Basin Research - Volume 10, Issue 4, 1998

The tectonic subsidence and gravity anomalies in the Malay and Penyu Basins, offshore Peninsular Malaysia, were analysed to determine the isostatic compensation mechanism in order to investigate their origin. These continental extensional basins contain up to 14 km of sediment fill which implies that the crust had been thinned significantly during basin development. Our results suggest, however, that the tectonic subsidence in the basins cannot be explained simply by crustal thinning and Airy isostatic compensation.

The Malay and Penyu Basins are characterized by broad negative free‐air gravity anomalies of between −20 and −30 mGal. To determine the cause of the anomaly, we modelled four gravity profiles across the basins using a method that combines two‐dimensional flexural backstripping and gravity modelling techniques. We assumed a model of uniform lithospheric stretching and Airy isostasy in the analysis of tectonic subsidence. Our study shows that the basins are probably underlain by relatively thinned crust, indicating that some form of crustal stretching was involved. To explain the observed gravity anomalies, however, the Moho depth that we calculated based on the free‐air gravity data is about 25% deeper than the Moho predicted by assuming Airy isostasy (Backstrip Moho). This suggests that the Airy model overestimates the compensation and that the basins are probably undercompensated isostatically. In other words, there is an extra amount of tectonic subsidence that is not compensated by crustal thinning, which has resulted in the discrepancy between the gravity‐derived Moho and the Backstrip Moho. We attribute this uncompensated or anomalous tectonic subsidence to thin‐skinned crustal extension that did not involve the mantle lithosphere. The Malay and Penyu Basins are interpreted therefore as basins that formed by a combination of whole‐lithosphere stretching and thin‐skinned crustal extension.

Pliocene–Quaternary basins of the Ionian islands evolved in a complex tectonic setting that evolved from a mid to late Cenozoic compressional zone of the northern external Hellenides to the rapidly extending Pliocene–Quaternary basins of the Peloponnese. The northern limit of the Hellenic Trench marks the junction of these two tectonic regimes. A foreland‐propagating fold and thrust system in the northern external Hellenides segmented the former Miocene continental margin basin in Zakynthos and permitted diapiric intrusion of Triassic gypsum along thrust ramps. Further inboard, coeval extensional basins developed, with increasing rates of subsidence from the Pliocene to Quaternary, resulting in four principal types of sedimentation: (1) condensed shelf‐sedimentation on the flanks of rising anticlines; (2) coarse‐grained sedimentation in restricted basins adjacent to evaporitic diapirs rising along thrust ramps; (3) larger basins between fold zones were filled by extrabasinal, prodeltaic mud and sand from the proto‐Acheloos river; (4) margins of subsiding Quaternary basins were supplied at sea‐level highstands by distal deltaic muds and at lowstands by locally derived coarse clastic sediment.

A numerical model linking a coral growth algorithm and an algorithm for flexural subsidence reproduces many of the characteristics of drowned foreland basin carbonate platforms. This model successfully matches the observed distribution and drowning age of drowned carbonate platforms in the Huon Gulf, Papua New Guinea, a modern submarine foreland basin. Analysis of equations describing flexural subsidence and eustatic sea‐level variations suggest that there are minimum convergence rates and periodicities of sea‐level variation required to drown foreland basin carbonate platforms. For convergence rates on the order of a few millimetres per year, sea‐level must vary on time‐scales of about 10⁵ years in order to induce a rate of relative sea‐level rise great enough to drown an otherwise healthy foreland basin carbonate platform.

The main Karoo Basin of South Africa is a Late Carboniferous–Middle Jurassic retroarc foreland fill, developed in front of the Cape Fold Belt (CFB) in relation to subduction of the palaeo‐Pacific plate underneath the Gondwana plate. The Karoo sedimentary fill corresponds to a first‐order sequence, with the basal and top contacts marking profound changes in the tectonic setting, i.e. from extensional to foreland and from foreland to extensional, respectively.

Sedimentation within the Karoo Foreland Basin was closely controlled by orogenic cycles of loading and unloading in the CFB. During orogenic loading, episodes of subsidence and increase in accommodation adjacent to the orogen correlate to episodes of uplift and decrease in accommodation away from the thrust‐fold belt. During orogenic unloading the reverse occurred. As a consequence, the depocentre of the Karoo Basin alternated between the proximal region, during orogenic loading, and the distal region, during orogenic unloading. Orogenic loading dominated during the Late Carboniferous–Middle Triassic interval, leading to the accumulation of thick foredeep sequences with much thinner forebulge correlatives. The Late Triassic–Middle Jurassic interval was dominated by orogenic unloading, with deposition taking place in the distal region of the foreland system and coeval bypass and reworking of the older foredeep sequences.

The out of phase history of base‐level changes generated contrasting stratigraphies between the proximal and distal regions of the foreland system separated by a stratigraphic hinge line. The patterns of hinge line migration show the flexural peripheral bulge advancing towards the craton during the Late Carboniferous–Permian interval in response to the progradation of the orogenic front. The orogenward migration of the foreland system recorded during the Triassic–Middle Jurassic may be attributed to piggyback thrusting accompanied by a retrogradation of the centre of weight within the orogenic belt during orogenic loading (Early Middle Triassic) or to the retrogradation of the orogenic load through the erosion of the orogenic front during times of orogenic unloading (Late Triassic–Middle Jurassic).

The Hikurangi Trough, off eastern New Zealand, is at the southern end of the Tonga–Kermadec–Hikurangi subduction system, which merges into a zone of intracontinental transform. The trough is mainly a turbidite‐filled structural trench but includes an oblique‐collision, foredeep basin. Its northern end has a sharp boundary with the deep, sediment‐starved, Kermadec Trench.

Swath‐mapping, sampling and seismic surveys show modern sediment input is mainly via Kaikoura Canyon, which intercepts littoral drift at the southern, intracontinental apex of the trough, with minor input from seep gullies. Glacial age input was via many canyons and about an order of magnitude greater. Beyond a narrow, gravelly, intracontinental foredeep, the southern trench‐basin is characterized by a channel meandering around the seaward edge of mainly Plio‐Pleistocene, overbank deposits that reach 5 km in thickness. The aggrading channel has sandy turbidites, but low‐backscatter, and long‐wavelength bedforms indicating thick flows. Levées on both sides are capped by tangentially aligned mudwaves on the outsides of bends, indicating centrifugal overflow from heads of dense, fast‐moving, autosuspension flows. The higher, left‐bank levée also has levée‐parallel mudwaves, indicating Coriolis and/or boundary currents effects on dilute flows or tail plumes.

In the northern trough, basin‐fill is generally less than 2 km thick and includes widespread overbank turbidites, a massive, blocky, avalanche deposit and an extensive, buried, debris flow deposit. A line of low seamounts on the subducting plate acts as a dam preventing modern turbidity currents from reaching the Kermadec Trench. Major margin collapse probably occurred in the wake of a large subducting seamount; this seamount and its wake debris flow probably dammed the trench from 2 Ma to 0.5 Ma. Before this, similar dams may have re‐routed turbidity currents across the plateau.