Monday, 11 August, 2025/ Edition 71

Metamorphic core complexes (MCCs) are important geological features, first discovered in the 1960s in the North American Cordillera. However, their genesis and tectonic evolution and whether they can support petroliferous basins have been debated and are still the focus of interesting studies. This edition of Core Elements introduces two recent studies on MCCs.

Rasoul Sorkhabi

Editor, Core Elements

Metamorphic Core Complexes Revisited

MCCs in North America (after Zuza and colleagues, Earth-Science Reviews, 2025)

An article by Andrew Zuza and colleagues in Earth-Science Reviews re-examines the tectonic formation of metamorphic core complexes (MCCs) in the North American Cordillera.

Word origin: Peter Coney in 1980 coined the term “metamorphic core complex,” modifying the earlier term of “metamorphic core zone” suggested by John Wheeler in 1966 for his study area of British Columbia in Canada.

Architecture of MCCs:

There are four primary components to a typical Cordilleran MCC system:

1. A metamorphic/crystalline core on the footwall (“lower plate”) of the MMC which comprises medium- to high-grade metamorphic rocks intruded by plutons.

2. A mylonitic shear zone which is also overprinted by brittle brecciation. The shear zone has thicknesses in tens to thousands of meters and displays high-stress, low-temperature (300–500 degrees C) deformation.  

3. A major normal-fault, usually low-angle or detachment fault system, above the mylonitic shear zone separates the “lower plate” from the overlying low-grade to unmetamorphosed “upper plate” (hanging wall) rocks.

4. A supra-detachment, syn-kinematic basin above the detachment fault commonly filled with clastic and volcanic deposits.

Tectonic Formation:

Andrew Zuza and colleagues suggest a two-stage model for the formation of MCCs in the North American Cordillera:

1. An early stage of buoyant diapirism due to crustal melting driven by asthenosphere rise during slab rollback of the Farallon plate and associated magmatism. This ductile Paleogene MCC exhumation is constrained by Ar40/Ar30 thermochronology of muscovite and biotite.

2. A later phase of lithospheric extension caused by regional gravitational relaxation due to change of plate boundary kinematics. This brittle extensional faulting corresponds to the Miocene Basin-and-Range.

MCC on Venus?

While MCCs have been mapped in various parts on Earth, Jon Spencer in 2001 suggested that some circular features termed as “coronae” on Venus may also be a type of MCC.

Coronea on Venus are associated with radial fractures and lava flows, and probably formed by buoyant mantle diapirs.

The largest corona on Venus is the 2100-km Artemis Corona, which is split into half by a deformation belt that contains numerous rounded ridges similar to antiforms on Earth.

The largest of these ridges, located at the center of the Artemis Corona, according to Spencer is probably a Venusian MCC.

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Supra-detachment Basin in Northern South China Sea

MCC in Kaiping Sag Basin (Zhong and colleagues, AAPG Bulletin, 2025)

An article by a group of Chinese geologists in AAPG Bulletin presents a petroleum basin atop a possible MCC.

Basins atop MCCs:

The authors characterize supra-detachment basins as those that form on the hanging wall of low-angel (less than 30 degrees), high displacement (greater than 5 km) normal faults in highly extended terrains.

Case Study:

The authors suggest the Kaiping Sag basin in the rifted margin of northern South China Sea as a supra-detachment basin. It is located at the Pearl River delta, and is reprotedly the first such basin to host oil fields.

This would make Kaiping Sag an interesting case study to understand kinematic relationships between MCC formation and petroleum systems.

Study Design:

The researchers used seismic images, well logs, and geochemical and petrological data on source rocks.

Basin Evolution: The Kaiping Sag basin in relation to the underlying MCC evolved in three stages:

1. Deposition of deep-water lacustrine source rocks represented by lower shale units of Wenchang Formation during intense rifting in Early Eocene.

2. Formation of MCC via a rolling-hinge structural process which created a domal structure in late Eocene, coeval with deposition of shale and sand in the upper part of Wenchang Formation.

3. The MCC altered the original attitude and position of the source rock layers.

Source and Reservoir Rocks:

• Total organic carbon measurements of samples from upper Wenchang Formation range from 0.11 to 0.55 percent and of samples from lower Wenchang Formation range from 0.6 to 2.11 percent.

• High ratios of V/(V+Ni) and low ratios of Th/U measured in the Wenchang samples suggest an anoxic water column.

• Interbedded sand layers in the upper Wenchang Formation and in the overlying Oligocene-age Enping Formation are reservoir rocks.

MCC impact on Source Rocks: The authors discuss three modes how the MCC affected the distribution of source rock layers in the Wenchang Formation:

• Mode I. Source rock disconnected by antithetic faults, mainly observed in the western part of the basin.

• Mode II. Source rock moderately continuous and thick V shaped, mainly in the central part of the basin.

• Mode III. Source rock continuous with original dipping of the basin but relatively thin beds, mainly in the eastern part of the basin.

• In addition, MCC created faults that linked source rocks to structural traps and thus influenced oil migrations pathways.

Go deeper: Read the full article in AAPG Bulletin. Note that the structural and seismic evidence for MCC in Kaiping Sag basin was presented in 2022 by Ye and colleagues in JGR Solid Earth.

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✉️ To get in touch with Rasoul, send an email to editorial@aapg.org.