Roger L. Larson

Geological consequences of superplumes

Superplumes are suggested to have caused the period of constant normal magnetic polarity in mid-Cretaceous time (124-83 Ma) and, possibly, the period of constant reversed polarity in Pennsylvania-Permian time (323-248 Ma). These times coincide with increases in world temperature, deposition of black shales, oil generation, and eustatic sea level in the mid-Cretaceous, and increased coal generation and gas accumulation in the Pennsylvanian-Permian, accompanied by an intracratonic Pennsylvanian transgression of epicontinental seas. These geologic anomalies are associated with episodes of increased world-wide ocean-crust production and mantle outgassing, especially of carbon and nutrients. These superplumes originated just above the core-mantle boundary, significantly increased convection in the outer core, and stopped the magnetic field reversal process for 41 m.y. in the Cretaceous and 75 m.y. in Pennsylvanian-Permian time.

Plate tectonic reconstructions of the Cretaceous and Cenozoic ocean basins

Abstract In this paper we present nine reconstructions for the Mesozoic and Cenozoic, based on previously published sea-floor spreading isochrons ∗ . The purpose of this study was 1. (1) to determine if the isochrons could be refitted to produce accurate plate tectonic reconstructions 2. (2) to identify areas of apparent mismatch between magnetic isochrons as a focus for further investigations, and 3. (3) to test the capabilities and accuracy of interactive computer graphic methods of plate tectonic reconstruction. In general, Tertiary and Late Cretaceous isochrons could be refitted with little overlap and few gaps; however, closure errors were apparent in the vicinity of the Bouvet and Macquarie triple junctions. It was not possible to produce Early Cretaceous reconstructions that were consistent with the previously published isochrons. In this paper we also propose that the Late Cretaceous and Early Tertiary plate reorganizations observed in the Indian Ocean were the result of the progressive subduction of an intra-Tethyan rift system.

World-Wide Correlation of Mesozoic Magnetic Anomalies, and Its Implications

In the course of correlating three sets of Mesozoic magnetic lineations in the western Pacific (the Phoenix, Japanese, and Hawaiian lineations), Larson and Chase (1972) determined a paleomagnetic pole for the Pacific plate for the Early Cretaceous. Using this pole we have derived a magnetic reversal model for the Hawaiian lineation set. We then have used this model to correlate the entire Hawaiian lineation set to the entire Keathley lineation set in the western North Atlantic. On the basis of these correlations and drill holes associated with the lineation patterns, we have extended the geomagnetic reversal time scale back to the base of the Late Jurassic (162 m.y. B.P.). A period of reversals occurred corresponding to the Hawaiian and Keathley lineations from 150 o t 110 m.y. B.P., and these reversals are bracketed by long periods of dominantly normal polarity (the Cretaceous and Jurassic magnetic quiet zones). This magnetic reversal time scale significantly alters previous notions of the timing and origin of sea-floor spreading features in the Atlantic Ocean. It implies that the Bay of Biscay opened sometime during the interval between 150 and 110 m.y. B.P.; that drift in the South Atlantic was initiated at sometime during the interval from 110 to 85 m.y. B.P. (probably close to 110 m.y. B.P.); and that the seaward portion of the marginal quiet zones of the eastern United States and northwestern Africa resulted from sea-floor spreading during the Late Jurassic period of dominantly normal magnetic polarity prior to 150 m.y. B.P. In the Pacific during the late Mesozoic, spreading was occurring from at least five spreading centers joined at two triple points. The vast majority of the Pacific Basin today is occupied by only the Pacific-plate side of these spreading patterns. This implies that an area equal to most of the Pacific Basin has been subducted beneath the surrounding continents since the Early Cretaceous. Our magnetic reversal time scale calls for a rapid pulse of spreading from about 110 to 85 m.y. B.P. at all the spreading centers in both the Atlantic and Pacific Oceans. This implies a pulse of rapid subduction around the rim of the Pacific that we relate to episodes of large-scale plutonism in eastern Asia, western Antarctica, New Zealand, the southern Andes, and western North America during the Late Cretaceous.

Onset of the Mid‐Cretaceous greenhouse in the Barremian‐Aptian: Igneous events and the biological, sedimentary, and geochemical responses

Basalts and biostratigraphy dated at 125–120 Ma from the Ontong Java and Manihiki Plateaus in the western Pacific evidence the largest volcanic event in Earth history in at least the past 160 m.y. The intervening Nova-Canton Trough rifted at about 121–118 Ma, and a number of guyots and seamounts formed concurrently or slightly later. Geological events that probably were responses to these volcanic/tectonic events occurred in the following chronostratigraphic order. Biotic fluctuations began at about 122.5 Ma. At about 122.0 Ma, 87Sr/86Sr began to decline slowly. Metal concentrations of Co, Mn, Pb, Yb, and Cu in sediments peaked at about 121.5–121.2 Ma. Changes in planktonic communities and sedimentation culminated in a nannoconid “crisis” just prior to 120.5 Ma and in the Selli black shale (OAE la) at about 120.5–119.5 Ma. A sharp drop in δ13C occurred at the beginning of the Selli event and rebounded into a longer positive excursion that reached a peak after the Selli event at about 119.5–118.5 Ma. At 120.5 Ma, 87Sr/86Sr declined rapidly and reached a minimum at about 116–113 Ma. We speculate that the intensity of these latter responses suggests a corresponding peak in volcanic/tectonic activity at about 121–119 Ma.

Late Mesozoic Evolution of the Western Pacific Ocean

A set of east-trending magnetic anomalies located in the western equatorial Pacific Ocean near the Phoenix Islands is Early Cretaceous in age. The use of magnetic reversal model studies shows that this lineated anomaly pattern correlates with one east of Japan that trends east, and with one west of Hawaii that trends northwest. These patterns were formed in their present relative positions, but about 40° (4,500 km) south of their present geographic locations. The configuration of these three contemporaneous sets of magnetic anomalies implies that the Late Mesozoic tectonic pattern consisted of five spreading centers joined at two triple points. In this interpretation, the oldest part of the Pacific Ocean lies just east of the Mariana Trench and is Early Jurassic in age. This Mesozoic system evolved into the Cenozoic spreading pattern recorded in the eastern Pacific Ocean. The details of this transition are open to speculation because it occurred during a period in the Late Cretaceous that lacked magnetic reversals. We propose a model that suggests the northern triple point jumped southeast about 2,000 km at 100 m.y. B.P., and that the Emperor Trough was a transform fault of large offset during the Late Cretaceous. The southern triple point migrated rapidly toward the south-southeast, approximately parallel to the Eltanin Fracture Zone–Louisville Ridge complex that we extend o t the westernmost of the Phoenix lineation fracture zones.

Mantle plumes control magnetic reversal frequency

Magnetic reversal frequency correlates inversely with mantle plume activity for the past 150 Ma, as measured by the volume production rate of oceanic plateaus, seamount chains, and continental flood basalts. This inverse correlation is especially striking during the long Cretaceous magnetic normal “superchron”, when mantle plume activity was at a maximum. We suggest that mantle plumes control magnetic reversal frequency by the following sequence of events. Mantle plumes rise from theD″ seismic layer just above the core/mantle boundary, thinningD″ to fuel the plumes. This increases core cooling by allowing heat to be conducted more rapidly across the core/mantle boundary. Outer core convective activity then increases to restore the abnormal heat loss, causing a decrease in magnetic reversal frequency in accord with model predictions for bothα2 andαω dynamos. When core convective activity increases above a critical level, a magnetic superchron results. The pulse of plume activity that caused the Cretaceous superchron resulted in a minimum increase in core heat loss of about 1200 GW over the present-day level, which corresponds to an increase in Joule heat production of about 120 GW within the core.

Valanginian Weissert oceanic anoxic event

Biotic changes in nannofossils and radiolarians associated with the Valanginian δ 1 3 C anomaly are documented at Ocean Drilling Program Hole 1149B in the Pacific Ocean: they are coeval and similar to those previously documented in the Tethys, suggesting a global perturbation of marine ecosystems. A marked increase in abundance of Diazomatolithus, absence of nannoconids, and a Pantanellium peak characterize the Valanginian δ 1 3 C excursion. Such changes are interpreted as being due to global enhanced fertility and a biocalcification crisis under conditions of excess CO 2 . The occurrence of organic C-rich black shales in the Southern Alps and in the Pacific in the interval corresponding to the δ 1 3 C excursion suggests a Valanginian oceanic anoxic event (OAE). Volcanism of the Parana-Etendeka large igneous province (ca. 132 Ma) was presumably responsible for an increase of CO 2 , triggering a climate change and accelerated hydrological cycling, possibly causing an indirect fertilization of the oceans. Widespread nutrification via introduction of biolimiting metals at spreading ridges could have significantly increased during the Gondwana breakup and simultaneous tectonic events in three separate oceans. There is no paleontological or δ 1 8 O evidence of warming during the Valanginian OAE. On the contrary, both nannofossils and oxygen isotopes record a cooling event at the climax of the δ 1 3 C excursion. Weathering of basalts and burial of organic C-rich black shales were presumably responsible for CO 2 drawdown and establishment of reversed greenhouse conditions.

Magnetic lineations in the Pacific Jurassic quiet zone

Magnetic anomalies of low amplitude (<100 gammas) are present in the Jurassic magnetic quiet zone of the western Pacific Ocean. These small anomalies are lineated and can be correlated among the Phoenix, Hawaiian and Japanese lineation patterns. Thus, they represent seafloor spreading that recorded some sort of magnetic field phenomena prior to magnetic anomaly M25 at 153 m.y. B.P. The most likely possibility is that they represent a series of late Jurassic magnetic field reversals that occurred during a period of anomalously low magnetic field intensity. We propose a time scale of magnetic reversals between 153 and 158 m.y. B.P. to account for these anomalies and suggest that the dipole magnetic field intensity increased by a factor of about four from 160 to 140 m.y. B.P. in the late Jurassic.

Pacific microplate and the Pangea supercontinent in the Early to Middle Jurassic

New biostratigraphic data based on radiolarians recovered from deep within the oceanic crustal section of Ocean Drilling Program Hole 801C in the western Pacific, along with existing radiometric information, date this oceanic crust as late Bajocian–early Bathonian (170–165 Ma). The overlying basal sediments at Hole 801C are essentially identical in age (middle Bathonian, 164–162 Ma) to the basal sediments at Deep Sea Drilling Program Hole 534A in the central Atlantic. We estimate the time of formation of the Pacific plate as 175–170 Ma and the time of initial separation of the Pangea supercontinent in the central Atlantic as 190–180 Ma. We also identify a time of extensive subduction-zone magmatism (175–159 Ma) at the eastern and western edges of Pangea. We suggest that the initial plate separation of Pangea increased subduction rates at its outer margins and altered the plate boundaries in the Pacific superocean, leading to formation of the Pacific plate.

Bathymetry, Magnetic Anomalies, and Plate Tectonic History of the Mouth of the Gulf of California

Bathymetric and magnetic anomaly profiles show that the East Pacific Rise crest and both the Tamayo and Rivera Fracture Zones presently define the boundary of the Pacific and North American plates at the mouth of the Gulf of California. New oceanic crust is forming at the rise crest and transform fault slip is occurring along the fracture zones at the rate of 6.0 cm/yr. The Middle America Trench north of the Rivera Fracture Zone became nearly dormant as a plate boundary 8 to 10 m.y. ago when the rise crest changed from the Pacific-Farallon to the Pacific-Rivera plate boundary. Two m.y. ago, this section of the trench ceased crustal subduction as the Rivera plate became attached to the North American plate and the rise crest became the Pacific-North American plate boundary. Baja California was dislocated from the North American plate 4 m.y. ago and began rafting away to the northwest at a velocity that has not been significantly altered since the initial breakaway. For the past 2 m.y., a coherent pattern of spreading has been recorded north of the Tamayo Fracture Zone.