En Echelon Fractures in 2D & 3D
There's usually a low-tech way to explain complex geological phenomena. PlayDoh has been used for years to simulate ductile deformation in layered rocks. Sandy beaches teach us how head-cutting in river channels works. Paper mache, vinegar, dish soap, and baking soda make cool volcanoes. Pretty intuitive stuff.
Sometimes things aren't so simple. For example, fracture geometry.
Here's a way to explain en echelon fracture geometry using a piece of cardboard. Fractures open in shear (Mode II-strike slip, Mode III-dip slip) and extension (Mode I-extension). En echelon fractures in real rocks in real outcrops can be particularly confusing. Reduce confusion by understanding that joints and fractures are 3D structures that commonly change shape along their lengths (along strike) due to the way they propagate.
Here, a piece of cardboard is used to represent an en echelon fracture in both 2D and 3D. We will cut the cardboard twice (intersect 3D fracture with 2D planes) in order to show how the same fracture can appear differently depending on the orientation of the intersecting surfaces (i.e., the plane of an outcrop). Outcrops show us 2D projections of 3D forms. The terms 'joint' and 'fracture' are used interchangeably.
Fractures propagate along curved fronts, so we make it curvy.
En echelon fractures appear along the cut and bent edge.
While you weren't looking I made a new, straight cut across the edge of the cardboard that previously had the arc in it. The arc's loss is your gain. Just like in the original Indiana Jones movie, when that guy's face melted off. That was awesome.
The piece of cardboard is standing on edge, cut side down on the pad of paper. Fractures appear to show left-lateral offset (shear). Is it shear or the result of a fracture opening (no shear or offset)?
The same fracture may appear differently depending on where the plane of the outcrop intersects it. Pollard and Aydin (1988) differentiate the "segments" from the "parent" surface in complex fractures (joints) like this one.
When many of the same type of feature (i.e, clastic dikes) are present in a single exposure, try to note all of the different ways they appear in 2D. A variety of forms hint that fracture geometry may change along strike. Forms that appear at the cut face may merge and change somewhere out of the plane.
Important conversation overheard at the outcrop: "Try not to look so bored, Karl. You're killing the drill. Karl is always slowing us down, Bridget. We have got to get him out of our group. I'm, like, gonna text my adviser as soon as we get back."
Real World Example #1: Clastic dikes are useful. Since dikes are fractures that have been filled with sediment, we know the fracture was connected prior to shearing, though it now appears to be segmented. Is the thin white dike that steps down to the right offset along a series of small, subhorizontal faults? Is it an en echelon fracture? Did sliding along multiple bedding planes occur after the dike was injected? Are there thin sills (no offset) that follow bedding and connect the dike segments together? Did shearing occur after the fracture opened (two stages of development)? Is the fracture dying out? Is something else going on?
Real World Example #2: One dike or two? If two dikes, does one intrude upward and the other downward? Approximately how many fill bands (stripes) are present in each feature? Can you identify a source bed? If it is one dike, was it cut obliquely and offset by a sub-vertical fault? Is it part of an en echelon fracture system?
Real World Example #3: Is this dike offset by small faults or part an en echelon fracture or something else? Now I'm getting bored.
The photo above shows a single dike that appears segmented in the 2D cut face. The top and bottom of the dike are not exposed. The left segment tapers downward. The middle segment is elliptical (tapers on both ends). The right segment tapers upward. The middle segment is most interesting because it shows that when the leading edge of a propagating fracture is curved - which is the case most of the time - the dike (a sediment-propped fracture) appears elliptical on the intersecting plane (the outcrop).
A decidedly low-tech illustration by Pollard and Aydin (1988) in GSA Bulletin v. 100, Figure 6
"A joint is a three-dimensional structure composed of two matching surfaces (joint faces), that are commonly idealized as smooth, continuous, and planar. However, virtually all joints have some roughness, minor and major discontinuities, and occasional curves and kinks. An observer commonly sees only the joint trace, whose geometry depends on the orientation and location of the exposure surface relative to the joint surface. For example, traces of the same joint may be nearly straight and continuous or segmented (Fig. 6). We will call the structure a single joint if continuous paths connect the segments and parent joint surface."
-- Pollard and Aydin (1988, p. 1186)
A figure from the Brittle Deformation chapter of Twiss and Moores. Example G is what's at issue here.