South China Sea documents the transition from wide continental rift to continental break up

The study addresses how wide continental rifting evolves into continental breakup, a transition that has remained poorly constrained due to limited imaging of distal passive margins and lack of natural exposures. Wide rifts (e.g., North American Cordillera, Aegean) exhibit distributed thinning, lower-crustal ductile flow, detachment faulting, and metamorphic core complexes (MCCs), contrasting with cold magma-poor margins (e.g., West Iberia–Newfoundland) where extension leads to serpentinization and mantle exhumation. The South China Sea (SCS) records Early Eocene (~52 Ma) to Early Oligocene (~30 Ma) extension and breakup, forming an ultra-wide passive margin fringed by highly extended continental ribbons. Using high-resolution seismic data from the northern SCS, the authors aim to document MCC-style architecture, characterize detachment fault geometry and sub/supradetachment deformation, and evaluate the thermo-mechanical conditions linking wide rifting to continental breakup.

Prior work classifies rifting into narrow and wide modes, with wide rifts involving significant lower-crustal flow and detachment faulting, often forming MCCs. Cold magma-poor margins such as West Iberia–Newfoundland show progressive cooling, embrittlement, and mantle exhumation, while hot magma-poor margins like the Woodlark Basin display ductile lower-crustal extension and magmatism without mantle exhumation. Although wide rifts and highly extended margins have each been extensively studied, few studies explicitly link MCC-style wide rifting to the continent–ocean transition and breakup. The SCS has been recognized as an ultra-wide margin developed through Eocene–Oligocene extension, but seismically resolvable evidence for MCCs and related detachment architecture in the distal margin has been lacking. Comparisons to exposed analogues (North American Cordillera, Aegean) and active back-arc extension (Woodlark Basin) provide a framework to interpret detachment corrugations, grooves, and doming observed in seismic data.

• Data acquisition and processing: A proprietary 3D seismic dataset (CNOOC, 2011) acquired with two airgun arrays (7750 in^3 total, 2000 psi), fired every 25 m, towed at 6 ± 0.5 m; twelve 600 m streamers with 480 channels at 12.5 m spacing; recorded 8192 ms at 1-ms sample rate. Reprocessed with pre-stack depth migration (PGS, 2012). Inline and crossline spacing 12.5 m × 12.5 m; vertical sample rate 5 m; area ~1500 km^2; record length 10 km; main frequency 30–45 Hz. Velocity model consistent with OBS93. Regional 2D seismic lines image to 8–12 s TWT (~25–35 km). 3D depth-migrated volume images to ~10 km depth at spatial resolution of tens of metres.
• Bathymetry mapping: Constructed using GMT v6 with GEBCO one-minute grid.
• Detachment fault mapping: Interpreted on the pre-stack depth-migrated 3D volume in GeoFrame v2012. Initial horizon seeded on a 50 × 50 grid (600 m × 600 m) and autotracked; refined around intrusions at 10 × 10 grid, followed by autotracking. No smoothing/filters to preserve subtle deformation. 3D visualization performed in GeoViz with NW viewing perspective and NW light source at ~45°.
• Seismic attributes: Geometric attributes (azimuth and variance/“coherence”) extracted with 3 × 3 sampling; azimuth used to distinguish E–W faults (red) vs N–S lineations (blue/yellow); variance computed within ±5 m around the mapped horizon to highlight discontinuities (faults, intrusion-related distortions). Apparent polarity (Hilbert envelope maxima) used to analyze reflection strength and polarity.
• Measurement of grooves and secondary faults: The detachment surface (~32 km wide × 35 km long) divided into 1 km × 1 km cells; groove and fault orientations manually measured and plotted with Stereonet v10.4.2.
• Comparative analysis: Seismic observations compared with well-studied exposed analogues from the North American Cordillera, Aegean MCCs, and the Woodlark Basin to infer processes and thermal-mechanical state.

• Identification of MCC-style architecture in the distal northern South China Sea (SCS): dome structures, a corrugated and grooved low-angle detachment (Liwan detachment fault), subdetachment crustal-scale nappe folds, and magmatic intrusions coeval with supradetachment basins.
• Highly extended crust (<15 km thick) fringes the margin; upper crust shows pronounced boudinage. Crustal thickness beneath the large dome transitions rapidly from ~7 s TWT in the north to ~3 s TWT in the south.
• Subdetachment deformation: 30–40 km long nappe folds; isoclinal, high-amplitude laminated reflectors interpreted as mylonitic fabrics wrapped around rising domes; concentric low-amplitude reflections with step-like geometries consistent with intrusive bodies, grading to high-amplitude mantling reflections.
• Detachment geometry: The Liwan detachment dips ~6–10° south; exhibits N–S corrugations parallel to grooves. Corrugations have 2–3 km crest-to-trough amplitudes, 10–15 km wavelengths, extending >35 km down-dip and plunging south; aspect ratio (length/width) ~2.5, within typical MCC dome ranges and distinct from the higher aspect ratios and smaller dimensions of the S-reflector corrugations on cold West Iberian margins.
• Grooves: Parallel, well-developed, trending ~005° in the north curving to ~010° in the south; individual groove lengths up to ~20 km. Supradetachment faults offset grooves/corrugations and trend ~085° (north), ~060° (central), and ~090° (south), indicating late-stage faulting.
• Supradetachment basin development: Southward stacking/younging of Eocene–Oligocene (Tg–T60) sediments records top-to-the-south shear. Oligocene (T70) onlaps indicate localized vertical subsidence of ~2–3 km at the southern end of the detachment. Lower Eocene units (Tg–T80) are missing beneath the Oligocene basin, and Upper Eocene units (T80–T70) are attenuated, suggesting localized pure-shear extension in the south during the Oligocene.
• Temporal evolution of deformation: Eocene extension dominated by simple-shear along the Liwan detachment; Oligocene localization of pure-shear coeval with uplift of E–W domes (D1, D2, D3), deep crustal extrusion, and strain localization during continental breakup.
• Thermal-mechanical implications: Evidence for hot, weak lower crust with partial melting and deep crustal ascent beneath the detachment, enabling MCC formation, upper crust boudinage, and middle/lower crust exhumation. The observed corrugation/doming geometries and dimensions imply temperature plays a key role in shaping detachment topography, differentiating hot margins (SCS, Woodlark) from cold magma-poor margins (e.g., West Iberia).
• Ocean–continent transition context: Upper crust necking and boudinage in the hyperextended margin evolves down-dip into the OCT and oceanic lithosphere without large-scale mantle exhumation, consistent with a hot magma-poor margin scenario.

The observations demonstrate that the northern SCS records a continuous evolution from wide continental rifting to extreme thinning and continental breakup within a thermally weakened lithosphere. The Liwan detachment fault’s corrugations, grooves, and domes, together with subdetachment nappe folds and magmatic intrusions, are diagnostic of MCC systems and match patterns in the North American Cordillera, Aegean MCCs, and Woodlark Basin. The aspect ratios and scales of corrugations/domes align with intrusive- or migmatite-cored MCCs, contrasting with the smaller, higher-aspect corrugations of cold West Iberian margins, underscoring the influence of elevated temperatures and low lower-crustal viscosities on fault-surface morphology and strain localization. Supradetachment basin architecture (southward younging, Oligocene onlap, and asymmetric depocentre) ties basin subsidence and sedimentation directly to detachment kinematics and evolving rheology: Eocene simple-shear along the detachment transitions to localized Oligocene pure-shear as deep crust extrudes and strain localizes approaching breakup. Collectively, these results bridge the gap between studies of wide rifts and highly extended continental margins, showing that MCC-style processes can persist into the hyperextended domain and OCT in hot magma-poor settings, without large-scale mantle exhumation typical of colder margins.

High-resolution 2D/3D seismic imaging of the northern South China Sea distal margin reveals a complete MCC system—corrugated and grooved low-angle detachment, domes, crustal-scale nappe folds, magmatic intrusions, and a kinematically linked supradetachment basin—within highly extended continental crust. These structures document the transition from wide continental rifting (Eocene, simple-shear dominated) to localized deformation and continental breakup (Oligocene, local pure-shear) in a hot, weak lithosphere with active lower-crustal flow and partial melting. The architecture and dimensions of the detachment corrugations and domes closely resemble those in well-exposed MCC provinces (Cordillera, Aegean) and active back-arc extension (Woodlark), providing in situ, seismically resolved evidence that wide rifting can evolve directly into passive margin formation in thermally weakened active margins.

The paper highlights that understanding the transition from wide rifting to breakup has been hampered historically by limited distal-margin imaging and lack of natural exposures. In this study, interpretations rely on high-resolution seismic imaging and comparison with exposed analogues; direct sampling of the detachment footwall and lower crust in the study area is not presented. The proprietary nature of the primary seismic datasets (CNOOC) may limit independent replication, although methodological details and supplementary information are provided.