Negative Inclination Research Articles

Paleomagnetism of the Middle Proterozoic Laramie anorthosite complex and Sherman Granite, southern Laramie Range, Wyoming and Colorado

We present the results of a combined paleomagnetic and 40Ar/39Ar geochronologic investigation of the Middle Proterozoic Laramie anorthosite complex and Sherman Granite in the southern Laramie Range of Wyoming and Colorado. Anorthosites and monzosyenites of the Laramie anorthosite complex yield a well‐defined characteristic magnetization of northeast declination (D) and moderate negative inclination (I), although antipodal normal and reverse polarity magnetizations are present at three sites. A grand mean direction from 29 of 35 sites in the complex is D = 44.6°, I = −48.7° (k = 77.4, α95 = 3.1°). Alternating field (AF) and thermal demagnetization behavior and rock magnetic experiments indicate that magnetization is carried by low‐Ti titanomagnetite of single or pseudo‐single domain character that occurs as elongate to rod‐shaped inclusions in plagioclase and potassium feldspar. The Sherman Granite contains a dual polarity magnetization that is less well defined than that of the Laramie anorthosite complex but similar in declination and inclination (D = 53.1°, I= −48.1°, k = 46.5, α95 = 7.6°, n = 8/14 sites); rock magnetic data indicate the primary carrier of remanence in Sherman Granite is magnetite. The 40Ar/39Ar geochronologic data from Sherman Granite hornblende, biotite, and microcline indicate that subsolidus cooling was moderate to relatively rapid through the range of temperatures over which magnetization was blocked and that the age of remanence is about 1415 Ma. Microcline data indicate that the Laramie anorthosite complex and Sherman Granite have probably not been thermally remagnetized. Paleomagnetic poles from the Laramie anorthosite complex and Sherman Granite are indistinguishable at the 95% confidence level, and individual virtual geomagnetic poles (VGPs) from both units are combined to provide a mean pole at 215.0°E, 6.7°S (K = 46.9, A95 = 3.5°, N = 37 VGPs). The location of this pole is similar to paleomagnetic poles derived from 1480 Ma to 1450 Ma intrusions elsewhere in North America, but it plots significantly north of those from Middle Proterozoic sedimentary strata of the Belt Supergroup and Sibley Group. In addition to the characteristic magnetization, samples from some sites in Sherman Granite contain a remanence of southeast declination and shallow negative inclination (D = 154.9°, I = −16.0°, k = 90.6, α95 = 9.7°, n = 4 sites). This secondary magnetization was probably acquired during late Paleozoic time.

Late Carboniferous—Late Permian paleomagnetic overprinting of Carboniferous granitoids in southern Bohemian Massif (Austria)

Paleomagnetic investigations on Early-Middle Carboniferous granitoids of the Austrian part of the South Bohemian Pluton (48.5°N, 14.2°E) yielded two groups of characteristic remanent magnetization (ChRM) directions, one being characterized by SSW-declinations and positive inclinations and labelled A1 ( D = 200°, I = 14°, k = 67, N = 9; virtual geomagnetic pole (VGP) 31°N, 170°E), the other group having the same declinations but with negative inclinations ( D = 201°, I = −21°, k = 88, N = 18; VGP 48°N, 162°E). Both magnetizations are considered as magnetic overprints owing to distinct recrystallization processes that occurred in different hydrothermal and tectonic conditions in Late Carboniferous and Late Permian times. In some cases the original magnetizations may remain after the remagnetization processes, but as a whole this study confirms that the paleomagnetic history of a magmatic crystalline basement (in this case of Late Carboniferous to Late Permian age) maybe differ significantly from its intrusion history (in this case Early to Middle or Late Carboniferous).

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.

The Erieau Excursion revisited

Paleomagnetic results from two cores collected from the central and eastern basins of Lake Erie contain the same remanence direction shift from positive to negative inclination at a depth of approximately 9 m. Confirmation of the existence of this boundary over a wider extent of Lake Erie may initially appear to support the interpretation of this feature as the record of a geomagnetic excursion: the Erieau Excursion. Many details in the paleomagnetic record of these cores indicate that this feature is not of geomagnetic origin: the lack of synchroneity of this event across the lake; the association of the reversed directions with deformed sediments; the lack of any record of the transitional magnetic field; and the absence of any correlation between the reversed directions from the two cores. In core EK, the apparent magnetic transition is coincident with the stratigraphic boundary between the lacustrine and glaciogenic sediments. The model most compatible with this new paleomagnetic data involves regional deformation of the substrate of Lake Erie by some mechanism prior to 9500 years ago. The most likely mechanism to have produced this regional deformation was subglacial deformation of the underlying sediments.

Direct detection of solar wind dynamic pressure effect on ground geomagnetic field

The scatter plot of the equatorial Dst index versus the square root of the dynamic pressure of the solar wind (P0.5) showed a clear upper envelope which linearly increases with increasing P0.5. This upper envelope was interpreted as showing the minimum level of the ring current intensity and thus dependence of the Dst index upon P0.5. The similar upper envelope was also found in scatter plots of the nighttime H‐component versus P0.5 at stations in low and middle latitudes. From inclination of these upper envelopes the coefficients for the assumed linear relationship between P0.5 and the ground magnetic field were obtained. The coefficients derived from the envelopes for the Dst index and the nighttime H‐component at different stations agree well each other and the latter showed no latitudinal variation. The scatter plots also showed a clear linear lower envelope with negative inclination. The ground state of the magnetosphere should be specified by an intersection of the two lines showing upper and lower envelopes in the scatter plot of Dst which corresponds to zero IMF‐ Bz and the minimum value of the dynamic pressure of the solar wind.

Crustal age between the Clipperton and Clarion Fracture Zones

Magnetic anomalies of 52–80 Ma age have been mapped in the equatorial Pacific in a region where they were previously unrecognized. These anomalies, now at 7° N are best modeled with crustal rocks of negative inclination, apparently because the crust was formed at a spreading ridge that was south of the magnetic equator in Late Cretaceous‐Early Tertiary time. Magnetic anomalies recorded along a 5900‐km trackline that follows a flowline of crustal generation show one long period, from 12 to 31 Ma, of apparent constant half‐spreading rate over the 0 to 80 Ma represented.

The Dala dolerites, central Sweden, and their palaeomagnetic signature

Abstract New palaeomagnetic results are presented from nine dolerites from central Sweden. Data are also presented from four porphyry sites and one granite site. The data obtained are compared with previously published data from the area. Based on the directions of the characteristic magnetizations isolated in the various rock units a division into five groups, A-E, has been made. Group A consists of dolerites of late Sveconorwegian (c. 1050–850 Ma) age and group B comprises dolerites belonging to the Central Scandinavian Dolerite Group (c. 1250–1150 Ma). Groups C and D are less clearly defined. ‘Group C, with a shallow negative northerly remanence inclination represents the Tuna dykes (c. 1370 Ma). The D direction, with a shallow positive northerly inclination, may represent an older generation of dolerites. The border between groups C and D is diffuse and may be the result of a prolonged mid-Proterozoic period with dolerite intrusions in Dalarna during which significant apparent polar wander occurred. Group E comprises dolerites with a steep positive inclination close to the direction of the Earth's present field. The porphyries and the granite studied carry group D and E remanence directions. This strengthens the evidence for a D-group generation of dolerites contemporary with the porphyries and the granite, c. 1650 Ma ago.

Impact of assertive and accusatory communication of distress and anger: A verbal component analysis

Forty undergraduate women were asked to imagine discussing an important problem in their relationship with a personally close partner and to rate likely emotional and behavioral reactions to four kinds of statements: 1) assertive communication of distress, 2) assertive communication of anger, 3) accusatory communication of distress, and 4) accusatory communication of anger. Accusatory “you” statements were rated as more aversive and evoked more negative emotional and behavioral response inclinations than did assertive “I” messages. Statements including “anger” words evoked more negative emotional and behavioral response inclinations than statements including “distress” words. Findings were related to research on cue-controlled aggression and the work of Staats, which predicts ways that verbal labels (language) exert control over emotions and behavior.

The Matachewan dyke swarm, Canada: an early Proterozoic magnetic field reversal

Consistent age relationships between oppositely magnetized dykes of the 2.45 Ga Matachewan dyke swarm suggest that only a single magnetic field reversal occurred during the period of igneous activity. The magnetic field throughout most of this time was characterized by a SSW declination and shallow negative inclination but reversed toward the waning stages of magmatism. The new paleomagnetic data provide the oldest known magnetic reversal for which the relative reversal sense is known.

New insights into the structure of An tracts and B'-B' bends in DNA

Energy calculations suggest that the currently available NOE distance constraints for An tracts in DNA are incapable of distinguishing between structures with a narrowed minor groove arising from a large propeller twist with a small inclination or from a small propeller twist with a large negative inclination. Furthermore, analysis of published data, together with energy estimations, strongly argue against bifurcated hydrogen bonding between A and T residues being the cause of the anomalous structural properties of An tracts. A conformational analysis of the B'-B' junction has been performed in which a single variable base pair has been inserted between two regions of B' structure. We have calculated low-energy structures for AnGAn,AnCAn,AnTAn,AnCTn, and TnCAn duplexes, where the An and Tn tracts were fixed in the anomalous B' conformation. Upon optimization, all these structures were found to contain a pronounced roll-like bending into the major groove at the site of the insertion. The important factors in the formation of these B'-B' bends are the destruction of the B' conformation and the concomitant widening of the minor groove at the junction region in order to reduce minor groove interstrand base clashes and improve interstrand stacking energy. If the B' conformation has strong negative inclination, the improved intrastrand stacking energy also contributes to the bending. In calculations of duplexes with An and Tn tracts in the B conformation instead of B', the bending disappears.

A paleomagnetic investigation of the Lower Ordovician St. George Group, Port-au-Port Peninsula, Newfoundland: implications for the Iapetus Ocean and evidence for Late Paleozoic remagnetization

To obtain an early Ordovician paleomagnetic pole position for North America, and to determine the paleolatitude of its margin of the lapetus Ocean prior to the Taconic orogeny, a study of the lower St. George Group (Tremadocian-Arenigian) of western Newfoundland was undertaken. Analysis of 84 oriented core samples from six localities on Port au Port Peninsula indicates that the NRM of the St. George rocks is the resultant of three distinct paleomagnetic directions. The first is a steep downward component commonly observed in the low temperature range of demagnetization (room temperature 250°C) in virtually all samples, that roughly corresponds to the present day axial dipole field direction in situ ( D = 350°, I = 68°). The second is a stable component with southerly declinations and shallow-intermediate negative inclinations ( D, I in situ = 176° , −20°; D, I tilt corrected = 173° , −5°, respectively) usually seen in the range of 450–600°C and found in approximately 46% of the specimens. The third is a stable southeasterly, shallow-intermediate positive component ( D, I insitu = 161° , 16°; D, I tilt correcied = 163° , 34°, respectively) generally unblocked within the 350–550°C or 450–600°C range which was observed in 40% of the specimens. These components of remanence are herein referred to as the Type 1, Type 2, and Type 3 components, respectively. The remaining 14% of the specimens were found to be either directionally unstable to thermal demagnetization or totally overprinted by the Type 1 component. The Type 2 component is interpreted to represent a post-folding, Late Paleozoic remagnetization possibly associated with the Alleghanian Orogeny. On the other hand, the Type 3 component is interpreted as an Early Ordovician, possibly pre-Taconic remanence and, in tilt corrected coordinates, yields a paleolatitude for this region of the North American craton of 19° S ± 6° .

Natural convection in porous media above a near horizontal uniform heat flux surface

The boundary layer in free convection above a uniformly heated semi-infinite flat plate, which is inclined at a small angle to the horizontal in porous media is discussed. For positive inclinations of the plate, series solutions, one valid near the leading edge and the other at large distances from it, are obtained. When the inclination is negative, a series solution valid near the leading edge is again obtained. A step-by-step numerical technique, based on a scheme by Keller, is used to complete the solution in the region where neither series is adequate. For the negative inclinations of the plate, the boundary layer separates and a region of reverse flow develops.

Convergence aspect of limit equilibrium methods for slopes

This note is concerned with the multiplicity of solutions for the factor of safety that may be obtained on the basis of the method of slices. Discontinuities in the function for the factor of safety are discussed and the reasons for false convergence in any iterative solution process are explored, with particular reference to the well-known Bishop simplified method (circular slip surfaces) and Janbu simplified or generalized method (slip surfaces of arbitrary shape). The note emphasizes that both the solution method and the method of searching for the critical slip surface must be considered in assessing the potential for numerical difficulties and false convergence. Direct search methods for optimization (e.g., the simplex reflection method) appear to be superior to the grid search or repeated trial methods in this respect. To avoid false convergence, the initially assumed value of factor of safety F0 should be greater than β1(=−tan α1 tan [Formula: see text]) where α1 and [Formula: see text] are respectively the base inclination and internal friction angle of the first slice near the toe of a slope, the slice with the largest negative reverse inclination. A value of F0 = 1 + β1, is recommended on the basis of experience. If there is no slice with a negative slope for any of the slip surfaces generated in the automatic, search process, then any positive value of F0 will lead to true convergence for F. It is necessary to emphasize that no slip surface needs to be rejected for computational reasons except for Sarma's methods and similarly no artificial changes need to be made to the value of [Formula: see text] except for Sarma's methods. Key words: slope stability, convergence, limit equilibrium, analysis, optimization, slip surfaces, geological discontinuity, simplex reflection technique.

Paleomagnetic record from glacial varves in Alto Río Mayo, Patagonia, Argentina

Abstract Paleomagnetic study of a 3-m thick sequence containing 302 glacial varves outcropping in Alto Rio Mayo valley shows strong intensities, a declination shifting gradually to the west and an increasing negative inclination. Virtual geomagnetic pole positions reveal a migration of the geomagnetic pole from NE Canada-SW Greenland to a position around NW Siberia. The deposit is believed to represent the interval between the recession of the last glacial maximum and a final ice front oscillation, with a readvance in the Alto Rio Mayo valley thought to have occurred shortly before 13 500 yr B.P.

Late Pleistocene geomagnetic excursion in Icelandic lavas: confirmation of the Laschamp excursion

In 1980 Kristjansson and Gudmundsson [1] reported a late glacial geomagnetic excursion in three hills in the Reykjanes peninsula, Iceland, with shallow negative inclinations and westerly declinations. They named it the Skalamaelifell excursion. More extensive field work has identified the same excursional paleomagnetic direction ( declination= 258°,inclination= −15°) at four additional outcrops in a 10 × 10 km area in the Reykjanes peninsula. The excursion lavas are olivine tholeiites with similar petrography and chemical compositions. Paleointensity determinations by the Thellier method average 4.2 ± 0.2 μT for 8 samples, more than an order of magnitude weaker than the present geomagnetic field in Iceland. Together, these results suggest extrusion of the excursion lavas in a very brief span of time, probably less than a few hundred years. K Ar dating of the excursion lavas gives a mean age for 19 determinations of 42.9 ± 7.8ka(2σ). Compilation of thirty K Ar ages of the Laschamp and Olby flows by three laboratories yield a new age for the Laschamp excursion in France of 46.6 ± 2.4ka(2σ). The age of the excursion in southwestern Iceland is statistically indistinguishable from the Laschamp excursion at the 95% confidence level, and both have very low paleointensities. Therefore, we suggest that the Laschamp and Olby flows in France and the Skalamaelifell units of Iceland recorded essentially the same geomagnetic excursion. Differences in the virtual paleomagnetic poles (VGPs) of these excursions may be due to (1) the probable non-dipole character of the geomagnetic field during the excursion, (2) rapid geomagnetic secular variation and possible small age differences of the extrusive rocks in France and Iceland, and/or (3) crustal magnetic anomalies which might dominate the local geomagnetic field during the excursion at either or both locations.

Higher-order effects in natural convection flow over uniform flux inclined flat plates in porous media

Higher-order boundary-layer effects for natural convection flow along inclined flat plates (of both positive and negative inclinations) with a uniform heat flux surface condition in a saturated porous medium are studied. Using the method of matched asymptotic expansions the three terms in inner and outer expansions have been obtained. It is shown that the first eigenfunction for this considered problem coincides with 0(ɛ2)-term in the inner expansion.

Paleomagnetic study of the Ordovician Table Head Group, Port au Port Peninsula, Newfoundland

A paleomagnetic study of the Ordovician Table Head Group in the Port au Port Peninsula of western Newfoundland reveals a simple two-component magnetization history comprising a reversely magnetized, stable southeasterly remanence with a, shallow to moderate inclination, and an unstable present-day overprint. Pole positions for the stable component, both with and without tectonic tilt correction, correspond with the Early to middle Ordovician pole positions for North America, suggesting this remanence is early. Although the nature of the geomagnetic field in the Ordovician is not well known, the polarity observed is consistent with that reported from other mid-Ordovician studies and appears to reflect a predominance of reverse polarity for this time interval. Evidence of significant rotation of any of the sites studied is absent, indicating that the continental margin in this region acted in an integral rather than a fragmented fashion during deformation.Previously published and new, but preliminary, results from the Early Ordovician St. George Group indicate the presence of two stable components of remanence. These components have similar south-southeasterly declinations but differ in inclination. The shallow to intermediate positive inclination component has a direction that is broadly compatible with Early Ordovician poles from North America. The shallow negative inclination component observed in rocks of similar age from other parts of western Newfoundland appears to be consistent with a later remagnetization of this unit.

Free convection heat transfer from an isothermal plate with arbitrary inclination

This paper presents a new formulation for the laminar free convection from an arbitrarily inclined isothermal plate to fluids of any Prandtl number between 0.001 and infinity. A novel inclination parameterξ is proposed such that all cases of the horizontal, inclined and vertical plates can be described by a single set of transformed equations. Moreover, the self-similar equations for the limiting cases of the horizontal and vertical plates are recovered from the transformed equations by settingξ=0 andξ=1, respectively. Heated upward-facing plates with positive and negative inclination angles are investigated. A very accurate correlation equation of the local Nusselt number is developed for arbitrary inclination angle and for 0.001 ≦Pr ≦∞.

Paleomagnetism and rock magnetism of Quaternary volcanic rocks and Late Paleozoic strata, VC‐1 core hole, Valles Caldera, New Mexico, with emphasis on remagnetization of Late Paleozoic strata

Paleomagnetic and rock magnetic data obtained from azimuthally unoriented core samples, collected at approximately 1‐ to 3‐m intervals, of Continental Scientific Drilling Program core hole VC‐1 have prompted reinterpretations of the Quaternary volcanic stratigraphy intersected by the bore and have aided in evaluating the thermal regime within late Paleozoic strata attending fluid circulation and mineral deposition during and after development of the Toledo and Valles calderas. The results from Quaternary units (Banco Bonito Obsidian: I = +35.4°, a95 = 2.8° (inclination only determinations), n = 33; Battleship Rock Tuff: D = 359.6°, I = +42.4°, a95 = 2.8°, n = 5 site means (surface sites); VC‐1 Rhyolite: I = +39.2°, a95 = 12.8°, n = 7; Upper VC‐1 Tuff: I = +37.2°, a95 = 10.7°, n = 13; Middle VC‐1 Tuff: I = +42.1°, a95 = 2.1°, n = 39; South Mountain Rhyolite: D = 350.9°, I = +49.9°, a95 = 3.4°, n = 10 (one surface site)) are consistent with isotopic age data, indicating that the entire moat volcanic sequence intersected is less than 650 kyr. Monitoring of natural remanent magnetization (NRM) intensity, NRM directions, directions of magnetizations isolated during progressive demagnetization, median destructive forces, and rock magnetization parameters has identified systematic variations within the thick Banco Bonito Obsidian and VC‐1 Tuff units. The Permian Abo Formation, Pennsylvanian to earliest Permian Madera Limestone, and Pennsylvanian Sandia Formation typically contain a moderate positive inclination magnetization component (Abo Formation: I = +52.2°, a95 = 7.4°, n = 16; Madera Limestone: I = +58.4°, a95 = 2.8°, n = 105; Sandia Formation: I = +53.9°, a95 = 4.8°, n = 21); when residing in magnetite, it is usually unblocked in the laboratory by 350°C; when carried by hematite it is unblocked by 550°C. A moderate negative inclination (e.g., Madera and Abo strata: D = 173.1°, I = −46.6°, a95 = 5.5°; n = 47 samples; assuming a north seeking positive inclination RM providing a useful means of core orientation), oppositely directed magnetization is occasionally isolated and removed at higher laboratory unblocking temperatures (up to 550°C in magnetite‐dominated and 600°C in hematite‐dominated lithologies). Where present, magnetizations of shallower inclination (e.g., Madera and Abo strata: D = 160.1°, I = −10.9°, a95 = 4.4°, n = 28) are removed over higher ranges of unblocking temperatures, and these are probably of late Paleozoic age. The moderate negative and positive inclination magnetizations are in all likelihood viscous partial thermoremanent magnetizations (TRMs) (VPTRMs). These were activated at moderate (∼300°C) temperatures between 1.45 and 0.97 Ma, attending and following the main stages of Toledo/Valles caldera development during the Matuyama, and at near‐present downhole temperatures (∼100°–180+ °C) during the Brunhes and possibly within the last 10 kyr, respectively. Importantly, the preservation of negative inclination RM components in magnetite‐dominated lithologies of the Madera Limestone dictates that temperatures during the Brunhes chron (past 730 kyr) did not exceed approximately 300°–350°C. The inferred temperatures for thermal activation of the VPTRMs, based upon the Middleton and Schmidt thermal activation curves for both magnetite and hematite derived following Walton, agree with independent estimates of thermal activity, using fluid inclusion and K‐Ar isotopic age determination data, during the Quaternary. The results further illustrate the potential usefulness of thermal activation theories for inferring temperatures of past geologic events, especially when they can be applied to magnetizations of similar age residing in both magnetite and hematite, and of paleomagnetic and rock magnetic studies in general in continuous coreholes in complex geologic environments.

The Magnetisation of the Elura Orebody, Cobar, NSW

Petrophysical sampling of the steeply plunging, roughly pipe-like Elura Zn?Pb?Ag orebody demonstrates a dominantly remanent magnetisation with steep negative inclination; the magnetisation is carried by central monoclinic pyrrhotite lenses. The remanence data suggests a mid-Cretaceous low grade thermal overprint possibly related to final cooling associated with minor uplift. Susceptibilities are high, averaging about 4200 x 10-6 (about 52800 x 10-6SI), and markedly anisotropic with k1:k2:k3 = 1.6:1.2:1.0. The anisotropy reflects a preferred crystallographic orientation of pyrrhotite grains ? the basal planes are vertically oriented suggesting plastic flow or crystal growth in a vertical direction. The high susceptibilities coupled with a Koenigsberger Ratio of 9.4 produce a ground magnetic anomaly 140 gammas (nT) in magnitude. This anomaly can be simply and correctly modelled by vertically plunging ellipsoids using equations that incorporate remanence, anisotropic susceptibility, and demagnetisation.