The crustal evolution of the Chatham Rise: Mid-Cretaceous Hikurangi Plateau collision and breakup between Zealandia and Antarctica : Die Krustenentwicklung des Chatham Rise: Die Kollision mit dem Hikurangi Plateau und der Aufbruch zwischen dem Zealandia-Kontinent und der Antarktis in der mittleren Kreidezeit
The breakup of supercontinents is often associated with the changing polarity of tectonic forces from lithospheric convergence to lithospheric divergence. The initiation of the last supercontinent disintegration occurred simultaneously with the breakup of Gondwana. During the mid-Cretaceous, the Eas...
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| Format: | Thesis |
| Language: | English |
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Universität Bremen
2020
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| Online Access: | https://dx.doi.org/10.26092/elib/102 https://media.suub.uni-bremen.de/handle/elib/4317 |
| Summary: | The breakup of supercontinents is often associated with the changing polarity of tectonic forces from lithospheric convergence to lithospheric divergence. The initiation of the last supercontinent disintegration occurred simultaneously with the breakup of Gondwana. During the mid-Cretaceous, the East Gondwana margin underwent a remarkably fast transformation from a long-lived active subduction margin to a passive continental rifted margin, which led to the separation of southern Zealandia from West Antarctica. Recent studies suggest that the cessation of subduction and onset of extension in southern Zealandia was initiated by the collision and subduction of the thick oceanic Hikurangi Plateau with the East Gondwana subduction zone. However, little is known about the crustal structure of the Chatham Rise, east off New Zealand, although the Chatham Rise played a central role in change in tectonic forces. In particular, the nature of the southern Chatham Rise margin and the SE Chatham Terrace, an area of anomalously shallow seafloor hosting abundant seamounts and guyots, is poorly constrained. To investigate the role of the Hikurangi Plateau collision and subduction on the onset of extension and rifting in southern Zealandia, geophysical data including wide-angle reflection and refraction seismic, multi-channel seismic reflection, and potential field data were acquired during RV Sonne cruise SO246 in 2016. Geophysical data were collected along four profiles across two sub-provinces of the Chatham Rise, the SE Chatham Terrace and adjacent oceanic crust. P-wave velocity and gravity modelling of the new geophysical data yield insights into the crustal structure and therefore the breakup mechanism of the southern Chatham Rise margin, constrain the extent of the Hikurangi Plateau underthrusted beneath the Chatham Rise, and enhance our understanding of the driving forces behind the abrupt change from subduction to rifting along the East Gondwana margin. Along the Chatham Rise, the P-wave velocity models highlight distinct differences in the crustal thickness between the eastern and western sub-provinces, but also reveal common characteristics in crustal composition. The crust of the western Chatham Rise is up to 25 km thick, whereas the eastern Chatham Rise is substantially thinner (14-18 km). Modelled P-wave velocities and densities suggest a similar geology for both parts. The Chatham Rise mainly consists of greywackes, meta-greywackes, and schist in the upper crust, and their high-temperature equivalents in the lower crust. This is consistent with its past position at an active continental margin. Seismic imaging and gravity data show that the 10-16 km thick Hikurangi Plateau is restricted to the lower crust of the western Chatham Rise. The geophysical data suggest that the Hikurangi Plateau does not reach as far south as previously proposed. Furthermore, southward thinning of the lower crustal layer along the westernmost profile, together with previously published data, indicates that a piece of the subducted oceanic Phoenix Plate is still present below the Chatham Rise and southern Zealandia. The crustal thickness of the SE Chatham Terrace varies between 5 and 8 km, which can be correlated to slightly thinner or thicker than Pacific oceanic crust (~6 km thickness). The velocity structure can be interpreted as similar to Pacific oceanic crust, but at the same time also shows characteristics of hyper-extended continental crust. Since graben structures are present, I interpret the SE Chatham Terrace as a broad continent-ocean transition zone, which consists of very thin continental crust modified by magmatic activity. Typical Pacific oceanic crust has been only found close to the easternmost Chatham Rise and is presumably not older than 88 Ma. The Pacific oceanic crust is separated from the Chatham Rise by a highly faulted area, which I interpret as exhumed lower continental crust. High-velocity lower crust (VP > 7 km/s) has been identified along the eastern Chatham Rise and at the easternmost Chatham Rise. These two areas of high-velocity lower crust are interpreted as magmatic underplating and intrusions. Seaward-dipping reflector sequences typical of volcanic-rifted margins are completely absent along the southern Chatham Rise margin. Moreover, the southern Chatham Rise margin is largely fault-controlled, but the geophysical data do not support the presence of exhumed and serpentinised upper mantle, which is typical for magma-poor margins. On this basis, I interpret the southern Chatham Rise margin as a unique hybrid rifted margin, which shows features typical of both, volcanic-rifted and magma-poor margins. Based on these observations, I developed a tectonic model that explains the multi-stage tectonic evolution of the southern Chatham Rise margin. Accordingly, the Hikurangi Plateau entered the subduction zone at ~110 Ma. Subsequently, convergence velocities slowed down until subduction ceased at ~100 Ma. The thicker crust of the western Chatham Rise is a result of the subduction and underthrusting of the Hikurangi Plateau, which most likely attenuated subsequent crustal extension along the western Chatham Rise. Slowing subduction in the sector of the Hikurangi Plateau led to development of subduction-transform edge propagator (STEP) faults on both sides of the plateau after 110 Ma. I suggest that the Hikurangi Plateau collision, together with fragmentation of the Phoenix Plate by these STEP faults, triggered and / or contributed to the previously hypothesized global-scale plate reorganisation event between 105 and 100 Ma. At the same time, rifting and crustal extension in southern Zealandia started. The rifting in southern Zealandia and the evolution of the southern Chatham Rise were likely the result of complex slab dynamics triggered by the Hikurangi Plateau subduction. First, rifting was initiated by shallowing of the subducted slab due to the higher buoyancy of the young and thick Hikurangi Plateau. Initial extension was oblique to the margin and arc, and led to the reactivation of former arc-parallel E-W thrust faults as normal faults. With prolonged extension, new generations of NE-SW normal faults started to form, and lower crust was exhumed along the easternmost tip of the Chatham Rise was initiated. Secondly, progressive eclogitisation of the land-ward Phoenix Plate slab is likely to have caused the slab to rollback after convergence ceased. This led to a prolonged episode of rifting during which extension focussed on the southern Chatham Rise margin (i.e. the SE Chatham Terrace and Bounty Trough). Finally, the style of extension changed after most of the Phoenix Plate slab became detached at around 90 Ma. The slab detachment opened a pathway for deep-seated and hot upwelling mantle, which resulted in (I) intrusions and magmatic underplating, (II) formation of the first oceanic crust along the easternmost tip of the Chatham Rise, (III) alkaline magmatism on the Chatham Island between 85 and 82 Ma and (IV) magmatic overprint of the SE Chatham Terrace leading to seamount formation. After 85 Ma, spreading segments became connected and the formation of the young Pacific-Antarctic Ridge led to the final separation of Zealandia from Antarctica. |
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