Plan de mercadeo

Platt (1993) extended the critical taper Coulomb approach into three dimensions for linear systems, undergoing oblique convergence, and calculated that the angle o f obliquity o f the motion o f both the rigid backstop (the region at the rear o f the converging system) and the deforming and deformable wedge are functions o f the mechanical properties o f the boundaries, the fluid pressure ratio and the wedge geometry. Addition of the third dimension adds the mechanical variable o f the shear traction on the rear of the wedge and the observational variable of the distribution o f the strike-parallel velocity within the wedge (Platt, 2000).

In an obliquely convergent Coulomb system there is a limiting value for /o (Figure 3.2), the angle o f obliquity o f the velocity o f the backstop relative to the underthrust slab, below which the wedge will move at the same velocity as the backstop, given by:

tan7o = ^o

^ ^ [^(l - A) - + fil

where is the coefficient o f friction along the backstop o f the wedge; and the fluid pressure ratio at the base o f the wedge; and 6 is the taper o f the accretionary wedge in radians (Platt, 1993).

For angles of obliquity above this limiting value, the wedge will move laterally at a limiting strike-parallel velocity, Vy, intermediate between the velocity o f the backstop and the underthrust plate, and dependent on its mechanical properties and geometry, given by:

lOil — A) — Otl*\/l + f^n V =|UoVcosyo'

In this case strike-parallel motion will occur on a strike-slip fault and strike-normal motion will be at a lower angle o f obliquity relative to the underthrust plate than exists between the backstop and underthrust plate.

Platt (2000) presented three equations relating four mechanical variables to the three observational variables (Yo, 6 and <5 - the angle between the maximum principal compressive stress at the upper rear o f the wedge and the strike-normal direction). This allows three o f the mechanical properties o f the wedge to be determined if one o f the mechanical properties is known (A, for example, could be determined by formation tests and sonic log measurements in a currently active wedge).

Unless the wedge is accreting new material, accompanied by internal shortening, there is no mechanism in a Coulomb wedge for strike-parallel motion to be distributed through the wedge (Platt, 2000). The wedge can therefore either move together with the upper plate, to which it is attached at the backstop, or detach to form a forearc sliver that will move independently o f the upper and lower plates. Platt (2000) states that the limiting angle is likely to be -45°.

The location of the partitioning strike-slip fault at the rear of the proto-wedge is not controlled by geology and can be located at any suitable point. The mechanical strength o f the upper plate is likely to have the greatest impact on the location o f the partitioning fault. In the case of the External Zones, it is reasonable that the Crevillente Fault Zone (CFZ) owes its period of dextral slip to partitioning oblique convergence. As such a partitioning fault the CFZ would be influenced in location by the underthrust Internal Zones, which extend below the External Zones to a short distance SE o f the CFZ (see Figure 5.1).

Platt et al. (2003) estimate 31 km of dextral strike slip occurred on the CFZ as a whole, based on large rotations (of the order o f 180° (Allerton et al., 1993) of km-sized blocks distributed in a 10 km wide zone around the Crevillente Fault. The episode of dextral slip on the CFZ after Early Miocene N7 (Section 6.4) is likely to account for at least a portion of it, although I cannot rule out earlier dextral strike slip. The CFZ as it is today did not exist until at least late Serravallian time (Section 6.4), but there is no reason that a through-going structure did not exist prior to division by the Socovos Fault before middle Serravallian time. While the CFZ today seems to die out west of La Paca

C hap ter 3 - M echanics

(Figure 6.1), it is possible that earlier structures extending the zone westward are concealed beneath the Middle Miocene and Pliocene sediments found there today. Reactivation of the CFZ strand after the Socovos Fault brought it to its current position seems to be dominantly sinistral in sense, with dextral faults o f apparently random orientations probably remnants of the earlier dextral deformation. The alternative method o f transferring 31 km o f dextral slip on the CFZ is for it to transfer its displacement westward onto thrust faults, which were active in latest Burdigalian to early Langhian time (Sections 4.3.2-4.3.6, for example), an explanation put forward by Platt et al. (2003).

Platt et al. (2003) calculated an additional 41 km of strike-parallel displacement within the eastern Subbetic from palaeomagnetic data. It is possible that as the thrust front advanced, dextral partitioning on the back o f the proto-wedge was relocated toward the foreland. Because the thrust wedge was deforming it is not necessary for there to be additional strike slip faults developed toward the foreland, as the component of velocity parallel to strike could be taken up on thrust faults.