Diapirism and magmatic intrusions in the Tazoult salt wall revealed by Magnetotellurics

Salt diapirs are key structures for understanding tectonic and sedimentary processes and are relevant for applications in resource exploration, carbon storage, and geological risk assessment. However, their study has been limited by challenges in acquiring detailed subsurface data, especially in offshore environments where access is restricted and costs are high. Onshore studies, therefore, play an important role in understanding salt tectonics by offering more accessible settings for investigation.

The Atlas Mountains in Morocco serve as an exceptional natural laboratory for studying salt tectonics. Formed through the tectonic inversion of an extensional basin, the Central High Atlas region hosts numerous diapiric structures related to evaporites deposited during the Triassic rifting phase. The Tazoult diapir stands out for its well-preserved surface exposure and accessibility. Magmatic activity associated with rifting further increased the complexity of Tazoult, with mafic intrusions emplaced both within and along its salt walls.

Seismic methods have traditionally been used to study salt diapirs, but they face limitations due to salt's high acoustic impedance and complex geometry, resulting in low-resolution images and interpretative challenges. Magnetotellurics (MT), on the other hand, offers a powerful alternative by leveraging electrical resistivity contrasts between salt and host rocks. This approach delineates internal geometries and formation processes while identifying salt extrusion features (e.g., salt glaciers), enabling the reconstruction of their geological and tectonic evolution.

To study the Tazoult diapir, we completed the acquisition of 102 wide-band MT soundings both within and around the diapiric structure, covering a frequency range of 10 kHz Hz to 0.001 Hz, over an area of approximately 40 × 40 km². The aim is to retrieve a high-resolution resistivity model to image the internal structure of the diapir, analyze salt extrusion geometries, and identify the salt source unit to better understand the geological and tectonic processes at a semi-regional scale. Preliminary transfer function analysis reveals significant resistivity contrasts at high frequencies within the diapir, identifying zones of high conductivity potentially linked to salt extrusion structures, as well as highly resistive zones associated with volcanic intrusions hosted within Tazoult. Mid-to-long periods exhibit (>1-10 s) a large split in transfer functions, indicating increased structure complexity. For longer periods (>100 s), very high apparent conductivities (10 S/m) have been identified, likely related to the salt source unit. These findings suggest a promising outlook for detailed imaging of the salt extrusion system.