Rift formation has long been the focus of attention for researchers, and numerous studies have been carried out in order to understand causes and modes of whole lithospheric extension (e.g., Neumann and Ramberg, 1978; Illies, 1981; Palmason,1982; Morgan and Baker, 1983; Neugebauer, 1983; Dewey and Hancock, 1987; Keen et al., 1987; Khain, 1992; Ziegler, 1992; Ruppel, 1995; Brun, 1999; Whitmarsh et al., 2001; Corti et al., 2003). The process of lithospheric rifting is classically considered to be a product of “active rifting” or “passive rifting”, depending upon which forces are involved at the inception of rifting. Continental rifting is conventionally described as a thinning process of the whole lithosphere, ultimately leading to rupture of the continent, onset of sea-floor spreading and the consequent formation of a mid-oceanic ridge. Rifting is the initial and fundamental process by which the separation of a continent into two tectonic plates takes place. Active rifting or mantle-activated rifting has been classically ascribed to the ascent of a mantle plume impinging upon the base of the lithosphere, with consequent heating and thinning of the lithosphere. Passive rifting has been classically considered the result of horizontal stretching of the continental lithosphere, in which far-field tectonic stresses, generated at the boundaries of the lithospheric plates, result in lithosphere extension.

We study from time the passive continental rifting leading to opening of the fossil Jurassic Ligurian Tethys oceanic basin, by investigating the structural ad petrologic features recorded in the mantle peridotites of the Alpine-Apennine orogeny (North-West Italy), that represent the direct exposure in nature of the mantle lithosphere of the basin. Passive lithosphere extension is testified by km-scale extensional shear zones, induced by far field tectonic forces, which thinned the sub-continental lithosphere and caused the passive upwelling of the asthenosphere (the passive a-magmatic rifting stage). After significant adiabatic upwelling, asthenosphere underwent decompression melting and the melts infiltrated through the extending lithospheric mantle by diffuse porous flow, frequently exploiting porosity bands of former shear zones (the passive magmatic rifting stage). Deformation and melt percolation interacted and mutually enhanced, strongly modifying the rheological characteristics of the mantle lithosphere along the axial zone of rifting (forming a weakened/softened mantle wedge). The passive rifting system changed to splitting and continental break-up, passing to an active rifting system in the case deeper/hotter asthenosphere actively upwelled within the axial zone of weakened mantle lithosphere.

thank u....

Giovanni B. Piccardo....

thank u https://www.researchgate.net/profile/Martin_Menzies

In addition to the previous answers, I want to add the following reasons of rifting:

1. Rifting above thermal plumes- hot spots at the base of the lithosphere, three rift arms at 120 degrees in which one arm shut off.

2. Flextural relating rifting- changing the radius of curvature of a plate, occurs where the lithosphere bends just prior to descending beneath a collision boundary.

3. Ridge-push and slap-pull forces........

4. Rifts formed in regions of thickened and elevated crusts....

5. Rifting occurs at release bends along continental strike- slip faults........

6. Back arc extension associated with convergence.......etc.

In my opinion passive rifting accounts for 80% of all rifts.  Can you tell me which rifting event you are particularly interested in?  There may be some additional information that I can provide.

How can we justify the statement that active rifting is operating on the Afar Triangle, which Is the junction point of three rift arms that have roughly 120 degree between the neighbor arms?  East Africa Rift System is mainly mechanical stretching induced rifting (Corti et al., 2013), although magmatic intrusion further weakens the lithosphere at the breaking stage. 

Kasturi,

Something more to add to Martin's answer, who defined the main different processes for rifting and rifted continental margins:

(a) ACTIVE RIFTING : bottom-up processes involving mantle upwelling/plumes;

(b) PASSIVE RIFTING : top-down processes involving plate motions drive passive rifting & the formation of non-volcanic (continental) margins;

(c) CONTINUUM - between volcanic and non-volcanic margins and active and passive rifting.

My experience of the Ligurian Tethys evolution evidence the further case of :

(d) TRANSITION - from early passive rifting to subsequent active rifting, as described and modelled by Huismans, R.S et al. 2001 (Huismans, R.S., Podladchikov, Y.Y., Cloetingh, S., 2001. Transition from passive to activerifting: relative importance of asthenospheric doming and passive extension of the lithosphere. J. Geophys. Res. 106 (11), 271–291).

In this case, the PASSIVELY extending mantle lithosphere is destabilized (i.e., rheologically weakened and softened) along the axial zone of extension by concomitant deformation / melt percolation (melt thermal advection) processes, that allow deeper/hotter asthenosphere to upwell ACTIVELY within this zone, inducing transition from passive to active regime.)

The horizontal stresses caused by the active asthenosphere upwelling may start to compete with the far-field tectonic forces and even drive the system causing a change from passive to active rifting forces (Huismans et al. 2001). The model may explain several key features of intra-continental rift basins, such as the late (post-rift) mantle related volcanism.

In the case of the Ligurian Tethys, the late post-rifting occurrence of the oceanic MORB volcanism, when the sea-floor was already mostly composed by syn-rifting-exhumed and exposed mantle peridotites, is well consistent with the suggestions of Huismans et al. (2001) of the late (post-rift) mantle-related volcanism. (see: Piccardo G.B., Padovano M. & Guarneri L. 2014, The Ligurian Tethys: Mantle processes and geodynamics ESR, online)

This model of transition from passive to active regimes seems to be applicable to the early rifting stages of modern major slow spreading oceans (like North Atlantic).

This is a very interesting question which is still open. It is very difficult to differentiate between active and passive mechanisms of rifting. In the recent manuscript (Article Radiogenic trigger and driver for continental rifting and in...

A continental rift requires extension that causes lithospheric thinning. Lithospheric thinning may continue to the point of breakup of the continental lithosphere and the generation of oceanic crust. However, many rifts become inactive before the formation of a mid-oceanic ridge and sea-floor spreading . Two distinct mechanisms have been suggested to interpret the dynamics of continental extension: (1) horizontal extension of the continental lithosphere due to plate motions; (2) dynamic uplift by a large mantle plume. Examples of the first mechanism by extension of continental lithosphere in a transfer zone between two master strike-slip faults initiated rifting of the Appalachian successor basins in eastern Canada (Waldron et al., 2015) . The second mechanism, hot upwelling from a mantle plume, initiated rifting in East Africa and caused the uplift of much of the African continent (Hammond et al., 2013).

Recent manuscript: Article Permian rifting processes in the NW Junggar Basin, China: Im...