Brad Field

Reactivation of tectonics, crustal underplating, and uplift after 60 Myr of passive subsidence, Raukumara Basin, Hikurangi-Kermadec fore arc, New Zealand: implications for global growth and recycling of continents

We use seismic reflection and refraction data to determine crustal structure, to map a fore-arc basin containing 12 km of sediment, and to image the subduction thrust at 35 km depth. Seismic reflection megasequences within the basin are correlated with onshore geology: megasequence X, Late Cretaceous and Paleogene marine passive margin sediments; megasequence Y, a similar to 10,000 km(3) submarine landslide emplaced during subduction initiation at 22 Ma; and megasequence Z, a Neogene subduction margin megasequence. The Moho lies at 17 km beneath the basin center and at 35 km at the southern margin. Beneath the western basin margin, we interpret reflective units as deformed Gondwana fore-arc sediment that was thrust in Cretaceous time over oceanic crust 7 km thick. Raukumara Basin has normal faults at its western margin and is uplifted along its eastern and southern margins. Raukumara Basin represents a rigid fore-arc block > 150 km long, which contrasts with widespread faulting and large Neogene vertical axis rotations farther south. Taper of the western edge of allochthonous unit Y and westward thickening and downlap of immediately overlying strata suggest westward or northwestward paleoslope and emplacement direction rather than southwestward, as proposed for the correlative onshore allochthon. Spatial correlation between rock uplift of the eastern and southern basin margins with the intersection between Moho and subduction thrust leads us to suggest that crustal underplating is modulated by fore-arc crustal thickness. The trench slope has many small extensional faults and lacks coherent internal reflections, suggesting collapse of indurated rock, rather than accretion of > 1 km of sediment from the downgoing plate. The lack of volcanic intrusion east of the active arc, and stratigraphic evidence for the broadening of East Cape Ridge with time, suggests net fore-arc accretion since 22 Ma. We propose a cyclical fore-arc kinematic: rock moves down a subduction channel to near the base of the crust, where underplating drives rock uplift, oversteepens the trench slope, and causes collapse toward the trench and subduction channel. Cyclical rock particle paths led to persistent trench slope subsidence during net accretion. Existing global estimates of fore-arc loss are systematically too high because they assume vertical particle paths. Citation: Sutherland, R., et al. (2009), Reactivation of tectonics, crustal underplating, and uplift after 60 Myr of passive subsidence, Raukumara Basin, Hikurangi-Kermadec fore arc, New Zealand: Implications for global growth and recycling of continents, Tectonics, 28, TC5017, doi: 10.1029/2008TC002356.

Antarctic ice sheet sensitivity to atmospheric CO2 variations in the early to mid-Miocene

Significance New information from the ANDRILL-2A drill core and a complementary ice sheet modeling study show that polar climate and Antarctic ice sheet (AIS) margins were highly dynamic during the early to mid-Miocene. Changes in extent of the AIS inferred by these studies suggest that high southern latitudes were sensitive to relatively small changes in atmospheric CO2 (between 280 and 500 ppm). Importantly, reconstructions through intervals of peak warmth indicate that the AIS retreated beyond its terrestrial margin under atmospheric CO2 conditions that were similar to those projected for the coming centuries. Geological records from the Antarctic margin offer direct evidence of environmental variability at high southern latitudes and provide insight regarding ice sheet sensitivity to past climate change. The early to mid-Miocene (23–14 Mya) is a compelling interval to study as global temperatures and atmospheric CO2 concentrations were similar to those projected for coming centuries. Importantly, this time interval includes the Miocene Climatic Optimum, a period of global warmth during which average surface temperatures were 3–4 °C higher than today. Miocene sediments in the ANDRILL-2A drill core from the Western Ross Sea, Antarctica, indicate that the Antarctic ice sheet (AIS) was highly variable through this key time interval. A multiproxy dataset derived from the core identifies four distinct environmental motifs based on changes in sedimentary facies, fossil assemblages, geochemistry, and paleotemperature. Four major disconformities in the drill core coincide with regional seismic discontinuities and reflect transient expansion of grounded ice across the Ross Sea. They correlate with major positive shifts in benthic oxygen isotope records and generally coincide with intervals when atmospheric CO2 concentrations were at or below preindustrial levels (∼280 ppm). Five intervals reflect ice sheet minima and air temperatures warm enough for substantial ice mass loss during episodes of high (∼500 ppm) atmospheric CO2. These new drill core data and associated ice sheet modeling experiments indicate that polar climate and the AIS were highly sensitive to relatively small changes in atmospheric CO2 during the early to mid-Miocene.

Early Miocene thin‐skinned tectonics and wrench faulting in the Pongaroa district, Hikurangi margin, North Island, New Zealand

Abstract The Pongaroa‐Akitio area, Northern Wairarapa, North Island, New Zealand, is part of the exposed East Coast Deformed Belt at the obliquely convergent plate boundary of the Hikurangi margin. The sedimentary succession includes an allochthonous unit of Early Cretaceous greywacke basement resting on latest Cretaceous rocks. Since the units basal contact is subparallel to the bedding of the strata it overlies, the allochthon is inferred to be an unrooted gliding nappe similar to allochthonous outliers described in Northland and the Raukumara Peninsula. The southward emplacement of this “Greywacke Nappe” is supported by structural markers in the body of the nappe and is well dated as earliest Miocene by the youngest rocks involved, which are earliest Miocene (Waitakian; c. Aquitanian), and because Otaian‐Altonian (c. Burdigalian) faults postdate nappe emplacement. This thin‐skinned tectonic phase immediately preceded inception of dextral strike‐slip faulting along northeast‐trending Otaian‐Altonian (B...

Paleoenvironmental changes across the Cretaceous/Tertiary boundary in the northern Clarence valley, southeastern Marlborough, New Zealand

Abstract Strata outcropping in Mead and Branch Streams, northern Clarence valley, provide important records of pelagic‐hemipelagic sedimentation through the Cretaceous‐Paleocene transition in a southern high‐latitude, upwelling system flanking a carbonate platform. The two stream sections, <10 km apart along‐strike, comprise similar stratigraphic successions with differences mainly due to Branch Stream being situated closer to land (outer shelf to upper bathyal) than the mid‐bathyal Mead Stream section. Age control is based on foraminiferal and radiolarian biostratigraphy. A Cretaceous/Tertiary (K/T) boundary clay is not preserved in either section. The siliceous microfossil record indicates that basal Paleocene sediments at Branch Stream, although slightly enriched in Ni and Cr, were deposited after a significant relative sea‐level fall. A basal Paleocene claystone at Mead Stream is not enriched in K/T impact‐derived elements and was probably deposited after sea‐level fall. Earliest Paleocene sediment may be preserved as burrow fill in an uppermost Cretaceous bioturbated zone, which is enriched in Ni and Cr and contains a foraminiferal assemblage indicative of Paleocene Zone P0. Zone PO‐Pα foraminiferal assemblages within the basal Paleocene sediments in both sections indicate that sea level fell within 100 000 yr of the K/T boundary event. The K/T boundary at both sites coincides with an abrupt change in lithofacies from calcareous to siliceous ooze. Biosiliceous sediment dominates the sedimentary record over the first 1.5 m.y. of the Paleocene, which corresponds to 45 m of strata at Branch Stream and 20 m in the more condensed Mead Stream section. A trend from diatom‐poor to diatom‐rich and finally radiolarian‐rich microfossil assemblages over the lower 5 m of Paleocene strata at Branch Stream is consistent with progressive deepening at the landward margin of an upwelling zone. A second regressive pulse at c. 64.5 Ma, followed by prolonged transgression from 5–50 m above the K/T boundary, is inferred from an initial increase in the frequency of mudstone beds, followed by a similar trend from diatom‐poor to radiolarian‐rich microfossil assemblages. Within the upper part of this interval, an increase in carbonate marks a return to lithofacies, and probably also paleodepth, equivalent to the underlying Cretaceous. In the deeper Mead Stream section, variation in diatom and radiolarian assemblages is mainly due to variable preservation in highly recrystallised lithologies. High overall abundance and little change in paleoproductivity indicators (Ba, δ13C) indicate that high biological productivity continued across the K/T boundary and through the biosiliceous episode. Siliceous plankton thrived in the Marlborough upwelling zone during the early Paleocene. Fluctuations in abundance and lithofacies can be related to significant changes in sea level, which may be the result of local tectonic or global climate changes. The delayed recovery of calcareous plankton after mass extinction at the K/T boundary, in both outer neritic and bathyal settings, indicates a relatively cool oceanic regime for the first 1.5 m.y. of the Paleocene.

Foraminiferal and carbon isotope stratigraphy through the Paleocene‐Eocene transition at Dee Stream, Marlborough, New Zealand

Abstract Dee Stream in the Clarence River valley of New Zealand bisects a well‐exposed section of marine sedimentary rocks deposited in the early Paleogene at high southern latitudes. One hundred metres of strata lying within this section and comprising cm‐dm well‐bedded, siliceous limestone with marly partings was mapped, logged, and sampled to establish a detailed foraminiferal and carbon isotope stratigraphy and to examine environmental changes across the Paleocene‐Eocene thermal maximum (PETM). Although low abundance and poor preservation of planktic and benthic foraminifera characterises much of the Paleocene, foraminifera and carbon isotopes clearly show that the section spans the upper Paleocene to lower Eocene planktic foraminiferal zones from Zone P4 to Subzone P6b, and the Subbotina triloculinoides to Pseudohastigerina wilcoxensis Zones. The δ13 C record correlates closely to other δ13 C curves generated from other key early Paleogene carbonate sequences. The Dee Stream logged section contains a1m thick PETM interval at 26.5 m at the base of Zone P5, or the Morozovella velascoensis Subzone. Here, benthic foraminifera undergo significant extinction, Morozovella aequa makes its first appearance, and the δ13 C of carbonate decreases by 2‰. The benthic foraminifer Bulimina tuxpamensis dominates benthic assemblages immediately following the onset of the PETM interval, suggesting dysoxic bottom waters during this event. In conjunction with other recently examined sections from the Marlborough region, the thick and apparently continuous Paleogene record at Dee Stream provides an important site for understanding environmental change on high‐latitude continental margins during the Paleogene, including the PETM.

Olistostromes marking tectonic events, East Coast, New Zealand

Abstract Three discrete, successive tectonic events are marked by olistostromes in the East Coast, North Island. An Eocene Mataikona event is characterised by in situ dismembered beds. A second, Owahanga event is Otaian and preceded wrench faulting coeval with the start of oblique subduction at the East Coast margin. This second event is characterised by extended emplacement of exotic material followed by gliding, gravity‐driven nappes that are probably related to obduction along the north coast of the North Island. The third olistostrome is attributed to an early Miocene Mara event synchronous with incipient strike‐slip faulting during Alton‐ian times and the commencement of oblique convergence at the Hikurangi margin. From the ages and paleoenvironments of the olistostromes, we propose the Eocene and earliest Miocene syn‐sedimentary structures were dragged along the deforming backstop of the Hikurangi prism.

The geological setting of the Darfield and Christchurch earthquakes

Abstract The 2010–2011 Canterbury earthquake sequence occurred near the southeastern margin of Neogene deformation associated with the Australia–Pacific plate boundary. Basement comprises indurated rocks of the Torlesse Composite Terrane, of Permian to Early Cretaceous age, overlain by 1–2 km of less-indurated Cretaceous–Neogene rocks and unconsolidated Quaternary sediments. Proximity to the subduction interface between Gondwana and the paleo-Pacific Ocean produced a Mesozoic-age structural grain in the basement rocks, aligned broadly east–west in the Canterbury to Chatham Rise areas. These structures provided an inherited weakness that was likely reactivated by present-day stress. Mid- to Late Cretaceous extension, marked by localised fault-bounded grabens, was followed by deposition of a Late Cretaceous to Paleogene passive-margin transgressive sedimentary sheet and minor intraplate basaltic volcanics. Mid-Cenozoic inception of the modern Australia–Pacific plate boundary heralded deposition of a regressive succession of Neogene sediments and further episodes of volcanism, most notably constructing the Late Miocene Banks Peninsula intraplate volcanoes. The east- to northeast-striking faults associated with the Darfield and Christchurch earthquakes are probably aligned with the Mesozoic structural grain within the Torlesse basement rocks.

Textural variations in Neogene pelagic carbonate ooze at DSDP Site 593, southern Tasman Sea, and their paleoceanographic implications

Abstract Changes in Neogene sediment texture in pelagic carbonate‐rich oozes on the Challenger Plateau, southern Tasman Sea, are used to infer changes in depositional paleocurrent velocities. The most obvious record of textural change is in the mud: sand ratio. Increases in the sand content are inferred to indicate a general up‐core trend towards increasing winnowing of sediments resulting from increasing flow velocity of Southern Component Intermediate Water (SCIW), the forerunner of Antarctic Intermediate Water. In particular, the intervals c. 19–14.5 Ma, c. 9.5–8 Ma, and after 5 Ma are suggested to be times of increased SCIW velocity and strong sediment winnowing. Within the mud fraction, the fine silt to coarse clay sizes from 15.6 to 2 μm make the greatest contribution to the sediments and are composed of nannofossil plates. During extreme winnowing events it is the fine silt to very coarse clay material (13–3 μm) within this range that is preferentially removed, suggesting the 10 μm cohesive silt boundary reported for siliciclastic sediments does not apply to calcitic skeletal grains. The winnowed sediment comprises coccolithophore placoliths and spheres, represented by a mode at 4–7 μm. Further support for seafloor winnowing is gained from the presence in Hole 593 of a condensed sedimentary section from c. 18 to 14 Ma where the sand content increases to c. 20% of the bulk sample. Associated with the condensed section is a 6 m thick orange unit representing sediments subjected to particularly oxygen‐rich, late early to early middle Miocene SCIW. Together these are inferred to indicate increased SCIW velocity resulting in winnowed sediment associated with faster arrival of oxygen‐rich surface water subducted to form SCIW. Glacial development of Antarctica has been recorded from many deep‐sea sites, with extreme glacials providing the mechanism to increase watermass flow. Miocene glacial zones Mi1b‐Mi6 are identified in an associated oxygen isotope record from Hole 593, and correspond with times of particularly invigorated paleocirculation, bottom winnowing, and sediment textural changes.

Orbitally controlled cyclicity around the Cretaceous/Tertiary boundary, northern South Island, New Zealand

Abstract Outcrop gamma ray measurements are used as a proxy for terrigenous clastic component in Cretaceous‐Paleocene carbonate‐chert successions in northern South Island, New Zealand, and show Milankovitch‐scale periodicity above the Cretaceous/Tertiary (K/T) boundary. Higher baseline values of the gamma ray records from sections in the south compared to those in the north reflect increased terrigenous clay content associated with shallowing in paleobathymetry towards the south. In each northern section, the gamma ray baseline level increases across the K/T boundary, consistent with an increase in terrigenous clay in the early Paleocene; this might record increased erosion due to destruction or change of vegetation or a fall in relative sea level. Peak amplitudes in the gamma ray record are higher for the early Paleocene, perhaps recording more pronounced climatic variability after the K/ T boundary event, and consistent with a South Atlantic record. Cycle frequencies in the Late Cretaceous are not well constrained due to the sparseness of biostratigraphic events but could be consistent with eccentricity. Biostratigraphic age control in the Paleocene constrains cycle frequency to c. 120 000 yr and suggests the early Paleocene climate in the high latitude, southwest Pacific, was modulated by orbital eccentricity.

Middle Miocene paleoclimate change at Bryce Burn, southern New Zealand.

Abstract New Zealands isolated position in the Southwest Pacific and the quality of its marine and terrestrial sedimentary record make it a valuable source of information for studies of global climate change. A section at Bryce Burn in Southland, South Island, provides a record of middle Miocene global cooling associated with buildup of an extensive semi‐permanent ice sheet on Antarctica. The section yielded δ18O, δ13C, lithological, clay mineral, foraminiferal, and pollen data that have been linked to a high‐resolution age model based on magnetostratigraphy and foraminiferal dating. The sequence exhibits evidence for a δ18O c. 0.6‰ positive baseline shift consistent with the age and magnitude of the E3/ Mi3 isotope excursion around 13.9 Ma, carbon maxima 4–6 δ13C excursions, probable orbitally controlled changes in clay mineralogy, and short‐term changes in terrestrial vegetation. This is the first time these stable isotopic events have been recognised in outcrop sequences in New Zealand. There appears to be no clear, long‐term shift in the data associated with middle Miocene oceanic cooling, apart from the stable isotope evidence, though Milankovitch‐scale, short‐term fluctuations are present.