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22.7.1 IntroductionIf you’ve ever driven across the United States, or havelooked at a relief map of the country, you can’t help butnotice that the nature of topography varies radically withlocation. Along the East Coast, the land rises to form thelong ridges of the Appalachian Mountains, whereas inthe west, numerous chains of rugged peaks togethercompose the broad North American Cordillera. Inbetween these two mountain belts, a region known geo-graphically as the Midcontinent, the land surface is rel-atively flat and low-lying (Figure 22.7.1). Roads windamong dramatic cliffs and deep valleys in the moun-tains, but shoot like arrows across the checkerboard offarmland and rangeland in the Midcontinent. Thesecontrasts in topography reflect contrasts in the charac-ter and geologic history of the continent’s crust.

Following the definitions of Table 22.7.1, we clas-sify the Midcontinent region as a continental-interiorplatform. Here, a cover of Phanerozoic strata wasdeposited in wide but shallow seaways over a base-ment of Precambrian (crystalline) rock. A continental-interior platform is one of two kinds of crustalprovinces that can occur in a craton. We define a cra-ton as continental crust that has not developed pene-trative fabrics and tight folds, and has not been sub-jected to regional metamorphism since the beginningof the late Neoproterozoic (i.e., since about 800 Ma).In addition to continental-interior platforms, a cratoncan include a shield, a region where Precambrianbasement rocks crop out extensively at the ground sur-face, either because cover strata were never depositedor because they were eroded away after deposition. InNorth America, much of the interior of Canada is ashield—not surprisingly, the region is known as theCanadian Shield.

The mountain ranges of the United States consist oftwo kinds of crust. Specifically, the Appalachians andthe North American Cordillera can be consideredPhanerozoic orogens, for during the Phanerozoic theseregions developed penetrative deformation and, inplaces, regional metamorphism. They were also thesite of widespread igneous activity and significantuplift. As a consequence of Phanerozoic tectonism,these orogens became warm and relatively weak. Whatis often overlooked is that the North AmericanCordillera also includes a large area of crust that isidentical in character to that of the continental-interiorplatform, even though it has been uplifted significantlyand has been locally subjected to deformation andigneous activity during the Phanerozoic. This crustunderlies the Rocky Mountains and Colorado Plateauof the United States. In this essay, you will see that thetectonic behavior of the Midcontinent resembles thatof the Rocky Mountains and Colorado Plateau, excepton a more subdued scale.1

Cratons differ from younger orogens not just interms of topography, but also in terms of physical char-acteristics such as strength, seismicity, heat flow, andcrustal thickness. Specifically, orogens have lessstrength,2 more seismic activity, higher heat flow, andthicker crustal roots, than do cratons. The contrast instrength reflects the contrast in heat flow, for warmerrock tends to be more plastic, and therefore weaker,

2 2 .7 PH A N E ROZOI C T ECTONI C S OF T H E U NIT E D S TAT E SMIDCON TIN E N T

22.7.1 Introduction 61522.7.2 Classes of Structures in the Midcontinent 61622.7.3 Some Causes of Epeirogeny 623

22.7.4 Speculations on Midcontinent Fault-and-Fold Zones 625

22.7.5 Closing Remarks 626Additional Reading 627

1Note that the Canadian Rockies, in contrast, comprise a fold-thrust beltand, as such, are part of a younger orogen.2The collision between India and Asia illustrates the strength contrastbetween a craton and a younger orogen. India consists of old, cold Pre-cambrian crust, while the southern margin of Asia consists of warm, rela-tively weak crust. During their collision, India pushed deeply into Asia.

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than cooler rock of the same composition. Rock thatmakes up cratons initially formed either at volcanicarcs bordering convergent plate boundaries, or at hot-spot volcanoes above mantle plumes. Thus, the conti-nental crust that eventually became the craton grew bysuccessive collision of these buoyant blocks. This factis well illustrated by studies in the Canadian Shield,where outcrops contain spectacular tectonite fabrics,folds, and faults that tell of a long and complex historyof accretionary orogeny. If crust of the cratons beganits life in orogenic belts, then how did it becomestrengthened and stabilized or, in other words, cra-tonized? Researchers suggest that cratonization comesin part from aging, because continental lithosphereloses heat and becomes stiffer as it ages. In fact, thelithospheric mantle beneath cratonic crust is thickerthan that of other kinds of continental crust.Researchers also suggest that relatively buoyantasthenosphere has stayed attached to the base of thelithosphere and insulates the lithosphere from warmerparts of the asthenosphere. This special asthenospherehas been called the root of the craton.

In this essay, we discuss the Phanerozoic tectonicsof the Midcontinent region of the United States. At

first glance, the title may seem like an oxymoron, forthe lack of topography and of penetrative structuresgives the impression that this region has been tectoni-cally stable during the Phanerozoic. However, whilethe Midcontinent has been relatively stable, it has notbeen completely stable. In fact, during the Phanero-zoic, faults in the region have been reactivated, andbroad regions of the surface have subsided to form sed-imentary basins or have been uplifted to form domes orarches. Manifestations of Phanerozoic tectonism in theMidcontinent might not be as visually spectacular asthat of North America’s mountain belts, but tectonismhas, nevertheless, occurred there.

22.7.2 Classes of Structures in the Midcontinent

To structural geologists raised on a diet of spectacularfolds, faults, and deformation fabrics exposed inPhanerozoic orogens, the Midcontinent may seem,at first glance, to be structureless. But, in fact, theregion contains four classes of tectonic structures:(1) epeirogenic structures, (2) Midcontinent fault-and-fold

SierraNevadas

Great V

alley

SnakeRiverPlain

Basin andRidge Province

Wind RiverRange

Bighorn Mountains

Black Hills

Rocky Mountainfront range

Rio Grande Rift

ColoradoPlateau

Great Plains

Ozarks

MississippiEmbayment

Ouachitas

Mississippi Delta

Adirondacks

AppalachianPlateau

Appalachians

Piedm

ont

Coast

al p

lain

Cascades

F I G U R E 2 2 . 7 . 1 Topographic relief map of the United States, showing the contrast betweenPhanerozoic marginal orogens and the Great Plains.

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zones, (3) intragranular strain, and (4) regional jointsystems. We’ll describe these in succession.

E P E I R O G E N I C S T R U C T U R E S When geologists firstmapped the Midcontinent, they noted that Paleozoicstrata of the region are nearly, but not exactly flat-lying. As a consequence, outcrop patterns of Paleozoicstrata on a regional geologic map of the MidcontinentUnited States display distinctive bull’s-eye patterns

(Figure 22.7.2). In some examples, the youngest strataoccupy the center of the bull’s-eye, while in others,they occur in the outer ring. Bull’s-eyes with theyoungest strata in the center define intracratonicbasins, in which strata are warped downward to form abowl shape. Those with the youngest strata in the outerring define intracratonic domes or, if elongate,intracratonic arches (by “intracratonic” we mean“within the craton”). As new data defining subsurface

T A B L E 2 2 . 7 . 1 C A T E G O R I E S O F C O N T I N E N T A L C R U S T

Active rift A region where crust is currently undergoing horizontal stretching, so that crustal thicknessesare less than average crustal thicknesses. In active rifts, continental crust has a thickness ofonly 20–25 km, the crust has been diced up by normal faults, and volcanism occurs. Examplesinclude the Basin and Range Province of the western USA, and the East African Rift.

Inactive rift A belt of continental crust that underwent stretching, and became a narrow trough that filledwith sediment, but never succeeded in breaking a continent in two. The Midcontinent rift of thecentral USA, and the Rhine Graben of Europe are examples.

Active orogen A portion of the continental crust in which tectonism (faulting ± igneous activity ± uplift)currently takes place or has taken place in the recent past (Cenozoic). Such orogens tend tobe linear belts, in that they are substantially longer than they are wide. Examples include theNorth American Cordillera and the European Alps.

Continental shelf A belt fringing continents in which a portion of the continent has been submerged by the sea.Water depths over shelves are generally less than a few hundred meters. Continental shelvesare underlain by passive-margin basins, which form subsequent to rifting, as a consequenceof the subsidence of the stretched continental crust that bordered the rift. Sediment washedoff the adjacent land buried the sinking crust. The stretching occurred during the rifting eventand predated formation of a new mid-ocean ridge. Examples include the East Coast and Gulfcoast of the USA.

Craton A portion of a continent that has been relatively stable since the late Neoproterozoic (since atleast about 800 Ma). This means that penetrative fabrics, regional metamorphism, andwidespread igneous activity have not occurred in the craton during the Phanerozoic. The crustof a craton includes the eroded roots of Precambrian mountain belts.

Shield A broad region, typically of low relief (though some shields have been uplifted and incisedsince the Mesozoic), where Precambrian crystalline rocks are exposed. In North America, ashield area (the Canadian Shield) is part of the craton. In South America, shield regions includecratons and Neoproterozoic orogens.

Continental platform A broad region where Precambrian rocks (basement) have been covered by a veneer ofunmetamorphosed Phanerozoic strata. Examples include the Midcontinent region of the USA,and large portions of northern Europe.

Inactive Phanerozoic Orogenic belts that were tectonically active in the Phanerozoic, but are not active today. Some orogen inactive orogens, however, have been uplifted during the Cenozoic, so they are topographically

high regions. Examples include the Appalachians of the USA and the Tasmanides of Australia.

Phanerozoic orogen A general name for active Phanerozoic orogens and inactive Phanerozoic orogens, takentogether.

Neoproterozoic orogen Orogen active at the end of the Precambrian. Examples include the Pan-African/BrasilianoOrogens of Gondwana.

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stratigraphy became available, primarily from correla-tion of well logs3 and seismic-reflection profiles,researchers realized that stratigraphic formationsthicken toward the center of a basin, and thin towardthe center of a dome. Specifically, the Phanerozoicsedimentary cover of continental interior platformsthickens to about 5–7 km in basins, because that iswhere subsidence occurred most rapidly during depo-sition. In domes or arches, cover decreases in thicknessto zero because these locations were emergent or wereshallow shoals during deposition. A drop in sea level ofthe shallow oceans that covered the interior couldexpose the crest of a dome or arch, while the center ofa basin could remain submerged. Thus, many moreunconformities occur in the Phanerozoic section ofdomes and arches than in the interior of basins.

In effect, the lateral variations in sediment thicknessthat we observe in the Midcontinent indicate that therehas been differential uplift and subsidence of regions

of the Midcontinent during deposition. In other words,some regions went up while others went down as sed-iments were accumulating. Vertical movement affect-ing a broad region of crust is called epeirogeny, andthe basins, domes, and arches that form as a result areepeirogenic structures. Significantly, the generalposition of basins and domes in the Midcontinent hasremained fixed through the Phanerozoic. For example,the Illinois Basin and Michigan Basin have alwaysbeen basins while the Ozark Dome and the CincinnatiArch have been relatively high since their initial for-mation in the Late Proterozoic or Early Cambrian.Thus, these epeirogenic structures represent permanentfeatures of continental-interior lithosphere.

Lack of stratigraphic evidence, either due to nonde-position or erosion, makes it impossible to constrainthe rate of uplift of arches and domes precisely, but wecan constrain rates of epeirogenic movement in basinsby studying subsidence curves, which are graphs thatdefine the rate at which the basement-cover contact atthe floor of the basin moved down through time. Geol-ogists generate subsidence curves by plotting sedimen-tary thickness as a function of time, after taking intoaccount the affect of compaction (Figure 22.7.3). Sub-sidence curves for intracratonic basins demonstrate

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NROD

BW

ND

AB

coastal plain

Phanerozoicorogen

Midcontinentplatform

Precambrian shield

basin

dome or arch

NROD

BW

ND

AB

0 1000

km

F I G U R E 2 2 . 7 . 2 Regional tectonic map of North Americashowing the distribution of Phanerozoic orogenic belts, shield,continental-interior platform, and basins and domes.

3When drilling for oil, exploration companies use a rotating drill bit, whichpenetrates into the earth by grinding the rock into a mixture of powder andsmall chips called “cuttings.” Circulating drilling “mud,” pumped downinto the hole, flushes the cuttings out of the hole. Since this process doesnot yield an intact drill core, the only way to determine the precise depth atwhich specific rock units lie in the subsurface is to lower instruments downthe hole to record parameters such as electrical resistively and gamma-rayproduction, parameters that correlate with rock type. A record of resistivityversus depth, or gamma-radiation versus depth, is called a well log.

Passive-marginstage

Riftingstage

Time

Subsuidencepulse

Basementsubsidence

F I G U R E 2 2 . 7 . 3 A graph illustrating how the rate ofsubsidence changes with time for an intracratonic basin. Rapidsubsidence occurs at first, probably in association with rifting.Then, as the basin subsides due to cooling, the rate ofsubsidence gradually decreases. Pulses of rapid subsidencemay occur during this time, perhaps due to the effects oforogeny along the continental margin. For example, an increasein stress may cause the lithosphere to weaken, souncompensated loads beneath the basin may sink.

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that basins have subsided overall through most of thePhanerozoic, probably due to slow cooling of lithos-phere beneath the basins, but emphasize that the rate of subsidence for a basin varies with time. For exam-ple, pulses of rapid subsidence occurred in the Michi-gan and Illinois Basins during Ordovician, LateDevonian–Mississippian, and Pennsylvanian–Permiantime. Notably, the timing of some, but not all,epeirogenic movements in the Midcontinent roughlycorresponds with the timing of major orogenic eventsalong the continental margin. For example, the time ofLate Paleozoic subsidence in the Midcontinent U.S.basins roughly corresponds with the time of theAlleghanian Orogeny (the collision of Africa with theeastern margin of North America).

M I D C O N T I N E N T F A U L T - A N D - F O L D Z O N E S Bedrockgeology of much of the Midcontinent has beenobscured by Pleistocene glacial deposits and/or thick

soils. In the isolated exposures that do occur, geolo-gists have found occurrences of folds and faults. Theregional distribution of such structures has slowlyemerged from studies of well logs (compiled on structure-contour and isopach maps), potential-field data (compiled on gravity- and magnetic-anomaly maps),and seismic-reflection profiles. Abrupt steps andridges on structure-contour maps or isopach maps, lin-ear anomalies on potential field maps, abrupt bends orbreaks in reflectors on seismic-reflection profiles—along with outcrop data—reveal that the Phanerozoicstrata of the Midcontinent United States has been dis-rupted locally by distinct belts of deformation that werefer to here as Midcontinent fault-and-fold zones(Figure 22.7.4).

Individual fault-and-fold zones range in size fromonly a few hundred meters wide and several kilometerslong, to 100 km wide and 500 km long. The north-northwest trending La Salle fault-and-fold zone of

WB

100° W

OA

NU

OD

RR

M-S

CALDIB

MCR

MB

40° N

BE

NM

UT

ND

intracratonic basin

probable Precambrian rift(narrow sediment-filled trough)

edge of the Rocky Mountains

edge of younger mountain belts

edge of the coastal plainapproximate trace of a majormidcontinent fold or fault

km0 500

F I G U R E 2 2 . 7 . 4 Map of the United States showing the distribution of Midcontinent fault-and-fold zones and of documented intracratonic rifts. BE = Beltian embayment, UT = Uinta trough, MCR = Midcontinent Rift, OA = Southern Oklahoma aulacogen, RR = Reelfoot Rift, LD = La Salledeformation belt, WB = Williston Basin, IB = Illinois Basin, MB = Michigan Basin, ND = Nashvilledome, CA = Cincinnati arch, M-S = Mojave-Sonora megashear.

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Illinois serves as an example of a larger zone—it effec-tively bisects the Illinois Basin (Figure 22.7.5). Largerzones include numerous non-coplanar faults that rangein length from <5 km to as much as 50 km. At theirtips, these faults overlap with one another in a relayfashion. Locally, a band of en echelon subsidiary faultsegments borders the trace of principal faults. In theupper few kilometers of the crust, major faults of aMidcontinent fault-and-fold zone dip steeply anddivide upwards into numerous splays. The resultingarray of faults resembles that of a flower structure. Atdepth, major faults decrease in dip (i.e., some faultsappear to be listric) and penetrate basement (Fig-ure 22.7.6). Some, but not all, faults clearly border nar-row rift basins that contain anomalously thicksequences of sediments and volcanics. The largest ofthese intracratonic rifts, the Midcontinent Rift, con-sists of two principal arms, one running from LakeSuperior into Kansas and the other running diagonallyacross Michigan (Figure 22.7.4). Faults along theserifts initiated as normal faults, but later reactivated asthrust or strike-slip faults.

620 W E S T E R N H E M I S P H E R E

IllinoisIllinois

La Salle B

elt

La Salle B

elt

Du

Quo

inD

u Q

uoin

WWaterloo/aterloo/

Dupo

Dupo

WWab

ash

Vab

ash

Valle

yal

ley

faul

t sys

tem

faul

t sys

tem

CottageCottage

DADA

Cin

cinn

ati

Cin

cinn

ati

Arc

hA

rch

CreekCreek

PennyrilePennyrile KentuckyKentucky

RoughRough

M EM EReelfo

ot

Reelfo

ot

OzarkOzarkDomeDome

AAvonvondiatremesdiatremes

Ste. Genevieve

Ste. Genevieve

MissouriMissouri M EM E

Cap au Gr sCap au Gr s

MediaMediaAnticlineAnticline

IowaIowa

Sandwich

Sandwich

Plum RiverPlum River

GroveGrove

IndianaIndiana

anticline tracesyncline tracemonocline trace

km

0 100

La Salle B

elt

Left step

Du

Quo

in

Waterloo/

Dupo

Wab

ash

Valle

yfa

ult s

yste

m

NNN

Cottage

DA

Cin

cinn

ati

Arc

h

Creek

Pennyrile

Rough

M EReelfo

ot

OzarkDome

Avondiatremes

Ste. Genevieve

M E

LincolnCap au Gr s

MediaAnticline

Sandwich

Plum River

Grove

fault; tick on down-thrown side

ILLINOIS

MISSOURI

TENNESSEE

KENTUCKY

INDIANA

IOWA

F I G U R E 2 2 . 7 . 5 Structure map of the Illinois Basin regionshowing the map traces of Midcontinent fault-and-fold zones.Note that the Cottage Grove Fault is bordered by short enechelon faults, indicative of strike-slip displacement.

Precambrian Basement

2

km (approx.)

Box anticline

Monocline

F I G U R E 2 2 . 7 . 6 Cross-sectional structure of aMidcontinent fault zone. (a) Seismic-reflection profile of theCottage Grove fault system; (b) Schematic cross sectionshowing the principal features of a Midcontinent fault zone.

(a)

(b)

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Cross sections indicate that major faults in the Mid-continent locally have vertical throws of as much as 2 km, but more typically throws are less—no morethan tens to hundreds of meters. Strike-slip compo-nents of displacement across continental-interior faultsare difficult to ascertain because of lack of exposedshear-sense indicators on fault surfaces and the lack ofrecognizable offset markers. But geologists have foundstrike-slip lineations on faults exposed in coal mines,and the en echelon map pattern of subsidiary faultsadjacent to larger faults resembles the en echelon fault-ing adjacent to continental strike-slip faults. Such fea-tures suggest that a component of strike-slip motionhas occurred on some Midcontinent faults. Where suchmotion has occurred, the faults can be considered to beoblique-slip faults. The occurrence of oblique slip, inturn, suggests that transpression or transtension hastaken place in some Midcontinent fault-and-foldzones.

In general, folds of the continental interior are mon-oclinal in profile, meaning that they have one steeplydipping limb and one very shallowly dipping to sub-horizontal limb. Locally, oppositely facing monoclinalfolds form back-to-back, creating “box anticlines”(Figure 22.7.6b). Though geologists have not yet

obtained many clear images of Midcontinent folds atdepth, several studies document that folds lie abovesteeply dipping faults and that structural relief on foldsincreases with depth.

Detailed study of spatial variations in the thicknessand facies of a stratigraphic unit relative to a fault-and-fold zone, documentation of the timing of uncon-formity formation, as well as documentation of localslump-related deformation, permits determination ofwhen the fold-and-fault zone was tectonically active.Such timing constraints suggest that the structures, ingeneral, became active during more than one event inthe Phanerozoic. Activity appears to have been partic-ularly intense during times of orogenic activity alongthe continental margin, but occurred at other times aswell. The most significant reactivation occurred dur-ing the late Paleozoic, at the same time as theAlleghanian Orogeny. This event triggered reactiva-tion of faults across the entire interior platform. Faultreactivation in the region that now lies within theRocky Mountains yielded localized uplifts (noweroded) that have come to be known as the AncestralRockies. Some authors now use this term for LatePaleozoic uplifts associated with faulting across thewidth of North America (Figure 22.7.7).

Midcontinent fault-and-fold zones do nothave random orientations,but rather display domi-nant trends over broadregions of the craton. As indicated by the mapin Figure 22.7.4, most ofthe zones either trendnorth-south to northeast-southwest, or east-west tonorthwest-southeast, andthereby outline rectilinearblocks of crust. Along-strike linkage of fault-and-fold zones seems todefine transcratonic beltsof tectonic reactivation,which localize seismicitytoday.

Of note, Midcontinentfolds closely resembleLaramide monoclines ofthe Colorado Plateau, andLaramide basement-coreduplifts of the U.S. RockyMountain region, though

F I G U R E 2 2 . 7 . 7 Late Paleozoic uplifts of the United States. The large ones west of the RockyMountain front have traditionally been referred to as the Ancestral Rockies.

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at a smaller scale. This is not surprising because, as wenoted earlier, the Colorado Plateau and Rocky Moun-tain region were once part of the craton’s interior plat-form. These regions differ from the Midcontinent onlyin that they were uplifted and deformed during the LateMesozoic–Early Cenozoic Laramide Orogeny. Duringthis event, slip on some faults of the region exceeded 15km, an order of magnitude more than occurred duringPaleozoic reactivation of faults in the Midcontinent.Because of substantial Laramide and younger uplift,and because of the dry climate of the Plateau, fault-and-fold zones of the Plateau are brazenly displayed. Butkeep in mind that, if the Midcontinent region werestripped of its glacial sedimentary blanket and itsprairie soils, it would look much like the ColoradoPlateau.

I N T R A G R A N U L A R S T R A I N A N D R E G I O N A L J O I N T I N GWhen you traverse a large fold in the Appalachianfold-thrust belt, you will find that outcrops of argilla-ceous (clay-rich) sandstones and limestones containwell-developed spaced cleavage to slaty cleavage. But,if you walk across a large fold in the Midcontinent (or

on the Colorado Plateau), you will find a distinct lackof cleavage. Layer-parallel shortening strains sufficientto form a regional cleavage apparently did not developin the Midcontinent. Microstructural studies, however,indicate that subtle layer-parallel shortening did, infact, develop in Midcontinent strata. This strain ismanifested by the development of calcite twinning, atype of intragranular, crystal-plastic strain that can beseen only with a microscope and has developed inlimestone.

Calcite twinning forms under the relatively lowpressure and temperature conditions that are character-istic of the uppermost crust. Regional studies of calcitetwinning in Midcontinent limestones indicate that themaximum shortening direction remains fairly constantover broad regions and trends roughly perpendicular toorogenic fronts, though more complex shortening pat-terns occur in the vicinity of Midcontinent fault-and-fold zones (Figure 22.7.8). Notably, strain and differ-ential stress magnitudes in the eastern Midcontinentdecrease progressively from the Appalachian-Ouachitaorogenic front to the interior—strain magnitudes are6% at the front but decrease to 0.5% in the interior,

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300 km

–3.6

–.29–2.38

–1.71–1.31

–3.5

–.51–3.88

–1.21

–18.0

–.67–.70

–4.9–4.2–10.8

–6.4–14.1

–.40–5.9

–2.5

–.38

–2.8

–5.2

–3.5

KeweenawProvince

Black Hills

Edge ofCretaceoussediments

SuperiorProvince

Appalachians

GrenvilleProvince

–3.5–2.3

Lara

mid

e fo

rela

nd

–2.7–1.1

–9.8

–4.2

–2.2

–.93 –.45

–.31 –.48

–.22

–.45

–.18–.83

–1.84

Shorteningdirection andmagnitude

–3.5

–1.1

–1.60

–1.6

–1.8

–3.14

–2.21–.49

–.46

–10.8

–.21

–.56

–1.51

Ouachitas

F I G U R E 2 2 . 7 . 8 Calcitetwinning strains in easternNorth America. Regional tectonicprovinces are labeled, except forthe Paleozoic cover sequenceinland from the Appalachian-Ouachita thrust front (bold,toothed line). Strains arepresented by orientation (shortlines) and magnitude in percent(negative is shortening);typically, maximum shorteningis horizontal and perpendicularto the Appalachian-Ouachitathrust front. Twinning data fromother tectonic provinces showpatterns that are unrelated toPaleozoic deformation.

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with associated differential stresses exponentiallydecreasing from ∼100 MPa to less than 20 MPa. Thispattern suggests that calcite twinning in strata of theeastern Midcontinent formed during the AlleghanianOrogeny, the Late Paleozoic collision of Africa withNorth America. In the western part of the Midconti-nent, twinning strains also generally lie in the range of0.5% and 3%, and the direction of maximum shorten-ing trends roughly at right angles to the Rocky Moun-tain front. This geometry implies that layer-parallelshortening in strata of the western Midcontinent devel-oped in association with compression accompanyingthe Sevier and Laramide Orogenies.

No one has yet compiled joint-trend data for theentire Midcontinent, but the literature does providedata from numerous local studies. Joint frequency dia-grams suggest that there are two dominant joint sets(one trending generally northwest and one trendinggenerally northeast) and two less prominent sets (onetrending east-west and one trending north-south) in theDevonian strata of northern Michigan. Similar, but notexactly identical, trends have been documented inOhio, Indiana, Illinois, and Wisconsin. Taken together,regional studies suggest that systematic vertical jointsets do occur in platform strata of the Midcontinent,and that in general there are east-west sets, northwestsets, north-south sets, and north-east sets, but that ori-entations change across regions and that different setsdominate in different locations. The origin of thisjointing remains enigmatic.

22.7.3 Some Causes of EpeirogenyOver the years, geologists and geophysicists have pro-posed many mechanisms to explain continental inte-rior epeirogeny. The candidates that may explainepeirogeny will be briefly discussed and are illustratedin Figure 22.7.9.

• Cooling of Unsuccessful Rifts. Intracratonic basinsmay form because of thermal contraction due to cool-ing of unsuccessful rifts that opened during the LateProterozoic. When active extension ceased, these riftscooled and subsided, much like the rifts that underliepassive margins. Such epeirogeny continued throughthe Phanerozoic at ever-decreasing rates.

• Variations in Asthenosphere Temperature. Modernseismic tomography studies of the Earth’s interiordemonstrate that the Earth’s mantle is thermally het-erogeneous. As lithosphere drifts over this heteroge-neous asthenosphere, it conceivably warms whencrossing hot asthenosphere and cools when crossingcool asthenosphere. These temperature changes

could cause isostatic uplift (when warmed) or sub-sidence (when cooled) of broad regions of thelithosphere.

• Changes in State of Stress. Changes in stress state inthe lithosphere may cause epeirogenic movement inmany ways. For example, as differential stressincreases, plastic deformation occurs more rapidly,so that the viscosity of the lithosphere effectivelydecreases. Thus, an increase in differential stressweakens the lithosphere. If this were to happen,denser masses in the crust (e.g., a lens of maficigneous rock below a rift), which were previouslysupported by the flexural strength of the lithosphere,would sink, whereas less dense masses (e.g., a gran-ite pluton) would rise. Thus, differential epeirogenicmovements may be localized by preexisting hetero-geneities of the crust, which is set free to move in anattempt to attain isostatic equilibrium by the weak-ening of the lithosphere that accompanies anincrease in differential stress. Some geologists havesuggested that changes in horizontal stress magni-tude may also cause epeirogeny by buckling thelithosphere, or by amplifying existing depressions(basins) or rises (arches).

• Flexural Response to a Load. Creation of a largeload, such as a volcano or a stack of thrust sheets,results in flexural loading on the surface of the con-tinent, and thus bending down of the continent’ssurface. Flexural loading due to emplacement ofthrust sheets leads to the development of asymmet-ric sedimentary basins, called foreland basins, onthe craton side of fold-thrust belts. In addition tocausing a depression to form, the levering effect ofthe loaded lithosphere may cause an uplift, or outerswell, to form on the cratonic-interior of the depres-sion. This effect may have caused Midcontinentuplifts, like the Cincinnati arch, to rise in responseto loading the continental margin by thrust sheets ofthe Appalachian orogen.

• Block Tilting. As noted earlier, Midcontinent fault-and-fold zones divide the upper crust into fault-bounded blocks. Changes in the stress state in thecontinental interior may cause tilting of regional-scale, fault-bounded blocks of continental crust rel-ative to one another. These could cause uplift orsubsidence of the corners and edges of blocks.

• Changes in Crustal-to-Lithosphere Mantle Thick-ness Ratio. Continental elevation is controlled,regionally, by isostasy. Since crust and mantle donot have the same density, any phenomenon thatcauses a change in the proportion of crust to lithos-pheric mantle in a column from the surface of theEarth down to the level of isostatic compensation

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Sinking mass in the mantle.

(f)

Flexural loading on the margin.

(e)

Relief amplification due to in-plane stress.

(d)

Thermal anomalies in the mantle.

(b)

Thermal subsidence of a rift.

(a)

Uncompensated load sinks due to increase in differential stress.

(c)

Jostling blocks above a mid-crustal detachment.

(h)

Epeirogeny due to changes in crust/mantle lithosphere ratio.

Starting condition

Crustal thickeningwithout lithosphericmantle thickening

(g)

No thickening

Elevation without litho-

Elevation withthickening

Thickenedlithosphericmantle

sphere mantlethickening

Sinking delaminatedlithospheric mantle

Thinned lithospheric mantle

Higher elevationbecause of mantlelithosphere thinning

Crust

Mantle lithosphere

Elevation withoutsubduction

Sinking mass

Increase in differential stressamplifies preexisting structures

Preexisting bumps and dimples

Upwelling hotasthenosphere

Rifting; creation of hot lithosphere

Rifting ceases; hot lithosphere subsides

Uncompensated loadheld up by thickelastic lithosphere

Increase in differentialstress causes adecrease in effectiveelastic thickness, sothe uncompensatedload sinks

Orogenic belt(vertical load)

F I G U R E 2 2 . 7 . 9 Models of epeirogeny. (a) Thermal cooling over an unsuccessful rift (before andafter); (b) Uplift related to thermal anomalies in the mantle; (c) Vertical movement of anuncompensated load due to changes in the elastic thickness of the lithosphere; (d) Amplification ofpreexisting bumps and dimples due to in-plane stress; (e) Flexural loading of a lithospheric margin; (f) Epeirogeny related to subduction; (g) Epeirogeny due to changes in the ratio of crustal thicknessto lithosphere mantle thickness; (h) Epeirogeny due to tilting of regional fault-bounded blocks.

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has the potential to cause a change in the elevationof the continent’s surface. For example, thickeningof the crust, perhaps due to plastic strain in responseto tectonic compression relative to the lithosphericmantle, would cause a rise in elevation; decreasingthe thickness of the lithospheric mantle in responseto delamination (separation of lithospheric mantlefrom the base of the plate) could also cause a rise incrustal elevation; adding basalt to the base of thecrust (a process called underplating) would therebythicken the crust and could cause uplift.

• Subduction. Subduction of oceanic lithosphere cancause epeirogenic movement because, as the sub-ducted plate sinks, it pulls down the overlying con-tinent. To picture this phenomenon, imagine abucket filled with honey (representing the asthenos-phere), in which an iron ball (representing oceaniclithosphere) has been suspended just below the sur-face (Figure 22.7.10). Now, place a film of plasticwrap (representing the continental lithosphere) overthe top of the honey, and then release the ball. As theball sinks (representing subduction of the oceaniclithosphere), the plastic wrap is pulled down. Thisdownward motion is epeirogenic subsidence.

22.7.4 Speculations on MidcontinentFault-and-Fold Zones

As noted above, stratigraphic evidence demonstratesthat Midcontinent fault-and-fold zones were reacti-vated multiple times during the Phanerozoic. But howdid the zones originate in the first place? Did the faultsinitiate during the Phanerozoic by brittle rupturing ofintact crust in response to compression caused byorogeny along the continental margin? If so, then thefaults started out as reverse or transpressional faults.Or did the structures form earlier in Earth history? We

favor the second proposal, and suggest that Midconti-nent fault-and-fold zones initiated as normal faultsduring episodes of Proterozoic extension. Thus, dis-placements in the zones during the Phanerozoic repre-sent fault reactivation. We base this statement on theobservation that Midcontinent fault-and-fold zoneshave the same trends as Proterozoic rift basins anddikes in the United States. They are not systematicallyoriented perpendicular to the shortening directions ofmarginal orogens.

If the above hypothesis is correct, then the fault-and-fold zones of the Midcontinent, as well as of theRocky Mountains and Colorado Plateau, are relicts ofunsuccessful Proterozoic rifting. Once formed, theyremained as long-lived weaknesses in the crust, avail-able for reactivation during the Phanerozoic, when theinterior underwent slight regional strain. A reversecomponent of displacement occurred on the faults ifreactivation was caused by regional shortening,whereas a normal sense of displacement occurred ifreactivation was caused by regional extension. Onfaults that were not perpendicular to shortening orextension directions, transpression or transtension ledto a component of strike-slip motion on faults.Regarldless of slip direction, displacement on thefaults generated fault-propagation folds in overlyingstrata. Note that reverse or transpressional motion rep-resents inversion of Proterozoic extensional faults, inthat reactivation represents reversal of slip on thefaults. In cases where the faults bounded preserved riftbasins, the inversion led to thrusting of the rift’s con-tents up and over the rift’s margin. But rift basins donot occur in all fault-and-fold zones, either becauseProterozoic displacements were too small to create abasin, or because the basin was eroded away duringlatest Proterozoic rifting, before Phanerozoic stratawere deposited.

The hypothesis that Midcontinent fault-and-foldzones are reactivated Proterozoic normal faults isappealing because it explains how these structurescould have formed with the orientations that they have,and how they formed without the development ofregional cleavage. Zone orientation simply reflects thetrends of preexisting Proterozoic normal faults, not theorientation of a regional stress field during thePhanerozoic. The lack of regional cleavage reflects the fact that development of these structures is notassociated with significant shortening above detach-ments within the Phanerozoic section. This hypothesisalso explains the timing of movement—faults werereactivated primarily during marginal orogenies or rift-ing events, when displacement of the continental mar-gin caused a slight strain in the interior, and this strain

F I G U R E 2 2 . 7 . 1 0 Schematic drawing showing the sinkingball model for epeirogeny caused by subduction (or dynamictopography). As the ball sinks through the honey (shaded area),the thin plastic film on the surface of the honey is pulled down.

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was accommodated by movement on faults. Perhaps away to envision Phanerozoic Midcontinent faultingand folding is to think of the upper crust in the cratonas a mosaic of rigid fault-bounded blocks that jostlerelative to one another in response to changes in thestress state of the continental interior (Figure 22.7.11).Depending on the geometry of the stress during a giventime period, blocks may move slightly apart, moveslightly together, or move laterally relative to oneanother. Movements tend to be transpressional ortranstensional, for the belts are not oriented appropri-ately for thrust, reverse, or strike-slip faulting alone tooccur.

The greatest amount of movement in Midcontinentfault-and-fold zones occurred in Late Paleozoic time,when Africa collided on the east, South America col-lided on the south, and a subduction zone had formedalong the southwest. This pulse resulted in the forma-tion of the Ancestral Rockies of the Rocky MountainsProvince as well in the kilometer-scale displacementsin fault-and-fold zones across the Midcontinent (Fig-ure 22.7.7). The intracratonic strain that formed at thistime is significantly less than that in Asia during theCenozoic collision of India with Asia. The contrast incontinental-interior response to collision probablyreflects the respective strengths of these two conti-nents. The interior of Asia consists of weak continen-tal crust of the Altaids Orogen, while the interior of theUnited States is a strong craton.

While major movements appear to have accompa-nied major marginal orogenies, movement on thesefaults can occur during nonorogenic times as well. Forexample, historic intraplate earthquakes (earthquakesoccurring in a plate interior, away from plate bound-aries) at New Madrid, Missouri, result from move-ments at the intersection of two Midcontinent fault-and-fold zones. This movement may be a response tothe ambient stress in the continental lithosphere,caused by ridge-push force and/or basal traction, or tostress resulting from epeirogenic movements.

22.7.5 Closing RemarksThe speculative tone used in this essay emphasizes thatgeologists need to obtain more data on structures incratonic interiors before we can confidently explainthem and assess their significance. However, it hasbecome increasingly clear that cratonic interiors werenot tectonically dead during the Phanerozoic. Rather,they were sensitive recorders of plate interactions,which may have caused jostling of upper-crustalblocks. Although we have focused on examples ofstructures in the Midcontinent of the United States,

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∗∗∗ ∗

∗∗

CordilleranCordilleranOrogenOrogen

AppalachianAppalachianOrogenOrogen

CanadianCanadianShieldShield

Coastal PlainCoastal Plain

CordilleranOrogen

AppalachianOrogen

CanadianShield

Coastal Plain

F I G U R E 2 2 . 7 . 1 1 (a) Map showing block model ofintracratonic tectonism in the United States. Stars indicateseismically active regions. The diagram in (b) illustrates howthrust and strike-slip motions can be reactivated in response toregional compression, while the diagram in (c) illustrates howuplift along a reverse fault leads to oblique slip along the side ofthe block.

(a)

(b)

(c)

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keep in mind that these structures can be viewed astype examples for continental-interior platformsthroughout the world. Finally, it is important to empha-size once more that the dramatic basement-coreduplifts that developed in the Rocky Mountains, as wellas in the Sierra Pampeanas of Argentina, and the TienShan of southern Asia, are similar in style to those ofthe U.S. Midcontinent; all may have been formed byreactivation of preexisting faults.

A DDITION A L R E A DING

Bally, A. W., 1989. Phanerozoic basins of North Amer-ica. In Bally, A. W., and Palmer, A. R., eds., TheGeology of North America—An Overview, the Geol-ogy of North America, v. A, Boulder, CO: Geologi-cal Society of America.

Bond, G. C., 1979. Evidence for some uplifts of largemagnitude in continental platforms. Tectonophysics,61, 285–305.

Cathles, L. M., and Hallam, A., 1991. Stress-inducedchanges in plate density, Vail sequences, epeiro-geny, and short-lived global sea level fluctuations.Tectonics, 10, 659–671.

Craddock, J., Jackson, M., van der Pluijm, B. A., andVersical, R. T., 1993. Regional shortening fabrics ineastern North America: far-field stress transmissionfrom the Appalachian-Ouachita orogenic belt. Tec-tonics, 12, 257–264.

Gurnis, M., 1992. Rapid continental subsidence fol-lowing the initiation and evolution of subduction.Science, 255, 1556–1558.

Howell, P. D., and van der Pluijm, B. A., 1990. Earlyhistory of the Michigan basin: subsidence andAppalachian tectonics. Geology, 18, 1195–1198.

Karner, G. D., 1986. Effects of lithospheric in-planestress on sedimentary basin stratigraphy. Tectonics,5, 573–588.

Lambeck, K., 1983. The role of compressive forces inintracratonic basin formation and mid-plate oroge-nies. Geophysical Research Letters, 10, 845–848.

Marshak, S., Nelson, W. J., and McBride, J. H., 2003.Phanerozoic strike-slip faulting in the continentalinterior platform of the United States: examplesfrom the Laramide orogen, Midcontinent, andAncestral Rocky Mountains. Geological Society ofLondon Special Publication (in press).

Marshak, S., and Paulsen, T., 1996. Midcontinent U.S.fault and fold zones: a legacy of Proterozoicintracratonic extensional tectonism? Geology, 24,151–154.

Park, R. G., and Jaroszewski, W., 1994. Craton tecton-ics, stress, and seismicity. In Hancock, P. L., ed.,Continental Deformation. Oxford: Pergamon Press.

Paulsen, T., and Marshak, S., 1995. Cratonic weakzone in the U.S. continental interior: the Dakota-Carolina corridor. Geology, 22, 15–18.

Quinlan, G. M., and Beaumont, C., 1984. Appalachianthrusting, lithospheric flexure, and the Paleozoicstratigraphy of the eastern interior of North Amer-ica. Canadian Journal of Earth Sciences, 21,973–996.

Sloss, L. L., 1963. Sequences in the cratonic interior ofNorth America. Geological Society of America Bul-letin, 74, 93–114.

Van der Pluijm, B. M., Craddock, J. P., Graham, B. R.,and Harris, J. H., 1997. Paleostress in cratonicNorth America: implications for deformation ofcontinental interiors. Science, 277, 792–796.

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