margot mcmechan [email protected]

Northern Resources Development

Margot [email protected]

Earth Sciences Sector


Transverse Structure and Tectonic Inversion Mosque Mountain area, southeastern Skeena Fold Belt


Evenchick, 2003

Mosque Mtn area

LOCATION

At NW end southwestern Sustut basin of preservation


• Paleocurrents lower Tango Creek SW directed (Eisbacher, 1974)– Basin of preservation

STRATIGRAPHY

• Major change in level sub-Sustut unconformity across Mosque Mountain area


MosqueMtn

Anomalous area with transverse northeast to east and orogen parallel northwest compressive structures

Evenchick et al., 2003


STRUCTURAL STYLE

• Similar for both northwest and northeast to east trends• FOLDS - dominantly detachment folds, formed above blind

detachments, wavelength largely controlled by thickness dominant member; different levels of detachment

Longer wavelength folds where thick sandstones occur in Tango, and

sst/cgl at base Brothers Peak

Thick sandstones near base TangoThick sandstones near base Tango

Thick sandstone/conglomerate base Brothers Peak, at Mosque MountainThick sandstone/conglomerate base Brothers Peak, at Mosque Mountain


• NW-trending folds common throughout area• NE- to E-trending folds occur mainly in

eastern part of area

Nearly orthogonal detachment folds Brothers Peak Fm, south of Mosque Mtn


• Fold interference evident on stereograms (widely dispersed poles, calt FA nearly orthogonal to mapped fold)

• In o/c as variable plunges and unusual orientation

Area where NE to E trending folds more common


REVERSE FAULTS

• Can be difficult to recognize • NW, N, NE and E-trends • Asymmetry subparallel hanging and/or

footwall folds indicate:– NE-directed motion on NW trending faults; – NW or SE-directed motion on NE trending

faults

View to SView to S


RELATIVE TIMING RELATIONSHIPS

View to NWView to NW

NE-trending crenulations of steep E limb SE plunging fold. View to N

NE-trending crenulations of steep E limb SE plunging fold. View to N

a

a

• Mainly from relationships between nearly orthogonal folds and reverse faults


• NO consistent timing relationship• NE of Mosque Mtn at least 3 episodes NE-trending

compression separated by movement on NW-trending structures

View to SEView to SE


SHORTENING

• NE directed approx 30%• NW-SE directed 5-10%

• Regional 44% NE directed, NE Skeena Fold Belt (Evenchick, 1991)

MosqueMtn

A

A’

B’

B

Late dextral oblique-slip and normal faults


EARLY PRE-SUSTUT STRUCTURE


POSSIBLE ORIGINS COMPRESSIVE STRUCTURAL

TRENDSNORTHWEST-TRENDS

– Express NE-directed shortening – Consistent with northeastward expansion of orogenic wedge

(Critical Taper Model of Davis et al., 1983; Evenchick 2001)


NORTHEAST- TO EAST- TRENDING– Express NW- to N-directed shortening highly oblique to

northeastward expansion of orogenic wedge

Potential explanations– En-echelon strike-slip faulting– Rotation about a vertical axis– Oblique ramp– Basement features

View to SWView to SW


En-echelon strike-slip faults Rotation about a vertical axis

Evenchick, 2003

Pre-mid Cretaceous

(95 Ma) Both have geometric and timing problems


Oblique Ramps

- cause out of plane stresses

- oblique folds, faults, cleavages, joints

in hanging wall

Surface geology, depth to magnetic basement calculations (Lowe et al., in press) rule out


Basement Features- transverse sedimentary taper

Structure curves out to foreland where sedimentary section thickens significantly (ie Helena Embayment, Crowsnest Deflection)

Soto et al., 2002

Constant thickness Thin Thick

Such thickening occurs under Mosque Mtn area


Basement Features – effecting preferred zones of detachment

Sedimentary taper, and distribution of preferred zones of detachment in Bowser could cause northeastward swing in structural trends,

- help explain why less shortening in Sustut south of Mosque Mtn area

Bowser

Northern Resources DevelopmentBasement Features- inversion of pre-Sustut “basement uplift”

NW end pre-Sustut basement uplift sloped wrong way for compressive out-of-plane deformation with NE-directed transport

-localization NW end Sustut preservational basin

- potential development oblique structures Mosque Mtn area along higher detachments

Inversion above NE-directed thrust resulted in

Bowser

Bowser


A STRUCTURAL MODEL

• Skeena Fold Belt characterized by detachment folding above multiple detachments and blind thrusts

• Mosque Mtn area nearly orthogonal trends – defm on alternatively active detachments and (blind) thrusts– NE- to E-trends out-of-plane

stress due to inversion or reactivation north end basement uplift or detachment deflection at N end

– NW-trends formed at times or above detachments that lacked those influences

(Evenchick, 1991)


SUMMARY

• Anomalous nearly orthogonal compressive structures, parallel and transverse to the regional structural trend, formed alternatively during post early Maastrichtian deformation of the Mosque Mtn area

• Structural inversion of the pre-Sustut “basement” uplift produced a SE-facing monocline that localized the NW limit of preserved Sustut strata to the Mosque Mtn area and caused detachment deflection, and out-of-plane stresses to form anomalous transverse structures in the Mosque Mtn area

• The NW end of a pre-Albian pre-Sustut “basement” uplift underlies the Mosque Mtn area

margot mcmechan [email protected]

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