Malcolm S. Pringle

Chronology and dynamics of a large silicic magmatic system: Central Taupo Volcanic Zone, New Zealand

The central Taupo Volcanic Zone in New Zealand is a region of intense Quaternary silicic volcanism accompanying rapid extension of continental crust. At least 34 caldera-forming ignimbrite eruptions have produced a complex sequence of relatively short-lived, nested, and/or overlapping volcanic centers over 1.6 m.y. Silicic volcanism at Taupo is similar to the Yellowstone system in size, longevity, thermal flux, and magma output rate. However, Taupo contrasts with Yellowstone in the exceptionally high frequency, but small size, of caldera-forming eruptions. This contrast reflects the thin, rifted nature of the crust, which precludes the development of long-term magmatic cycles at Taupo. 11 refs., 4 figs., 1 tab.

Origin and evolution of a submarine large igneous province: the Kerguelen Plateau and Broken Ridge, southern Indian Ocean

Oceanic plateaus form by mantle processes distinct from those forming oceanic crust at divergent plate boundaries. Eleven drillsites into igneous basement of Kerguelen Plateau and Broken Ridge, including seven from the recent Ocean Drilling Program Leg 183 (1998–99) and four from Legs 119 and 120 (1987–88), show that the dominant rocks are basalts with geochemical characteristics distinct from those of mid-ocean ridge basalts. Moreover, the physical characteristics of the lava flows and the presence of wood fragments, charcoal, pollen, spores and seeds in the shallow water sediments overlying the igneous basement show that the growth rate of the plateau was sufficient to form subaerial landmasses. Most of the southern Kerguelen Plateau formed at ~110 Ma, but the uppermost submarine lavas in the northern Kerguelen Plateau erupted during Cenozoic time. These results are consistent with derivation of the plateau by partial melting of the Kerguelen plume. Leg 183 provided two new major observations about the final growth stages of the Kerguelen Plateau. 1: At several locations, volcanism ended with explosive eruptions of volatile-rich, felsic magmas; although the total volume of felsic volcanic rocks is poorly constrained, the explosive nature of the eruptions may have resulted in globally significant effects on climate and atmospheric chemistry during the late-stage, subaerial growth of the Kerguelen Plateau. 2: At one drillsite, clasts of garnet–biotite gneiss, a continental rock, occur in a fluvial conglomerate intercalated within basaltic flows. Previously, geochemical and geophysical evidence has been used to infer continental lithospheric components within this large igneous province. A continental geochemical signature in an oceanic setting may represent deeply recycled crust incorporated into the Kerguelen plume or continental fragments dispersed during initial formation of the Indian Ocean during breakup of Gondwana. The clasts of garnet–biotite gneiss are the first unequivocal evidence of continental crust in this oceanic plateau. We propose that during initial breakup between India and Antarctica, the spreading center jumped northwards transferring slivers of the continental Indian plate to oceanic portions of the Antarctic plate.

Rift relocation — A geochemical and geochronological investigation of a palaeo-rift in northwest Iceland

A dominant process in the evolution of Iceland is the repeated eastward relocation of the spreading axis in response to westward migration of the plate boundary relative to the plume centre. Two major former rifts can be identified in western Iceland: the Snaefellsnes rift zone, which last erupted tholeiitic lavas at about 7 Ma, and an older spreading system, lava flows from which can be traced some 100 km along a SW-NE strike in the extreme northwest of Iceland. The extinction of the latter is marked by a 14.9 Ma unconformity with a laterite-lignite horizon representing a maximum 200 k.y. hiatus in the lava succession. Lavas below the unconformity dip northwest towards the older axis from which they were erupted, whereas lavas above the unconformity dip southeast towards their source in the younger Snaefellsnes axis. Thus, two nearly complete rift relocation cycles are preserved in western Iceland, each lasting about 8 m.y. as measured between rift extinction events, and for around 12 m.y. from initial propagation to extinction. In this paper we present major- and trace-element analyses, Sr, Nd and Pb isotope data, and40Ar/39Ar dates on basalt samples from above and below the unconformity in northwest Iceland. The Icelandic Tertiary and Quaternary plateau basalts are remarkably homogeneous in composition, in contrast to the much more diverse compositions found in the presently active rift zone. However, basaltic lava flows beneath the unconformity in northwest Iceland show a wider range of incompatible element and radiogenic isotope ratios than do the younger plateau basalts. At least two mantle components, one depleted and the other less depleted with respect to bulk Earth, are required to explain the composition of post-15 Ma Icelandic basalt. The depleted end-member is chemically and isotopically distinct from the N-MORB source. Basalt from the northwest palaeo-rift, however, contains a significant North Atlantic N-MORB component, suggesting that depleted upper mantle can influence the composition of Icelandic basalt in a dying rift that is too far from the plume centre to be dominated by plume mantle. This may account for the periods of low magma productivity represented by troughs between the V-shaped ridges on the Reykjanes Ridge. We suggest that temporal variation in the composition of Icelandic basalt is better explained by crustal accretion and rift relocation processes than by variations in plume composition and temperature.

The longevity of the South Pacific isotopic and thermal anomaly

Abstract The South Pacific is anomalous in terms of the Sr, Nd, and Pb isotope ratios of its hot spot basalts, a thermally enhanced lithosphere, and possibly a hotter mantle. We have studied the Sr, Nd, and Pb isotope characteristics of 12 Cretaceous seamounts in the Magellans, Marshall and Wake seamount groups (western Pacific Ocean) that originated in this South Pacific Isotopic and Thermal Anomaly (SOPITA). The range and values of isotope ratios of the Cretaceous seamount data are similar to those of the island chains of Samoa, Tahiti, Marquesas and Cook/Austral in the SOPITA. These define two major mantle components suggesting that isotopically extreme lavas have been produced at SOPITA for at least 120 Ma. Shallow bathymetry, and weakened lithosphere beneath some of the seamounts studied suggests that at least some of the thermal effects prevailed during the Cretaceous as well. These data, in the context of published data, suggest: (1)|SOPITA is a long-lived feature, and enhanced heat transfer into the lithosphere and isotopically anomalous mantle appear to be an intrinsic characteristic of the anomaly. (2)|The less pronounced depth anomaly during northwesterly plate motion suggests that some of the expressions of SOPITA may be controlled by the direction of plate motion. Motion parallel to the alignment of SOPITA hot spots focusses the heat (and chemical input into the lithosphere) on a smaller cross section than oblique motion. (3)|The lithosphere in the eastern and central SOPITA appears to have lost its original depleted mantle characteristics, probably due to enhanced plume/lithosphere interaction, and it is dominated by isotopic compositions derived from plume materials. (4)|We speculate (following D.L. Anderson) that the origin of the SOPITA, and possibly the DUPAL anomaly is largely due to focussed subduction through long periods of the geological history of the earth, creating a heterogeneous distribution of recycled components in the lower mantle

Age and duration of the Matuyama-Brunhes geomagnetic polarity reversal from 40Ar39Ar incremental heating analyses of lavas

Abstract Constraints on the timing of geomagnetic polarity reversals have come mainly from KAr, or more recently 40 Ar 39 Ar , age determinations of lavas or their K-rich phenocrysts that erupted prior or subsequent to particular geomagnetic events. We have obtained 40 Ar 39 Ar isochron ages using incremental heating techniques on groundmass separates, phenocryst-poor whole rock samples, or plagioclase, from eight basaltic to andesitic lavas that erupted during the Matuyama-Brunhes (M-B) polarity transition at four geographically dispersed sites. These eight lavas range from 784.6 ± 7.1 ka to 770.8 ± 5.2 ka (1 σ errors); the weighted mean, 778.7 ± 1.9 ka, gives a high-precision age that is remarkably consistent with revised astronomical age estimates for the M-B polarity transition [6,12,13]. Despite uncertainties in absolute calibration of 40 Ar 39 Ar ages relative to mineral standards used as neutron fluence monitors, our age determinations are consistent with five other 40 Ar 39 Ar studies focused on the M-B transition. These results confirm that the earlier KAr based geomagnetic polarity time scale underestimated the age of the M-B reversal by about 6%. None of the eight isochron ages are distinguishable from one another at the 95% confidence level. However, we are tantalizingly close to testing for the duration of this reversal. One lava at the base of a sequence of transitionally magnetized flows in Chile and the uppermost lava in a similar sequence on Maui are only just indistinguishable in age at the 95% confidence level and preserve different magnetic orientations. We suggest that the ∼ 12 kyr difference in age represents an upper limit for the duration of the reversal and is similar to the period of low magnetic field intensity associated with records of the M-B reversal from deep sea sediment cores. Together with the short duration (∼ 2 kyr) of the directional reversal observed in several different marine sediment sections, our data suggest that reversal of the fields direction could have occurred at slightly different times depending on the position of the recording site.

Dating transitionally magnetized lavas of the late Matuyama chron : Toward a new 40Ar/39Ar timescale of reversals and events

The K-Ar based geomagnetic polarity timescale was constructed using data from lavas and tuffs that bracketed, but rarely dated, the transitions between polarity intervals. Subsequent 40Ar/39Ar dating indicated that the ages of some polarity transitions had been underestimated by about 6%. Although the accepted ages of the polarity chron boundaries have increased, their precise temporal definition remained uncertain. We have taken a different approach and used incremental-heating techniques to obtain 18 new 40Ar/39Ar ages from basaltic lavas within flow sequences at Punaruu Valley, Tahiti, and Haleakala volcano, Hawaii. These lavas record transitional paleomagnetic directions corresponding to four mid-Pleistocene polarity reversals or events. Three lavas from Punaruu Valley previously thought to record the Cobb Mountain Normal Polarity Subchron (CMNS) gave a mean age of 1.105 ± 0.005 Ma, indicating that they were erupted about 76 kyr after the CMNS; this period of transitional field behavior is designated the Punaruu event. In addition, seven new 40Ar/39Ar ages from the Punaruu Valley indicate that the Jaramillo Normal Polarity Subchron (JNS) lasted about 67 kyr, starting at 1.053 ± 0.006 Ma and ending 0.986 ± 0.005 Ma. This agrees with astronomical estimates but conflicts with JNS ages proposed by Spell and McDougall [1992] and Izett and Obradovich [1994] on the basis of 40Ar/39Ar dating of rhyolite domes in the Valles Caldera. Indistinguishable 40Ar/39Ar ages of seven lavas, including one from Punaruu Valley and six from Haleakala that record broadly similar intermediate paleodirections, suggest that the Kamikatsura event occurred at 0.886 ± 0.003 Ma. Moreover, these data indicate that the Kamikatsura event occurred 20–40 kyr after another geomagnetic event, most probably taking place at 0.92 Ma. We designate this earlier field behavior the Santa Rosa event, adopting its name from that of a transitionally magnetized rhyolite dome which happened to figure prominently in the original definition of the end of the JNS in the 1968 study of Doell et al. [1968]. The discovery of these new short-lived polarity events during the Matuyama reversed chron suggests that the 400 kyr period between 1.18 and 0.78 Ma experienced no less than 7 and perhaps more than 11 attempts by the geodynamo to reverse. This newly determined higher frequency of geomagnetic activity illustrates vividly the importance of obtaining precise age control directly from transitionally magnetized rocks.

Volcanism in the Sumisu Rift, I. Major element, volatile, and stable isotope geochemistry

A bimodal volcanic suite with KAr ages of 0.05–1.40 Ma was collected from the Sumisu Rift using alvin. These rocks are contemporaneous with island arc tholeiite lavas of the Izu-Ogasawara arc 20 km to the east, and provide a present day example of volcanism associated with arc rifting and back-arc basin initiation. Major element geochemistry of the basalts is most similar to that of basalts found in other, more mature back-arc basins, which indicates that back-arc basins need not begin their magmatic evolution with lavas bearing strong arc signatures. Volatile concentrations distinguish Sumisu Rift basalts from island arc basalts and MORB. H_2O contents, which are at least four times greater than in MORB, suppress plagioclase crystallization. This suppression results in a more mafic fractionating assemblage, which prevents Al_2O_3 depletion and delays the initiation of Fe_2O_3_((tot)) and TiO_2 enrichment. However, unlike arc basalts,Fe^(3+)/ΣFe ratios are only slightly higher than in MORB and are insufficient to cause magnetite saturation early enough to suppress Fe_2O_3_(tot) and TiO_2 enrichment. Thus, major element trends are more similar to those of MORB than arcs. H_2O, CO_2 and S are undersaturated relative to pure phase solubility curves, indicating exsolution of an H_2O-rich mixed gas phase. High H_2O/S, high δD, and low (MORB-like) δ^(34)S ratios are considered primary and distinctive of the back-arc basin setting.

Uplift of the western margin of the Andean plateau revealed from canyon incision history, southern Peru

We explore canyon incision history of the western margin of the Andean (Altiplano-Puna) plateau in the central Andes as a proxy for surface uplift. (U-Th)/He apatite data show rapid cooling beginning at ca. 9 Ma and continuing to ca. 5.1 Ma in response to incision. A minimum of 1.0 km of incision took place during that interval. The youngest apatite date and a volcanic fl ow perched 125 m above the present valley fl oor dated at 2.261 ± 0.046 Ma ( 40 Ar/ 39 Ar) show that an additional ~1.4 km of incision occurred between ca. 5.1 and 2.3 Ma. Thus, we infer that a total of at least 2.4 km, or 75% of the present canyon depth was incised after ca. 9 Ma. (U-Th)/He zircon data collected along the same transect imply that the western margin of the plateau was warped upward into its present monoclinal form, rather than uplift being accommodated on major surface-breaking faults.

Constraints on the exhumation and erosion of the High Himalayan Slab, NW India, from foreland basin deposits

Petrography, Sr–Nd isotope compositions and single-grain laser 40Ar–39Ar ages of detrital white mica from Early–Middle Miocene molasse of the Dharamsala and Lower Siwalik Formations of Northern India, dated by magnetostratigraphy, determine the sediment sources, their metamorphic grade and exhumation rates in the Himalayan palaeo-hinterland. Deposition of the Lower Dharamsala Member (21–17 Ma) occurred during the period of rapid isothermal decompression and crustal anatexis (24–18 Ma) of the metamorphic core. This episode of decompression is thought to be coeval with thrusting on the Main Central Thrust and normal faulting on the South Tibetan Detachment System. The sediment composition and detrital mica ages indicate erosion from the rapidly exhumed metamorphic slab of the High Himalayan Crystalline Series. Deposition of the Upper Dharamsala Member (17–13 Ma) and basal Siwalik Group (13–12.5 Ma) spanned the period in which thrusting transferred south from the Main Central Thrust. The sediment composition and detrital mica ages contrast strongly with those of the Lower Dharamsala, indicating erosion from sedimentary and low grade rocks. The isotopic composition indicates that these rocks were part of the High Himalayan Series unaffected by Tertiary metamorphism, i.e. from upper structural levels of the High Himalayan Slab. This suggests that a major reorganisation of the orogenic wedge occurred at 17 Ma involving forward propagation of the MCT and cessation of rapid exhumation of the metamorphic slab.

Short-lived and discontinuous intraplate volcanism in the South Pacific: Hot spots or extensional volcanism?

[1] South Pacific intraplate volcanoes have been active since the Early Cretaceous. Their HIMU-EMIEMII mantle sources can be traced back into the West Pacific Seamount Province (WPSP) using plate tectonic reconstructions, implying that these distinctive components are enduring features within the Earth’s mantle for, at least, the last 120 Myr. These correlations are eminent on the scale of the WPSP and the South Pacific Thermal and Isotopic Anomaly (SOPITA), but the evolution of single hot spots emerges notably more complicated. Hot spots in the WPSP and SOPITA mantle regions typically display intermittent volcanic activity, longevities shorter than 40 Myr, superposition of hot spot volcanism, and motion relative to other hot spots. In this review, we use 40 Ar/ 39 Ar seamount ages and Sr-Nd-Pb isotopic signatures to map out Cretaceous volcanism in the WPSP and to characterize its evolution with respect to the currently active hot spots in the SOPITA region. Our plate tectonic reconstructions indicate cessation of volcanism during the Cretaceous for the Typhoon and Japanese hot spots; whereas the currently active Samoan, Society, Pitcairn and Marquesas hot spots lack long-lived counterparts in the WPSP. These hot spots may have become active during the last 20 Myr only. The other WPSP seamount trails can be only ‘‘indirectly’’ reconciled with hot spots in the SOPITA region. Complex age distributions in the Magellan, Anewetak, Ralik and Ratak seamount trails would necessitate the superposition of multiple volcanic trails generated by the Macdonald, Rurutu and Rarotonga hot spots during the Cretaceous; whereas HIMU-type seamounts in the Southern Wake seamount trail would require 350–500 km of hot spot motion over the last 100 Myr following its origination along the Mangaia-Rurutu ‘‘hotline’’ in the Cook-Austral Islands. These observations, however, violate all assumptions of the classical Wilson-Morgan hot spot hypothesis, indicating that long-lived, deep and fixed mantle plumes cannot explain the intraplate volcanism of the South Pacific region. We argue that the observed short-lived and discontinuous intraplate volcanism has been produced by another type of hot spot-related volcanism, as opposed to the strong and continuous Hawaiian-type hot spots. Our results also indicate that other geological processes (plate tension, hotlines, faulting, wetspots, self-propagating volcanoes) may act in conjunction with hot spot volcanism in the South Pacific. In all these scenarios, intraplate volcanism has to be controlled by ‘‘broad-scale’’ events giving rise to multiple closely-spaced mantle plumelets, each with a distinct isotopic signature, but only briefly active and stable over geological time. It seems most likely that these plumelets originate and dissipate at very shallow mantle depths, where they may shoot off as thin plumes from the top of a ‘‘superplume’’ that is present in the South Pacific mantle. The absence of clear age progressions in most