Dextral

Dextral describes a strike-slip fault on which the block on the opposite side of the fault from the observer moves to the right. The term comes from the Latin dexter, meaning right. Another name for the same geometry is right-lateral. If you stand on one side of the fault and look across it, the opposing block has moved to the right relative to you. The San Andreas Fault in California is the most famous dextral fault in the world. Its counterpart, sinistral (left-lateral), applies when the opposing block moves to the left. Recognizing fault sense is fundamental to understanding how a basin formed, whether a trap is compressional or extensional, and how much total displacement has accumulated on a fault system over geological time.

Key Takeaways

Dextral (right-lateral) and sinistral (left-lateral) are the two senses of movement on strike-slip faults. The sense is consistent regardless of which side of the fault you stand on: if the fault is right-lateral and you cross to the other side and look back, the opposite block still appears to have moved to the right.
Fault sense is determined from offset geological markers: a stream channel, a dike, a rock unit boundary, or a fold axis that crosses the fault will be offset in the direction of fault movement. Measure the offset and note which way the marker moved on the far side to determine dextral or sinistral.
Dextral faults form in specific regional stress fields. Where a region is under north-south compression, northwest-southeast-trending faults tend to be dextral and northeast-southwest-trending faults tend to be sinistral. This pattern follows from the Andersonian theory of faulting, which predicts fault orientations at 30 to 45 degrees to the maximum principal stress direction.
En echelon fold arrays are associated with dextral faulting. As two adjacent dextral faults interact, the zone between them can be compressed or extended depending on the geometry, producing chains of anticlinal folds (restraining stepovers) or pull-apart basins (releasing stepovers) that are prospective for hydrocarbons.
The Queen Charlotte Fault system along the coast of British Columbia and southeast Alaska is a major dextral transform fault, one of the most seismically active fault systems in Canada. The 1949 Queen Charlotte earthquake (magnitude 8.1) was the largest recorded in Canada's history and resulted from dextral motion on this fault.

What Does Dextral Mean in Geology?

Stand in the middle of a road and imagine it is a geological marker (a river, a dike, or a rock formation boundary) that runs across a strike-slip fault. The fault cuts the road in front of you. On your side, the road runs straight to the fault. Now walk to the fault and look across. If the road continues to the right of where you expect it to be, the fault is dextral (right-lateral). If it continues to the left, the fault is sinistral (left-lateral). That is the entire definition: which way did the far block move?

This simple test applies to any linear geological feature that crosses a fault. Stream channels are commonly used because erosion cuts them continuously and they preserve offset well in the landscape. A stream channel offset 500 metres to the right across a fault records 500 metres of right-lateral displacement. The cumulative displacement over the fault's history represents millions of years of movement at millimetres per year.

In the subsurface, geologists apply the same test to seismic reflection horizons, formation boundaries on well logs, and the traces of other faults visible on seismic. A fault that offsets a seismic horizon to the right in map view is dextral. Identifying the sense correctly is the first step in reconstructing the kinematic history of a basin and predicting where structural traps may have formed.

Fast Facts

The San Andreas Fault has accumulated approximately 315 kilometres of total right-lateral (dextral) displacement since it became active. The fault moves at an average of about 45 to 50 millimetres per year. A geologist can trace pink granites and rhyolites from the Tehachapi Mountains in southern California to matching rocks near Morro Bay in the central Coast Ranges, 315 kilometres to the northwest, confirming the cumulative offset. The 1906 San Francisco earthquake produced up to 6 metres of right-lateral surface rupture in a single event. The rate of slip, though often described as gradual, is punctuated by sudden jumps during major earthquakes.

Dextral Faults and Petroleum Traps

Strike-slip faults, whether dextral or sinistral, create a variety of petroleum trap types depending on the fault geometry and the regional stress field. Understanding the sense of movement is the first step in predicting which type of trap to expect.

Along a dextral fault, a releasing stepover (where the fault steps to the left in the direction of far-block motion) creates a pull-apart basin between the two fault segments. The releasing block drops down, the basin subsides, and rapid deposition of organic-rich sediment can create source rock. Many of the oil-producing basins along the San Andreas system in California (the Los Angeles Basin, the San Joaquin Valley) have pull-apart origins controlled by dextral fault geometry.

A restraining stepover (where the fault steps to the right in the direction of far-block motion) creates compression between the fault segments, pushing rock upward into a positive flower structure or a pop-up. If this structural high is draped by a seal and underlain by a source kitchen, it forms a structural trap. The ridge-like structural highs along the Whittier Fault in southern California are examples of restraining stepover traps in a dextral fault system.

In wrench fault systems (fault zones where the dominant motion is strike-slip but with some vertical component), both releasing and restraining stepovers occur along the same fault system at different locations. Mapping which segments are restraining (structural highs, prospective for traps) versus releasing (basins, prospective for source rock) is a critical exploration step in any area with significant strike-slip deformation.

Dextral Faulting in the Canadian Context

The Queen Charlotte Fault system offshore British Columbia is a dextral transform fault analogous in many ways to the San Andreas. The Pacific Plate moves northwest relative to the North American Plate along this fault at approximately 55 millimetres per year. The fault has generated earthquakes of magnitude 6 to 8.1 and produces the largest tsunamis along Canada's Pacific coast.

Within the BC Foothills and Alberta Rocky Mountain thrust belt, dextral strike-slip faults occur as transfer faults that accommodate lateral variations in the amount of shortening along the fold-and-thrust belt. These transfer faults can create local structural complexity that traps gas in tight formations. The Foothills gas fields of British Columbia, some of which are among the largest in Canada, sit in a deformation zone influenced by both thrust and strike-slip motion.

Dextral is synonymous with right-lateral. The antonym is sinistral (left-lateral). Related terms include sinistral (left-lateral fault sense, in which the block across the fault moves to the left; the mirror image of dextral; examples include the North Anatolian Fault in Turkey and the Alpine Fault in New Zealand), strike-slip fault (a fault on which movement is primarily horizontal and parallel to the fault strike; dextral and sinistral describe the sense of movement on such faults), pull-apart basin (an extensional basin that forms at a releasing stepover between two segments of a strike-slip fault; the subsiding block between the faults is the "pull-apart"), flower structure (a seismic reflection pattern associated with strike-slip faults, in which faults splay upward from a deep master fault into a flower-like geometry; positive flower structures (transpressional) are associated with restraining stepovers on dextral or sinistral faults), and transform fault (a specific type of strike-slip fault that forms a plate boundary; all transform faults have either dextral or sinistral sense depending on the relative motion of the two plates).

How Identifying a Dextral Relay Zone Unlocked a Foothills Gas Field

An exploration team was interpreting a 3D seismic survey in the Rocky Mountain Foothills of northeast British Columbia, targeting a Cretaceous tight gas sandstone at 3,200 metres. The seismic showed a prominent northeast-trending anticline, typical of the fold-and-thrust belt, with what appeared to be lateral terminations at both ends. The southern termination was unusually abrupt, truncating the anticline over a very short lateral distance.

A structural geologist on the team identified the termination as a northwest-trending fault crossing the anticlinal axis. The fault showed apparent offset of the anticline crest by 400 metres to the right in map view, consistent with right-lateral (dextral) motion. The fault was a transfer fault accommodating lateral variation in thrust displacement.

At the releasing stepover between the dextral fault and the thrust anticline, the team mapped a subtle local depression in the crest of the anticline. Rather than being a trap killer, the stepover had created a secondary structural closure in the extensional zone between the two fault traces, sitting between the main anticline crest and the dextral fault. A sidetrack well was proposed to test this secondary closure specifically.

The sidetrack encountered a 28-metre gas pay in the Cardium equivalent sandstone at the secondary closure. The discovery well flowed at 2.2 million cubic feet per day on a 20-millimetre choke, exceeding the pre-drill estimate for the primary anticline. The dextral relay zone, initially seen as a complication in the anticline geometry, had created a second structural trap that the exploration team almost missed by focusing only on the main fold crest.