1. Northern Resources Development Margot McMechan mmcmecha@nrcan.gc.ca Earth Sciences Sector Northern Resources Development Transverse Structure and Tectonic Inversion Mosque Mountain area, southeastern Skeena Fold Belt

2. Northern Resources Development Evenchick, 2003 Mosque Mtn area LOCATION At NW end southwestern Sustut basin of preservation

3. 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

4. Northern Resources Development Mosque Mtn Anomalous area with transverse northeast to east and orogen parallel northwest compressive structures Evenchick et al., 2003

5. 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 Tango Thick sandstone/conglomerate base Brothers Peak, at Mosque Mountain

6. 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

7. 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

8. 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 S

9. Northern Resources Development RELATIVE TIMING RELATIONSHIPS View to NW 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

10. 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 SE

11. Northern Resources Development SHORTENING NE directed approx 30% NW-SE directed 5-10% Regional 44% NE directed, NE Skeena Fold Belt (Evenchick, 1991) Mosque Mtn A A’ B’ B Late dextral oblique-slip and normal faults

12. Northern Resources Development EARLY PRE-SUSTUT STRUCTURE

13. Northern Resources Development POSSIBLE ORIGINS COMPRESSIVE STRUCTURAL TRENDS NORTHWEST-TRENDS –Express NE-directed shortening –Consistent with northeastward expansion of orogenic wedge (Critical Taper Model of Davis et al., 1983; Evenchick 2001)

14. 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 SW

15. 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

16. 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

17. 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

18. 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

19. Northern Resources Development Basement 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

20. 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)

21. 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