shallow tilt‐corrected directions resides in approximately 95% of the total number of samples analyzed; this component passes fold tests at the 99% significance level in three formations. The overall mean direction of component A is 350/−3 (tilt‐corrected, a95=7.2). Component B, isolated from 19 samples in two formations (Cl and D3), yields negative fold tests at the 95% significance level and is, therefore, younger than the Mesozoic folding. The overall site mean direction of component B is 354/−4 (in situ, a95=8.4). The tilt‐corrected direction of component A clearly resembles the in situ direction of component B, indicating that both components are likely to be of approximately the same age. We infer that they are both remagnetizations, associated with the deformation. A review of the Paleozoic paleomagnetic data reported by other authors from adjacent areas shows that similar remagnetization situations may prevail. It is concluded that the northeast part of south China has experienced pervasive remagnetizations at the time of folding. The direction of this folding‐induced remagnetization yields a new well‐defined paleopole position (56°N, 314°E) for south China. Although this paleopole is significantly different from Mesozoic poles reported to date, we infer that a Triassic to Jurassic age is most likely for this remagnetization.

Tilting, burial, and uplift of the Guadalupe Igneous Complex, Sierra Nevada, California

Research Article| October 01, 1993 Tilting, burial, and uplift of the Guadalupe Igneous Complex, Sierra Nevada, California PETER J. HAEUSSLER; PETER J. HAEUSSLER 1Earth Sciences Board and Institute of Tectonics, University of California, Santa Cruz, California 95064 Search for other works by this author on: GSW Google Scholar SCOTT R. PATERSON SCOTT R. PATERSON 2Department of Geological Sciences, University of Southern California, Los Angeles, California 90089-0740 Search for other works by this author on: GSW Google Scholar Author and Article Information PETER J. HAEUSSLER 1Earth Sciences Board and Institute of Tectonics, University of California, Santa Cruz, California 95064 SCOTT R. PATERSON 2Department of Geological Sciences, University of Southern California, Los Angeles, California 90089-0740 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1993) 105 (10): 1310–1320. https://doi.org/10.1130/0016-7606(1993)105<1310:TBAUOT>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation PETER J. HAEUSSLER, SCOTT R. PATERSON; Tilting, burial, and uplift of the Guadalupe Igneous Complex, Sierra Nevada, California. GSA Bulletin 1993;; 105 (10): 1310–1320. doi: https://doi.org/10.1130/0016-7606(1993)105<1310:TBAUOT>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract It is often incorrectly assumed that plutons have a relatively uneventful structural history after emplacement. The 151 Ma Guadalupe Igneous Complex (GIC) in the Foothills Terrane, California, was involved in three post-emplacement events: (1) ∼30° of southwestside-up tilting during ductile regional faulting and contraction, (2) burial of the pluton from ∼4 to 12 km during crustal thickening of the wall rocks, and (3) uplift with only minor tilting in the Late Cretaceous. Tilting of the pluton is indicated by (1) southwest to northeast gradational changes from layered gabbros and diorites to granites and granophyres; (2) northeastward dips of layering in gabbro, internal contacts, and bedding of overlying coeval(?) volcanic rocks; (3) northeastward decrease in wall-rock metamorphic grade; and (4) paleomagnetic data from 14 localities across the pluton. We argue that tilting occurred between 146-135 Ma during southwest-northeast-directed regional contraction. This contraction is indicated by widespread folds and cleavages and by reverse motion on the Bear Mountains fault zone (BMFZ), a large northeast-dipping shear zone that bounds the GIC on its southwest side. Burial of the GIC, which overlapped in time but outlasted tilting, is suggested by (1) post-emplacement contractional faulting, folding, and cleavage development; (2) analyses of strains associated with widespread cleavage that indicate vertical thickening of ∼100% and (3) microstructural and mineral assemblage data that indicate shallow emplacement of the GIC, in contrast to mineral assemblage and limited geobarometric data from adjacent 120-110 Ma plutons that indicate moderate emplacement levels. Late Cretaceous uplift is indicated by 95-75 Ma sedimentary rocks that unconformably overlie the 120-110 Ma plutons.This geologic history is interesting for several reasons. First, although the GIC participated in extensive post-emplacement deformation, it lacks internal structural evidence of these events, except locally along the Bear Mountains fault zone. Second, the agreement between paleomagnetic and structural evidence for tilting suggests that no large latitudinal displacement of the GIC is required. Third, the paleomagnetic data also help to define the geometry of the magma chamber now represented by the GIC. Lack of streaking of paleomagnetic site-mean directions demonstrates that the pluton acted as a single unit after cooling through the blocking temperature (450-560 °C) of low-titanium titanomagnetite; however, variations in the dip of internal layering and contacts, from 70° at the base to 30° near the top of the pluton, indicate that not all of these features were horizontal and planar when they formed. We propose that this variation in dip of layering is most consistent with sidewall crystallization of magma resulting in drape of layering along the walls of the intrusion. Therefore, internal layering within this pluton does not record paleohorizontal. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Paleomagnetic constraints on eruption patterns at the Pacaya composite volcano, Guatemala

Pacaya volcano is an active composite volcano located in the volcanic highlands of Guatemala about 40 km south of Guatemala City. Volcanism at Pacaya alternates between Strombolian and Vulcanian, and during the past five years there has been a marked increase in the violence of eruptions. The volcano is composed principally of basalt flows interbedded with thin scoria fall units, several pyroclastic surge beds, and at least one welded tuff. Between 400 and 2000 years BP the W-SW sector of the volcano collapsed producing a horseshoeshaped amphitheater (0.65 km3) and providing a window into the cone's infrastructure. Lava flows and tephra exposed in the amphitheater are more then 200 m thick and when combined with flows erupted recently represent between 30 and 40% of the cone's history. Pacaya is ideally suited for a paleomagnetic study into the timing and duration of eruption episodes at a large, composite volcano. We drilled 27 paleomagnetic sites (25 aa flows, 1 dike, and 1 welded tuff) from four lava-flow sequences with between 4 and 14 sites per sequence. The four sequences represent initial through historic activity at Pacaya. We resolved, what appear to be, 22 time-independent paleomagnetic sites by averaging together directions from successive sites where the sitemean directions were indistinguishable at the 95% level of confidence. However, mean-sequence directions of individual lava-flow sequences yielded unusually high Fisher precision parameters (k=44–224) and small circles of 63% confidence (a63=1.6–6.1°) suggesting as few as three or four time-independent sites were collected. This indicates that activity as Pacaya is strongly episodic and that episodes are characterized by voluminous outpouring of lavas. Modelling the data using Holocene PSV rates confirms this and shows that differences in within-sequence directions (6–11.5°) are consistent with emplacement of lava-flow sequences in less than 100 years to as many as 300 years. Relatively larger differences in directions (18–23°) between subjacent lava-flow sequences indicates that repose is at least 300–500 years and could be even longer.