Extreme hydrothermal conditions at an active plate-bounding fault

Area of Science:

• Geophysics
• Structural Geology
• Tectonics

Background:

• Temperature and fluid pressure are critical factors in rock deformation, mineralization, and earthquake distribution on geological faults.
• Intraplate continental crust typically exhibits hydrostatic fluid pressure and a geothermal gradient of ~31°C/km.
• High temperatures (>300-450°C) at depths >10-15 km promote aseismic creep due to quartz and feldspar plasticity, reducing earthquake frequency.

Purpose of the Study:

• To investigate the temperature and fluid pressure conditions in the upper part of the Alpine Fault, a major plate-bounding fault.
• To understand the hydrothermal conditions influencing frictional-mechanical processes and earthquake rupture cycles at active faults.

Main Methods:

• Drilling a borehole into the Alpine Fault's hanging wall to a depth of 893 meters.
• Measuring pore fluid pressure and temperature gradients within the borehole.
• Analyzing data to model the interplay of fault slip, fluid flow, and landscape development.

Main Results:

• The borehole revealed a pore fluid pressure gradient exceeding hydrostatic levels by 9±1%.
• An average geothermal gradient of 125±55°C/km was recorded in the fault's hanging wall.
• These extreme conditions are attributed to rapid fault movement and topographically driven fluid flow, concentrating heat.

Conclusions:

• Active plate-bounding faults can develop highly anomalous fluid pressure and temperature gradients in the seismogenic zone.
• Positive feedbacks between fault slip, rock alteration, and landscape evolution drive these extreme hydrothermal conditions.
• These conditions significantly impact the mechanical behavior of faults and earthquake potential.