% % Lecture%13%
!
1!
Folding%I:%Folds%and%Their%Classification%
%%%
Ch.%11,%p.%219*226%
%%
!
1.!Folds:%Plastic%deformation%is%the%underlying%mechanism%by%which%rocks%are%folded.%Folds%fall%into%the%realm%of%
ductile%deformation.%Recall,%this%is%simply%a%nongenetic,%scale%of%observation%term,%implying%deformation%without%
obvious%brittle%processes%or%loss%of%cohesion%(i.e.,%solid*state%flow).%Deformation%is%distributed%through%the%rock%on%
the%granular%scale.%
%%%%%%%%%%%%%%%%%%%%%%%%
[Figure.)Caledonides)folding)in)Greenland)(cliff)is)800)m)high)]%
)
)
2.!Fold!Scales:%Folds%are%wave*like%undulations%of%formerly%planar%features%in%rocks,%such%as%beds.%They%occur%at%all%
scales:%microscopic%to%outcrop%to%regional.%They%may%constitute%much%of%fold*and*thrust%belts%(orogenic%belts)%that%
can%extend%for%100s%of%km%(e.g.,%Appalachians;%Alps).%
%%%%%%%%%%%%%%%%%%%%%%%%%
[Figure.)Folded)landscape)in)the)Appalachians)and)a)cross)section)of)folds)in)the)European)Alps]%
%
)
3.!Fold!Characteristics:%Before%considering%the%processes%by%which%folds%form%(including%physical%conditions%and%
rock%mechanical%properties),%we%will%simply%look%nongenetically%at%the%range%of%fold%geometries,%terminologies,%and%
criteria%for%classification,%based%on%observable%characteristics%of%folds.%
%%%%%%%%%%%%%%%%%%%%%%%%%%
[Figure.)Typical)fold)geometries]%
%
)
4.!Fold!Types:%There%are%three%fundamental,%basic%shapes%of%folds:%
%%
• %%A%monocline%consists%of%a%single%warp%along%otherwise%planar%layers,%with%no%change%in%dip%direction.%
• %%A%synform%is%a%concave*upward%fold.%%
• %%An%antiform%is%concave*downward.%
%
Whenever%bedding%is%initially%NOT%overturned%(most%commonly%the%case),%synforms%define%synclines%(youngest%
rocks%at%the%center)%and%antiforms%define%anticlines%(oldest%rocks%at%the%center).%Overturned%rocks%create%antiformal%
synclines%and%synformal%anticlines.%%
%%%%%%%%%%%%%%%%%%%%%%%%%%
[Fig.)11.8.)Basic)fold)shapes])[Figure.)Synform)and)antiform])
%
%
5.!Fold!Terminologies:%Synclines%and%anticlines%consist%of%two%fold%limbs%that%meet%along%a%hinge%line.%This%may%be%
an%abrupt%change%at%a%hinge%point,%or%a%broader%hinge%zone.%The%change%in%curvature%along%each%limb%is%the%
inflection%line.%
%%%%%%%%%%%%%%%%%%%%%%%%%%%
[Fig.)11.1.)Basic)terminologies)for)fold)components])
%
6.!Hinge!Points:%Hinge%lines%are%typically%not%observable%in%a%cross%section%view.%%[Figure.)Hinge)points]%
%
7.!Fold!Hinge!Zones:%Fold%hinges%may%be%abrupt%hinge%points,%such%as%along%kink%bands%or%chevron%folds,%but%are%
more%commonly%broader%hinge%zones,%such%as%within%concentric%folds.%%%%%%%%
%%%%%%%%%%%%%%%%%%%%
[Fig.)11.2.)Hinge)zone)variability])[Figure.)Chevron)folds)in)coastal)California])
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