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Structural geology focuses on secondary tectonic structures like folds and faults in sedimentary layers. Geologists map these structures, measure orientations, and analyze data to understand larger-scale processes. This is closely linked to tectonics and plate tectonics, as plate motions create these structures. For example, extensional faults (normal faults) occur during crustal stretching, while reverse and thrust faults form during shortening. The Keystone Thrust in Red Rock Canyon, Nevada, illustrates how older Cambrian rocks lie atop younger Jurassic rocks due to thrust faulting, indicating crustal shortening during the Sevier orogeny. Experiments, like shortening wet plaster, visually demonstrate fault formation and development, showing fault sequences and imbricate structures. Normal faults, indicative of extension, are observable in Arches National Park, Utah, where displacement is relayed between faults creating relay ramps and zones. Different deformation styles exist depending on depth and temperature. Ductile deformation, like that seen in elongated pebbles in Portugal ("chocolate-bage" structure), occurs at greater depths and higher temperatures. Folds in gneiss, requiring temperatures above 350°, indicate depths of 12-15km. Structural geologists use structural data to infer tectonic history. Convergent plate boundaries typically result in reverse faults, thrusts, folds, and mountain ranges, while divergent boundaries and continental breakup produce normal faults and extensional shear zones. Analyzing structures often requires extensive fieldwork, modeling, and data collection to fully understand the tectonic processes involved.

IAM a structural geologist and as a
‏structural geologist I'm interested in
‏structures that form secondary so-called
‏tectonic
‏structures structures that form by
‏folding and faulting of say sedimentary

‏layers

‏ and there is a whole lot whole
‏range of structures you can go out in
‏the field and you can map them you can
‏measure up the orientations
‏geometries go back and plot the data in
‏stereonets and and things like that and
‏that's a lot of fun its important but

‏it doesn't stop there the most important
‏part is to use the structural
‏information to try and understand what's
‏going on on a larger scale and that's
‏why structural geology is very closely
‏linked with um

‏tectonics including plate
‏tectonics where the Motions in the crust
‏in the lithosphere the motion of plates
‏is is what creates
‏structures so we can use structures if
‏We new What were looking at if we

understand how the structure is
formed um we can use that information
and try and say something about for
example plate motions in the
past as an example we can take faults
and there is different types of faults

tol example there are extensional faults
that's normal faults that are involvec
Guring the extension and stretening on
the crust and there are reverse fulta

the crust and there are reverse fults
and thrust fults that form when the
crust is being
shontened and rignenow we are just
outside of Las Vegas in the Red Rock.
Canyon area were looking at some red

and yellow sandstones in the background
now that's Jurassic Sandstone the
Jurassic atte formation on top of thase
sandstones back there high in the
mountain there there is uh there's
another type of rock another rock unit

นาลด (s oider inar ls Camonian and
devonian and those reers trose oid rockis
are resting on top of the relatively
Younger rocks Jurassic rocks so pre so
so 50 Cambrian rocks on Jurassic rocks

between those units there is whats
called thrust
fold
um so that thrust fold formed when a
part of the crust during snortening was nipped off and put on top of the another
part of the crust so you get older rocks
un on top of younger rock so thats one
of the characteristics of thrust faults just understanding that relationship
enables you to say something about the
size and the magnitude of the shortening
of this part of the United
States sometimes after the Jurassic
probably in the Cretaceous and this is actually the Sevier orogy and that
particular thrust that separates the un
the upper plate the the camer from the
the Jurassic is called the Keystone
Thrust in this area let's let s have a look at the plaster experiment where we
produce reverse faults and thrust faults
a little bit like what we have here in
Red Rock Canyon outside Las
Vegas this is an experiment where we are shortening wet plaster in a box
shortening by by pushing one side toward
the other together and the nice thing
about experimems is that we can see not
only the end result but we can see the entire development from the beginning to
the end and in this case we can see
which of the reverse faults developed
Tirst and which developed last and now
they deform and change orientation
during the shortening so in this case we have sort of a normal um insequence U
frosting history and the result is nice
imbricate
structure so thats about shortening anc
the structures that form during
snortening of the crust We couid also extend the erust and
like I said we get normal faults
extensional
faults and we have to go somewhere else
to well there are extensional faults
here too but let's go to to place in Utah to show you what an extensional
fault uh looks like
the geology of Southern Utah is
beautifully
exposed and uh right here we're flying
over archers National Park and we can uh we can see sandstones of Jurassic age
with uh several sets of
fractures if we go down on the ground we
will see not only fractures but also
faults hey look at those beautiful structures
on the other side of the
road um those are faults affecting
sedimentary layers sandstones
shells that were once continuous layers
and now they have been affected by these faults in a very interesting way in this
case we have normal faults uh normal
faults are extensional
structures uh that means that at some
point in time um forces were trying to
pull these Rock layers apart and they
partly succeeded on these faults formed so that's useful information for us as
geologists what happened back in
time this place in Canon lens is a
really interesting place where we can
see that the displacement on one fult is
decreasing the same time another fault
picks up the displacement so the
displacement is basic basically relayed
from one fault to to another and we have
a relay Zone and in between these two faults there's what's called a relay
ramp where layers have been
folded and and uh in this particular
case we actually have two sets of relay
structures and we have actually what's
called a grain stepover structure where one grain dies out and another one
continues um both being parallel with an
offset in some cases we have stretching
in two directions like in these
turbidites in Portugal where we have
quartz veins a vertical set of quartz veins and horizontal set of quarts veins
and together these imply vertical and
horizontal stretching so we have a
structure that is called chocolate bage
structure and um The Strain feeli that
in applies is a flattening stream so we've seen sandstones and
sediments being deformed forming faults
and fractures in this case the sediment
has not
fractured uh it has deformed in a very
different way uh these Pebbles and cobbles that were once part of a
conglomerate has now been stretched out
into these very elongated objects
um almost cigar or or um walking stick
shaped uh
geometries and so so so the Rock in this case behaved like a very soft material
and were talking um ductile or plastic
deformation and this happens deeper down
in the crust where the temperature gets
high and and rocks uh start to to
flow so a very different expression of defamation temperature crustal depth are
Very important parameters when it comes
to the way that rocks respond to to
forces and
stresses um in the crust so here's another example of rocks
that have formed in the ductile fashion
we're looking at Nies and we're looking
at at like fairly nice
fold so to fold an ice like this we need
temperatures of say 350° or more so we know that we've been
down at at least 12 15 km in the
crust so we're in the in the in the
ductile
regime the aim is to
understand um the history behind the
formation of these structures time when the structures
formed uh many possibilities um we talk
about tectonics so structural geology
and tectonics are sort of walking hand in
hand uh pretty much all the time we use
structure to understand the tectonic
history so tectonics is more about large
scale
motions un often times plates plate motions and when plates move they create
structures so uh if we know the plate
motions we could un we would expect
certain types of structures to form
depending on how the plates move I mean
if plates uh converge you would expect to find uh what's called reverse faults
thrusts uh folds uh of large scale folds
and and Mountain chains orogenic
belts whereas when when plates move
apart and especially when continents
internally start to break up move apart one part moving one way the other part
moving the other way we expect um in the
Upper Crust at least normal faults
what's it's called extensional
faults and those kind of things
extensional Shear zones so we
could um anticipate certain types of
structures depending on the plate motion
picture or the kinematic picture we
could also go the other way around which
is what we often times do as structural geologist we go in the field we map both
the structures and then we say something
about the large scale
picture and that's that's that's
exciting um it usually takes a lot of uh
data collecting um a lot of field work for example and maybe you need to do
some modeling numerical modeling maybe
some physical modeling to actually
understand what is going on uh structure
wise so there are a whole range of
different types of structure some form high up in the crust some some form
deeper down but they also form um they
also depend on on the kinematic picture
and the stress
picture fults are