Michael S. Petronis

Paleomagnetic and 40Ar/39Ar geochronologic data bearing on the structural evolution of the Silver Peak extensional complex, west-central Nevada

The Silver Peak extensional complex, located in the Silver Peak Range of west- central Nevada, is a displacement-transfer system linking the Furnace Creek–Fish Lake Valley fault system and transcurrent faults of the central Walker Lane. Late Neogene, northwest-directed motion of an upper plate, composed of lower Paleozoic sedimentary rocks and late Tertiary volcanic and volcaniclastic strata, exhumed a lower-plate assemblage of metamorphic tectonites with Proterozoic and Mesozoic protoliths. Paleomagnetic investigation of Miocene–Pliocene pyroclastic and sedimentary rocks of the upper plate and Miocene mafic dikes in the lower plate reveals modest horizontal- axis tilting (northwest-side-up) and vertical-axis rotation (clockwise) within the extensional complex. Eight to ten samples from each of 123 sites were demagnetized; 95 sites yielded interpretable results. Dual- polarity results from one population of mafic dikes in the lower-plate assemblage indicate moderate, northwest-side-up tilting (declination D = 329°, inclination I = 37°, α95 = 4.3°, number N = 30 sites; in situ) (α95 = the confidence limit for the calculated mean direction expressed as an angular radius from the calculated mean direction). Some dikes yield exclusively normal-polarity results that are interpreted to indicate modest clockwise vertical-axis rotation ( D = 021°, I = 57°, α95 = 4.3°, N = 19 sites; in situ) concurrent with uplift of the lower-plate rocks, and nine sites yield magnetization directions that are north-directed with positive inclinations of moderate steepness, similar to an expected Miocene field. Late Miocene pyroclastic rocks in the upper plate yield normal-polarity magnetizations suggestive of moderate, clockwise, vertical-axis rotation ( D = 032°, I = 53°, α95 = 8.8°, N = 10 sites). The apparent clockwise rotation is unlikely to result from incomplete sampling of the geomagnetic field, because the overall dispersion of the VGP (virtual geomagnetic pole) positions is high for the latitude of the site location. Middle Miocene sedimentary rocks probably were remagnetized shortly after deposition. Of eight 40Ar/39Ar determinations from mafic dikes in the lower plate, five groundmass concentrates yield saddle-shaped age spectra, and one separate provided a plateau date of low confidence. Isochron analysis reveals that all six groundmass concentrates contain excess Ar. If rapid cooling and Ar retention below ∼250 °C are assumed, the preferred age estimate for mafic intrusions is provided by isochron dates and suggests emplacement between 12 and 10.5 Ma. The 40Ar/39Ar age-spectrum data are consistent with existing fission-track cooling and K-Ar isotopic age information from lower-plate granitic rocks and indicate rapid cooling of the lower-plate assemblage from well above 300 °C to 100 °C between 13 and 5 Ma. Rapid cooling may explain the overall distribution of paleomagnetic results from lower-plate intrusions such that the earliest acquired magnetizations reflect both northwest-side-up tilt and clockwise rotation and the younger magnetizations reflect northwest-side-up tilt. Overall, the paleomagnetic data from the Silver Peak extensional complex are interpreted to suggest that vertical-axis rotation of crustal-scale blocks, associated with displacement transfer in the central Walker Lane, may play an integral part in accommodating strain within a continental displacement-transfer system.

Palaeomagnetic and anisotropy of magnetic susceptibility data bearing on the emplacement of the Western Granite, Isle of Rum, NW Scotland

The Western Granite is the largest of several granitic bodies around the margin of the Rum Central Igneous Complex. We report palaeomagnetic and anisotropy of magnetic susceptibility (AMS) data that bear on the emplacement and deformation of the Western Granite. The collection includes samples from 27 sites throughout the Western Granite, five sites in adjacent feldspathic peridotite, and two sites in intermediate to mafic hybrid contact aureole rocks. Palaeomagnetic data from 22 of the 27 sites in the granite provide an in situ group mean D = 213.2°, I = −69.5°, α95 = 5.5° that is discordant to an early Paleocene reverse polarity expected field (about 184°, −66°, α95 = 4.3°). The discrepancy is eliminated by removing an inferred 15° of northwest-side-down tilting about a best fit horizontal tilt axis trending 040°. Data from the younger peridotite and hybrid rocks of the Rum Layered Suite provide an in situ group mean of D = 182.6°, I = −64.8°, α95 = 4.0°, which is statistically indistinguishable from an early Paleocene expected field, and imply no post-emplacement tilting of these rocks since remanence acquisition. The inferred tilt recorded in the Western Granite, which did not affect the younger Layered Suite, suggests that emplacement of the ultrabasic rocks resulted in roof uplift and associated tilt of the Western Granite to make space for mafic magma emplacement. Magnetic fabric magnitude and susceptibility parameters yield two subtle groupings in the Western Granite AMS data set. Group 1 data, defined by rocks from exposures to the east and south, have comparatively high bulk susceptibilities (Kmean, 29.51 × 10−3 in SI system), stronger anisotropies (Pj, 1.031) and oblate susceptibility ellipsoids. Group 2 data, from rocks in the west part of the pluton, have lower values of Kmean (15.89 × 10−3 SI) and Pj (1.014), and triaxial susceptibility ellipsoids. Magnetic lineations argue for emplacement of the granite as a tabular sheet from the south–southeast toward the north and west. Moderate to steeply outward-dipping magnetic foliations, together with deflection of the country rock bedding in the north, are consistent with doming accompanying magma emplacement.

Magmatic flow paths and palaeomagnetism of the Miocene Stoddard Mountain laccolith, Iron Axis region, Southwestern Utah, USA

Abstract The Stoddard Mountain laccolith is part of a complex of Early Miocene laccoliths intruded along the western edge of the Colorado Plateau in the Iron Axis region of Southwestern Utah. Most Colorado Plateau laccoliths (e.g. Henry and La Sal Mountains) are considered to be fed by a central axial feeder system. However, detailed mapping in the Iron Axis region suggests that the Stoddard Mountain laccolith was fed laterally from the west. Structural and field data suggest the quartz monzonitic magma initially migrated laterally eastward at ∼1 km depth as a sill before spreading laterally north-south where it inflated to ∼1–1.5 km thickness. To test the model of a lateral feeder system, data were collected from 32 palaeomagnetic sites and 76 AMS stations (763 accepted specimens) sampled over the ∼54 km2 exposed part of the N-S oval-shaped laccolith. The in situ AMS fabrics, inferred to correlate with magmatic fabrics, typically show NE trending lineations in the north and S-SE trending lineations in the south part of the intrusion. The palaeomagnetic data are interpreted to indicate a very minimal amount of post-emplacement deformation of the intrusion. The overall lack of westerly-directed and steep magnetic lineations argues against emplacement via a central axial feeder system.

Controls on emplacement of the Caledonian Ross of Mull Granite, NW Scotland: Anisotropy of magnetic susceptibility and magmatic and regional structures

ABSTRACT Anisotropy of magnetic susceptibility (AMS) was measured from 139 sites across the exposed portion of the Caledonian Ross of Mull Granite, Argyllshire, NW Scotland, to investigate the internal architecture of the pluton. Field and petrographic observations support the results of the AMS study, and a complementary suite of rock magnetic experiments was used to quantify the magnetic mineralogy of the granite. AMS data reveal a fabric that records a partial tectonic overprint of an emplacement-related magma inflow fabric. The partially preserved inflow pattern indicates a south to north emplacement of subhorizontal sheets that coalesced to form a tabular pluton. The AMS data are remarkably consistent across the intrusion, with clear north-south strikes and trends dominating the magnetic foliation and lineation patterns, respectively. These AMS fabrics are discordant to internal zonation in the granite. In the southernmost part of the granite, AMS data are parallel to bedding-cleavage orientations preserved in many large (>100 m) Proterozoic Moine blocks. The scarcity of Moine blocks in the northern part of the intrusion and the prevalence of randomly oriented stoped blocks suggest that this part of the intrusion is nearer to the ceiling of the pluton. The partial tectonic overprint, consistent with east-west compression, occurred during the latter stages of emplacement and mainly affected the younger granite facies (RM2), while the oldest granite facies (RM1) and certain magmatic structures (diorite enclaves) preserve the original emplacement fabric. An alternative explanation interprets the magnetic lineations as an intersection fabric between a shallowly dipping emplacement fabric and a steep tectonic fabric. We argue that emplacement was not associated with eastward orogenic collapse of the Scandian Moine nappes, as proposed by previous workers, but occurred either before orogenic collapse or during a period of compressional reactivation.

Assessing vertical axis rotations in large-magnitude extensional settings: A transect across the Death Valley extended terrane, California

Models for Neogene crustal deformation in the central Death Valley extended terrane, southeastern California, differ markedly in their estimates of upper crustal extension versus shear translations. Documentation of vertical axis rotations of range-scale crustal blocks (or parts thereof) is critical when attempting to reconstruct this highly extended region. To better define the magnitude, aerial extent, and timing of vertical axis rotation that could mark shear translation of the crust in this area, paleomagnetic data were obtained from Tertiary igneous and remagnetized Paleozoic carbonate rocks along a roughly east-west traverse parallel to about 36°N latitude. Sites were established in ∼7 to 5 Ma volcanic sequences (Greenwater Canyon and Browns Peak) and the ∼10 Ma Chocolate Sundae Mountain granite in the Greenwater Range, ∼8.5 to 7.5 Ma and 5 to 4 Ma basalts on the east flank of the Black Mountains, the 10.6 Ma Little Chief stock and upper Miocene(?) basalts in the eastern Panamint Mountains, and Paleozoic Pogonip Group carbonate strata in the north central Panamint Mountains. At the site level, most materials yield readily interpretable paleomagnetic data. Group mean directions, after appropriate structural corrections, suggest no major vertical axis rotation of the Greenwater Range (e.g., D = 359°, I = 46°, α_(95) = 8.0°, N = 12 (7 normal (N), 5 reversed (R) polarity sites)), little post-5 Ma rotation of the eastern Black Mountains (e.g., D = 006°, I = 61°, α_(95) = 4.0°, N = 9 N, 6 R sites), and no significant post-10 Ma rotation of the Panamint Range (e.g., D = 181°, I = −51°, α_(95) = 6.5°, N = 9 R sites). In situ data from the Greenwater Canyon volcanic rocks, Chocolate Sundae Mountain granite, Funeral Peak basalt rocks, the Little Chief stock, and Paleozoic carbonate rocks (remagnetized) are consistent with moderate south east-side-down tilting of the separate range blocks during northwest directed extension. The paleomagnetic data reported here suggest that the Panamints shared none of the 7 Ma to recent clockwise rotation of the Black Mountains crystalline core, as proposed in recent models for transtensional development of the central Death Valley extended terrane.

Distinguishing diapirs from inflated plutons: an integrated rock magnetic fabric and structural study on the Roundstone Pluton, western Ireland

Granitoid plutons account for much of the continental crust and are critical in the generation of several economic resources. Despite over a century of research, the fundamental process by which large felsic bodies intrude remains controversial. Two contrasting models persist: (1) mass ascent and subsequent cooling of magma (diapirism); (2) sheet ascent and subsequent emplacement (laccoliths). The latter hypothesis is different from the former as distinct ascent and emplacement processes must occur and be identifiable; traditional field methods rarely allow this important distinction to be made. We present field, petrographic and anisotropy of magnetic susceptibility data from the Roundstone Pluton, Connemara, western Ireland, which was previously considered a diapiric intrusion. The new data reveal anomalous subvertical magmatic lineations in the core of this circular intrusion as well as a suite of magmatic and submagmatic fabrics that parallel NNW–SSE country rock faults and coeval G2 sheets within the pluton. We deduce that these structures reflect a distinct centralized ascent process along a NNW–SSE conduit. Lateral emplacement and pluton inflation followed, ultimately forming a punched laccolith. We conclude that the combined application of the above techniques provides a unique insight into pluton architecture that removes ambiguity between contrasting diapiric and inflated pluton models. Supplementary material: Complete rock magnetic data tables and sample site coordinates (Irish Grid) are available at http://www.geolsoc.org.uk/SUP18837.

Three-dimensional magma flow dynamics within subvolcanic sheet intrusions

Sheet intrusions represent important magma conduits and reservoirs in subvolcanic systems. Constraining the emplacement mechanisms of such intrusions is crucial to understanding the physiochemical evolution of magma, volcano deformation patterns, and the location of future eruption sites. However, magma plumbing systems of active volcanoes cannot be directly accessed and we therefore rely on the analysis of ancient systems to inform the interpretation of indirect geophysical and geochemical volcano monitoring techniques. Numerous studies have demonstrated that anisotropy of magnetic susceptibility (AMS) is a powerful tool for constraining magma flow patterns within such ancient solidified sheet intrusions. We conducted a high-resolution AMS study of seven inclined sheets exposed along the Ardnamurchan peninsula in northwest Scotland, and examined how magma flow in sheet intrusions may vary along and perpendicular to the magma flow axis. The sheets form part of the Ardnamurchan Central Complex, which represents the deeply eroded roots of an ∼58-m.y.-old volcano. Our results suggest that the inclined sheets were emplaced via either updip magma flow or along-strike lateral magma transport. It is important that observed variations in magnetic fabric orientation, particularly magnetic foliations, within individual intrusions suggest that some sheets were internally compartmentalized, i.e., different along-strike portions of the inclined sheets exhibit subtle differences in their magma flow dynamics. This may have implications for the flow regime and magma mixing within intrusions.

Discerning magmatic flow patterns in shallow-level basaltic dykes from the NE rift zone of Tenerife, Spain, using the Anisotropy of Magnetic Susceptibility (AMS) technique

Abstract The anisotropy of magnetic susceptibility (AMS) technique is a rapid petrophysical method used to infer magma flow directions within dykes as well as other igneous intrusions. Samples for AMS study were collected from dykes along the upper part of the NE Rift Zone (NERZ) of Tenerife, Canary Islands, Spain. Of the analysed dykes, 28 have interpretable normal magnetic fabrics. These 28 dykes are therefore suitable to assess the magma flow direction using the imbrication of the magnetic foliation plane from paired dyke margins and/or the overall trend and plunge of the magnetic lineations. AMS fabrics show downwards and upwards flow that could be related to flank and summit eruptions. Overall, however, the direction and sense of magma flow does not follow a specific trend across the NERZ, suggesting that the dykes are supplied by local shallow-level reservoir(s) underneath the ridge or are responding to variations in the local stress field across the axis of the rift zone. The variability of the AMS fabrics suggests a rather complicated propagation mode of magma within the dykes of the NERZ, contrasting with the common assumption of uniform magma propagation within rift zones. Our data therefore support the notion that magma propagation beneath active volcanic systems is inherently more complex than simple subvertical flow from source to final emplacement level, which bears on volcanic hazards worldwide.

Pre-Teide Volcanic Activity on the Northeast Volcanic Rift Zone

The northeast rift zone of Tenerife (NERZ) presents a partially eroded volcanic rift that offers a superb opportunity to study the structure and evolution of oceanic rift zones. Field data, structural observations, isotopic dating, magnetic stratigraphy, and isotope geochemistry have recently become available for this rift and provide a reliable temporal framework for understanding the structural and petrological evolution of the entire rift zone. The NERZ appears to have formed in several major pulses of activity with a particularly high production rate in the Pleistocene (ca. 0.99 and 0.56 Ma). The rift underwent several episodes of flank creep and eventual catastrophic collapses driven by intense intrusive activity and gravitational adjustment. Petrologically, a variety of mafic rock types, including crystal-rich ankaramites, have been documented, with most samples isotopically typical of the “Tenerife signal”. Some of the NERZ magmas also bear witness to contamination by hydrothermally altered components of the island edifice and/or sediments. Isotope geochemistry furthermore points to the generation of the NERZ magmas from an upwelling column of mantle plume material mixed with upper asthenospheric mantle. Finally, persistent isotopic similarity through time between the NERZ and the older central edifices on Tenerife provides strong evidence for a genetic link between Tenerife’s principal volcanic episodes.

Late Caledonian transpression and the structural controls on pluton construction; new insights from the Omey Pluton, western Ireland

The Galway Granite Complex is unique among the British and Irish Caledonian granitoid terranes, as it records punctuated phases of magmatism from approximate to 425-380 Ma throughout the latest phase of the Caledonian Orogeny. Remapping of the Omey Pluton, the oldest member of this suite, has constrained the spatial distribution and contact relationships of the plutons three main facies relative to the nature of the host rock structure. The external contacts of the pluton are mostly concordant to the limbs and hinge of the Connemara Antiform. New AMS data show that a subtle concentric outward dipping foliation is present, and this is interpreted to reflect pluton inflation during continued magma ingress. Combined field, petrographic and AMS data show that two sets of shear zones (NNW-SSE and ENE-WSW) cross-cut the concentric foliation, and that these structures were active during the construction of the pluton. We show that regional sinistral transpression at approximate to 420 Ma would have caused dilation along the intersection of these two fault sets, and suggest that this facilitated centralised magma ascent. Lateral emplacement was controlled by the symmetry of the Connemara Antiform to ultimately produce a discordant phacolith. We propose that regional sinistral transpression at approximate to 420 Ma influenced the siting of smaller intrusions over NNW-SSE faults, and that the later onset of regional transtension caused larger volumes of magma to intrude along the E-W Skird Rocks Fault at approximate to 400 Ma.