margot mcmechan [email protected]
DESCRIPTION
Transverse Structure and Tectonic Inversion Mosque Mountain area, southeastern Skeena Fold Belt. Earth Sciences Sector. Northern Resources Development. Margot McMechan [email protected]. LOCATION. Mosque Mtn area. At NW end southwestern Sustut basin of preservation. Evenchick, 2003. - PowerPoint PPT PresentationTRANSCRIPT
Northern Resources Development
Margot [email protected]
Earth Sciences Sector
Northern Resources Development
Transverse Structure and Tectonic Inversion Mosque Mountain area, southeastern Skeena Fold Belt
Northern Resources Development
Evenchick, 2003
Mosque Mtn area
LOCATION
At NW end southwestern Sustut basin of preservation
Northern Resources Development
• Paleocurrents lower Tango Creek SW directed (Eisbacher, 1974)– Basin of preservation
STRATIGRAPHY
• Major change in level sub-Sustut unconformity across Mosque Mountain area
Northern Resources Development
MosqueMtn
Anomalous area with transverse northeast to east and orogen parallel northwest compressive structures
Evenchick et al., 2003
Northern Resources Development
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
Northern Resources Development
• 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
Northern Resources Development
• 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
Northern Resources Development
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
Northern Resources Development
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
Northern Resources Development
• 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
Northern Resources Development
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
Northern Resources Development
EARLY PRE-SUSTUT STRUCTURE
Northern Resources Development
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)
Northern Resources Development
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
Northern Resources Development
En-echelon strike-slip faults Rotation about a vertical axis
Evenchick, 2003
Pre-mid Cretaceous
(95 Ma) Both have geometric and timing problems
Northern Resources Development
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
Northern Resources Development
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
Northern Resources Development
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
Northern Resources Development
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)
Northern Resources Development
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